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Tempel 1
Icarus 187 (2007) 123–131 www.elsevier.com/locate/icarus
Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1 K.O. Mason a,b,∗ , M. Chester c , A. Cucchiara c , C. Gronwall c , D. Grupe c , S. Hunsberger c , G.H. Jones d , S. Koch c , J. Nousek c , P.T. O’Brien e , J. Racusin c , P. Roming c , P. Smith a , A. Wells c,e , R. Willingale e , G. Branduardi-Raymont a , N. Gehrels f a Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK b Particle Physics & Astronomy Research Council, Polaris House, North Star Ave., Swindon, Wilts SN2 1SZ, UK c Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA d Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e Department of Physics & Astronomy, University of Leicester, University Rd., Leicester, UK f NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA
Received 31 March 2006; revised 14 September 2006 Available online 22 November 2006
Abstract We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29–2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant rebrightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4 ± 0.2) × 1032 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190 ± 20 km2 . © 2006 Elsevier Inc. All rights reserved. Keywords: Comets; Comet Tempel-1; Comets, composition; Comets, coma; Comets, dust
1. Introduction The Swift observatory (Gehrels et al., 2004) flies in a lowEarth orbit and is designed to localise and respond rapidly to Gamma-ray bursts (GRB). As well as a wide-field hard X-ray/ Gamma-ray camera (the Burst Alert Telescope; Barthelmy et al., 2005), Swift carries two narrow field telescopes that are sensitive in the optical/UV and X-ray bands, respectively. These are the Optical/UV Telescope (UVOT; Roming et al., * Corresponding author.
E-mail address: [email protected] (K.O. Mason). 0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.09.021
2005) and the X-ray Telescope (XRT; Burrows et al., 2005). Designed to provide multiwavelength follow-up of GRB afterglows, these telescopes provide a multiwavelength observing facility on a flexible and rapidly manoeuvrable spacecraft. We have used this capability to make extended observations of Comet 9P/Tempel 1 during and after the collision between it and the Deep Impact probe (A’Hearn et al., 2005). Willingale et al. (2006) describe the analysis of XRT Comet Tempel observations. We concentrate in this letter on data from the UVOT, which we used for imaging and timeresolved photometry of the comet in a passband centered on 250 nm.
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Fig. 1. Effective area as a function of wavelength for the UVW1 filter on Swift UVOT. Also shown, on a relative scale, is the photon spectrum of Comet 73P/Schwassmann–Wachmann 3-C taken with the UVOT in May 2006 (lighter curve, courtesy of Wayne Landsman), multiplied by the UVW1 effective area at each wavelength. This illustrates the actual response of the filter to a cometary spectrum. The dotted curve is the solar spectrum normalised to the continuum flux of Comet 73P/3-C.
2. Observations and analysis UVOT has a 0.3 m diameter primary mirror and utilises an intensified CCD detector to record individual photon events with a precision of 11 ms within a maximum field of view of 17 × 17 arcmin. The spatial pixel size on the sky is 0.5 arcsec, and the point spread function of the instrument has a FWHM of about 2 arcsec. Data on Comet Tempel were collected over a field of view of 8 arcmin on a side, chosen to restrict the telemetry flow to the ground to manageable levels. A series of broadband filters and grisms can be inserted into the optical path of UVOT by rotating a filter wheel. For the observations of Comet 9P/Tempel 1, we used an ultraviolet passband filter (designated UVW1). The wavelength response of the telescope with this filter is shown in Fig. 1. The sensitivity peaks at about 250 nm, with a full width at 10% response of about 130 nm. The UVW1 filter includes emission bands of the species CS, and OH, thus providing coverage of one of the direct dissociation products of H2 O (e.g., Feldman et al., 1993; Smith et al., 1980). We do not have a spectrum of Comet 9P/Tempel 1 in the waveband covered by the UVW1 filter, but we did obtain a spectrum of Comet 73P/3-C with the UVOT UV grism in May 2006. We show the photon spectrum from Comet 73P/3-C in Fig. 1 for illustration, multiplied by the response of the filter. Broad OH emission and a weaker CS feature are distinguished above a solar-like continuum due to dust scattering. We can anticipate a similar response to Comet 9P/Tempel 1. An initial observation of the Comet 9P/Tempel 1 was made on 2005 June 29 to test out the observing procedure used during the encounter and to assess the signal from the comet. This lasted for about 35 min. We started observing the comet in earnest on 2005 July 4 03:50 UT, approximately one Swift orbit before the Deep Impact collision at 05:52:02 UT seen from
Earth (A’Hearn et al., 2005). The Swift observations consist of continuous coverage for intervals of between a few minutes and about 40 min long every Swift orbit, interrupted by periods when the comet was occulted by the Earth, or the Swift satellite was in regions of high background. We observed the comet on every available Swift orbit for about 2 days following the impact, and then at a decreasing frequency interspersed with observations of GRB and other Swift targets. Occasionally the observations were also interrupted by bright stars that entered the UVOT field of view as it tracked the comet across the sky. Such stars have the potential to damage the sensitive UVOT detector, so observations were not scheduled during these times. The apparent track of the comet exhibits significant periodic effects (∼20 arcsec) due to parallax across the baseline of Swift’s orbit about Earth. This was dealt with empirically during data analysis by forming an image of the comet every 200 s and, from that, determining the centroid of the comet on the sky. We then interpolated between these time-resolved values to yield a comet track that was used to correct the position of individual photon events within an image, referenced to the start of the observation. This took out the effects both of Earth-orbit parallax and the relative motion of the comet and the Earth in their solar orbits. A consequence of this process is that the background stars appear trailed in the resulting comet images. An example of the UVOT imaging data is shown in Fig. 2. 2.1. Impact Swift was monitoring the comet at the time of the collision with the Deep Impact probe. The Swift observing window in question started at about 2005 July 4 05:28 UT and continued until just after 06:00 UT, some 8 min after the Deep Impact probe crashed into the comet nucleus, allowing for light travel delay as viewed from Earth. We have formed a light curve from the UVOT data for this Swift orbit by collecting counts within 2 arcsec radius of the comet nucleus, and these are presented at 20 s time resolution in Fig. 3. The physical scale is 649 km/arcsec at the time of the encounter. The background count rate has been subtracted from the data. This is computed from an annular region with inner and outer radii 18 arcsec and 36 arcsec, respectively, centered on the nucleus. This region contains some residual emission from the outer coma, but was chosen as a compromise between minimising the level of coma contamination and minimising the probability of contamination of the background signal by bright field stars. The data of Fig. 3 are consistent with a constant count rate prior to the moment of impact, and a gradual rise thereafter. The best fit such model is shown in the figure and has an acceptable χ 2 of 1.3 per degree of freedom (d.o.f.); without the rise in counts towards the end of the observation, the χ 2 is 2.1 per d.o.f. There is no evidence for a flash of light at the time of the impact. Specifically, the time bin that contains the impact does not exhibit an excess count with respect to its neighbours. To place an upper limit on such a flash, we have computed the standard deviation of the data about the best fit
Swift UV observations of the Deep Impact encounter
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Fig. 2. Example of an image of Comet 9P/Tempel 1 taken with the Swift UVOT in the UVW1 filter. These data were taken about one day after collision with the Deep Impact probe. Logarithmic intensity scaling is used and the image is 5 arcmin on a side, with North up and East to the left. The image is formed from the time-resolved UVOT event data by compensating for the apparent motion of the comet on the siderial sky (see text). Thus the background stars appear as non-linear trails.
model, and this is 0.18 counts per second. The translation from counts to flux depends on the spectrum of the emission. For a solar-type (G5) spectrum, each count corresponds to 1.45 × 10−16 erg cm−2 Å−1 at an effective wavelength of about 3000 Å. For a hotter spectrum, corresponding to an emission temperature of about 20,000 K, each count is equivalent to 3.5 × 10−16 erg cm−2 Å−1 at an effective wavelength of about 2500 Å. Thus the 5-sigma upper limit on a possible flash is 1.3 × 10−16 erg cm−2 Å−1 for a cool spectrum like the Sun, increasing to 3.2 × 10−16 erg cm−2 Å−1 for a hot spectrum. We have also checked for a flash in a larger aperture, 6 arcsec radius, allowing for the possibility that the impact site was partially obscured from Earth at the time of collision, but that light generated in the collision might be scattered within the inner coma. The count rate from a 6 arcsec aperture is significantly higher, allowing us to reduce the time bin to 10 s. The standard deviation of the data is 0.56 c/s, which yields a 5-sigma upper limit of 4.1 × 10−16 erg cm−2 Å−1 for a solar spectrum, and 9.8 × 10−16 erg cm−2 Å−1 for a hot spectrum.
2.2. Light curve evolution To examine how the coma of the comet evolves following the collision between the nucleus and the Deep Impact probe, we examine the signal in a 6 arcsec radius aperture (3891 km at the comet) centred on the nucleus. The background-subtracted light curve from this region is shown in Fig. 4. The main portion of the figure shows the overall light curve between June 29 and July 22. Each data point in this plot represents the average signal over a Swift orbit. The inset shows detail around the time of the Deep Impact probe collision with the comet, at 100 s time resolution. The count rate data were corrected for the change in Earth–Comet and Sun–Comet distance during the course of the observations. We use the values for July 1 as the fiducial point. As noted previously, the count rate from the nuclear region of the comet begins to rise immediately after the time of collision as seen from Earth, 05:52:02 UT (A’Hearn et al., 2005). Coverage is interrupted at about 06:00 UT when Swift slewed to another target as the comet entered a constrained viewing region. When Swift observations resume, at about 07:00 UT, the
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Fig. 3. Light curve taken from a 2 arcsec radius aperture centered on the nucleus of Comet 9P/Tempel 1 during the Swift orbit that included the encounter with the Deep Impact probe. The time resolution is 20 s. The background count rate has been determined from an annulus with inner radius 18 arcsec and outer radius 36 arcsec centered on the nucleus, and has been subtracted from the data. The time of impact seen from Earth is marked. The solid line is a best fit model which has a constant count-rate prior to impact, and a linear increase beyond.
Fig. 4. The light curve measured in the UVW1 (250 nm) filter of UVOT within 6 arcsec of the nucleus of Comet 9P/Tempel 1. The main plot shows the data between 2005 June 29 and 2005 July 22. The data points are the signal averaged over each interval of continuous Swift visibility (usually one every Earth-orbit during which observations were made). The upward arrows are spaced with a period of 40.8 h, and aligned to coincide with the peak of the initial outburst. The horizontal dashed line at 2 c/s marks the approximate pre-impact count rate for reference. The inset shows the data from the time immediately around the Deep Impact collision on an expanded time scale. The time of impact as seen from Earth is marked. The resolution of the data in this case is 100 s.
count rate has increased from about 2 per second prior to collision to over 5 counts per second, peaking in the next Swift orbit at about 09:00 UT, i.e. ∼3 h post-impact. Thereafter the count rate decays slowly, reaching 50% of peak excess above the pre-collision level about 9 h after the collision. The decay of the UV emission can be approximated by a power-law form F ∝ t α with a decay index of α = −0.55. There is a significant
re-brightening of the signal, with an amplitude of about 0.3 c/s, peaking about 40 h after the initial outburst. This is consistent with the reported 40.83 ± 0.33 h spin period of the comet nucleus (A’Hearn et al., 2005) and suggests enhanced emission of material as the impact site, or a nearby active site, comes back into sunlight. The upward arrows in Fig. 4 are spaced every 40.8 h and aligned with the peak of the initial outburst.
Swift UV observations of the Deep Impact encounter
Fig. 5. Data in the interval between about 2005 July 6 and 2005 July 12 folded modulo a 40.8 h period after subtraction of a mean power-law decay function. Data from four separate cycles are distinguished by different symbols.
To investigate the re-brightening further, we have subtracted a mean power-law decay function from the UVOT data, and fold the residual data on the 40.8 h period. Fig. 5 shows the result for four cycles of data covering the interval between about July 6 and July 12. Though the data become increasingly noisy and poorly sampled with time, as the comet fades and the density of Swift coverage decreases, there is plausible periodic variability on the spin period beyond the initial re-brightening, close in phase to the initial impact site. 2.3. Ejecta expansion: Photometric study To investigate how the material ejected by the collision with the Deep Impact probe propagates through the coma, we analyse the data in two independent ways: a purely photometric analysis, and the search for morphological changes in the UVOT images. First, we consider the outburst light curves at different radial distances from the cometary nucleus. Fig. 6 shows the background-subtracted light curve from the central 6 arcsec region, and from annuli 6–12 arcsec (3891–7782 km) and 12– 18 arcsec (7782–11673 km) from the nucleus. The latter show a delayed rise and a broader peak compared to the central aperture, consistent with expectations for an expanding cloud. We have constructed a simple model for the ejection of material due to the collision with the Deep Impact probe and its subsequent expansion into the coma. This allows us to estimate the ejection velocity. We assume that the volume of ejecta emitted from the cometary nucleus reaches a peak rapidly, within 1000 s of the collision, and then decays with time with a Gaussian form and 1/e width σd . We assume that the resulting material expands spherically away from the nucleus with a characteristic velocity v and a Gaussian spread about this mean of σv . This is incorporated into a Monte Carlo code which computes the footprint of the cloud on the sky as a function of time from the impact. The model parameters are then varied to yield a best fit to the data using a gradient-expansion non-linear least squares algorithm.
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A model with a single characteristic velocity is a poor fit to the data, yielding a χ 2 /d.o.f. of 12, and a characteristic velocity of v = 370 m/s with a Gaussian width to the velocity distribution of σv = 150 m/s. This model does not reproduce the breadth of the light curve in the two outer annuli. A much better fit is obtained if we allow for two characteristic velocities in the ejecta, each with a Gaussian distribution about the mean. This yields χ 2 /d.o.f. of 7, still statistically unacceptable, but a substantial improvement over the single velocity fit. It reproduces the main qualitative characteristics of the data, including the evolution in the breadth of the curve as a function of radius. The model has a distinct double-peaked form in the 12–18 arcsec annulus, which is a direct consequence of the dual velocity systems. The best fit model is shown in Fig. 6. The lower velocity ejecta system has v1 = 190 m/s and σ1 = 32 m/s, whereas the higher velocity system has v2 = 550 m/s and σ2 = 180 m/s. The fraction of the total light contributed by the lower velocity system is about 30%. The characteristic decay time in the rate of ejecta emission from the nucleus, σd = 10,000 s, i.e. 2.75 h. It should be noted that the model fit above also includes an effective opacity term which is significant within the central region close to the nucleus. Without this, the predicted central light curve is systematically about 20% brighter relative to those from the outer annuli. Physically, this could be ascribed to a combination of simple optical depth effects, the finite timescale for photodissociation of molecular species, and the release of dust grains embedded in icy material as the ice ablates. We model this using a simple radial dependant opacity term. Physically we would expect the ‘optical depth’ of the cloud to vary with time as well as radius. However our simple modelling is insensitive to these subtleties. 2.4. Ejecta expansion: Imaging of morphology changes We have also studied the expansion of the impact ejecta directly by means of time resolved imaging. A separate image of the comet was constructed for each observing period. Changes in the morphology of the comet’s coma were searched for using several image enhancement techniques. The most successful employed the subtraction from each image of an azimuthallysymmetric radial coma profile derived from a pre-impact image. The pre-impact image was normalised to the peak brightness prior to subtraction. Fig. 7 shows the resulting enhanced images. Immediately after impact, the image of the comet becomes more point-like as the region immediately around the nucleus brightens, leading to a dip in the surrounding brightness relative to the reference (pre-impact) image. At 4.6 h after the impact we begin to resolve a knot of excess emission at a position angle of about 190◦ . Thereafter the plume of emission is seen to expand in the hemisphere of the nucleus facing the impactor’s direction of motion. The plume’s projected leading edge velocity is measured to be 190 ± 30 m/s, consistent with one of the velocity systems identified in the photometric modelling results presented above, and with other observations of the dust plume (Meech et al., 2005; Schleicher et al., 2006), including by the Deep Impact flyby spacecraft itself (A’Hearn et al., 2005). We find that the
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Fig. 6. Outburst light curves for the Deep Impact collision with Comet 9P/Tempel 1 for three different radial intervals on the sky with respect to the comet nucleus. The data are the average signal over each interval of continuous visibility from Swift. Red points are taken from within 6 arcsec of the nucleus, blue points from an annulus with an inner radius 6 arcsec and an outer radius 12 arcsec, and green points from an annulus with an inner radius 12 arcsec and an outer radius 18 arcsec. The continuous curves are model fits of an expanding cloud to the data (see text).
Fig. 7. Montage of inner coma UVOT images obtained on 2005 July 4. Each frame measures 60 arcsec across, i.e. 38,910 km at the comet, at a pixel resolution of 324 km and FWHM of ∼1300 km. Celestial north is upwards, and east to the left. The direction towards the Sun, and the arrival direction of the impactor spacecraft are shown. The Sun–comet–Swift phase angle was 40.9◦ –41.0◦ during this sequence. All frames have been enhanced by the subtraction of a synthetic pre-impact coma image, which was normalised to the peak flux of each image. These are ‘negative’ images, i.e. positive excursions with respect to the reference profile are dark, and negative excursions are light. The mean epoch of each image measured in hours since the impact is indicated.
plume covers position angles 150–300◦ , compared to the impact direction at position angle 241◦ . To check that our results are self-consistent, we repeat the photometric analysis of the previous section, but this time dis-
tinguish the data according to the position angle on the sky with respect to the nucleus. The results are contained in Fig. 8, which shows the light curves from the 12–18 arcsec (7800–11,700 km) photometric annulus in the four position angle quadrants, PA =
Swift UV observations of the Deep Impact encounter
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Fig. 8. The lightcurve of Comet 9P/Tempel 1 measured with the Swift UVOT UVW1 filter in an annulus between 12 and 18 arcsec (7800–11,700 km) from the nucleus, as a function of quadrant on the sky and labelled by position angle. The data for quadrants PA = 0◦ –90◦ (filled symbols) and PA = 180◦ –270◦ are plotted as a histogram for clarity of comparison.
0 − 90◦ , 90◦ –180◦ , 180◦ –270◦ and 270◦ –360◦ . The opposing quadrants PA = 0–90◦ and PA = 180–270◦ are highlighted by being plotted as histograms. We find that the relative strength of the signal from the lower velocity (190 m/s) component, which in Fig. 6 dominates the emission in this annulus later than about 0.5 days after the impact, is much less pronounced in the PA = 0–90◦ quadrant than elsewhere. Conversely it is most pronounced in the opposite quadrant, PA = 180–270◦ . This is consistent with our imaging analysis which showed a plume of relatively slowly moving material centered roughly on the direction of the Deep Impact probe impact, at PA 241◦ . The evolution of the plume recorded with UVOT is qualitatively consistent with that observed by Sugita et al. (2006) using the mid-infrared camera on the Subaru telescope. They report three images of the comet taken between 07:09 UT and 09:17 UT on July 4, i.e. between about 1 and 3 h after impact. The measured size of the plume is about 2 arcsec, i.e. the resolution limit of the UVOT, in the middle Subaru image taken at 08:04 UT, about 2 h after impact. This is consistent with the fact that we first clearly resolve the plume with UVOT about 4.6 h after impact. The Subaru spectroscopy demonstrates that the plume was dominated by dust (silicate) emission at this time. 3. Discussion and conclusions The UVOT instrument aboard Swift has provided information on the near-UV development of Tempel 1’s coma following the Deep Impact collision. The lack of a positive fireball detection at the moment of impact, even with the high time resolution afforded by UVOT, agrees with the negative results of others (e.g. Meech et al., 2005; Schleicher et al., 2006). The increase in brightness in the minutes after impact was however clearly captured. Although the noise and error levels in the data are too high for a definitive comparison, we note that at 05:55–
05:56 UT, there is a suggestion that the post-impact growth in brightness did temporarily halt (Figs. 3 and 4). This is consistent with the observations by the OSIRIS camera aboard Rosetta, which also captured a similar feature (Keller et al., 2005). The latter was suggested to reflect the return of an ejecta curtain to the nucleus surface, in the case of a gravity-controlled impact. Schulz et al. (2006), utilising the Optical Monitor (OM) aboard XMM-Newton, reported the disappearance of small grains imaged by the instrument’s ∼230 nm filter around 2 h after impact. Although the Swift UVOT filter includes this bandpass, we cannot confirm that this was also seen in our observations. We find evidence for a periodic modulation of the UVOT signal consistent with the reported 40.8 h rotation period of the comet nucleus (A’Hearn et al., 2005), approximately aligned to the initial outburst phase. There is some indication from our data that the first peak of the modulation after the main outburst is more distinct than subsequent maxima, which might suggest decaying activity triggered by the collision. However, we only have good coverage after the impact, so we cannot be certain from our data whether this is connected to the impact event. Jehin et al. (2006) have studied the emission from CN and NH as a function of time, including before, during and after the Deep Impact collision. They conclude that there is a persistent modulation in the signal at the rotation period of the nucleus, which can be modelled by three active regions on the nucleus, the strongest of which is at a similar phase to the Deep Impact probe collision site. Though the density of coverage is not high, they find no evidence that this modulation pattern is disturbed by the impact. Küppers et al. (2005) show a light curve obtained in a dust-dominated filter at 648 nm. This also shows evidence for modulation consistent with the 40.8 h period prior to the
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collision, with a similar phasing. Thus the modulation observed using UVOT might well be the result of pre-existing active sites on the nucleus. The coincidence in phasing will make it difficult to establish whether the activity has been enhanced by the impact. Two velocity systems were required to model our ejecta photometry: 190 m/s, σ = 32 m/s, and 550 m/s, σ = 180 m/s. The former value agrees well with the dust expansion velocity derived both from analysis of the UVOT observations of the coma morphology, and others’ results (Meech et al., 2005; Küppers et al., 2005; Schleicher et al., 2006). The 500 m/s value is that expected for ejected gas (e.g., Schulz et al., 2006). It therefore appears that UVOT captured the ejection of both solid grains and gas components. Support for this interpretation comes from the relative strength of the two components as determined by the modelling. This is consistent with the ratio of emission line flux (gas) to reflected solar continuum (dust) in the spectrum of Comet 73P/Schwassmann–Wachmann 3-C in the UVW1 filter passband (Fig. 1). There is no evidence of the higher velocity material in the differential imaging data, which was designed to enhance non-axisymmetric structure. This may suggest that the higher velocity material (gas) emerged more spherically symmetrically from the nucleus. Küppers et al. (2005) have discussed a model for the time evolution in the number of OH molecules in the coma resulting from the outburst. They balance the creation of OH by photodissociation of the parent H2 O molecules with the photodissociation of the OH molecules themselves, and compare the results with their time-resolved measurements of OH derived from the OSIRIS wide angle camera on the ESA Rosetta spacecraft. The photodissociation timescale for H2 O is 7.67 × 104 s and for OH is 1.32 × 105 s at 1 AU. In this way they estimate that (1.5 ± 0.5) × 1032 water molecules were produced by the impact. We can apply the same methodology to our data to more accurately constrain the OH content at early times, and in turn derive a more precise estimate of the total number of water molecules by application of the Küppers et al. (2005) model. Our modelling of the UVOT time resolved photometry indicates that the count rate due to the high velocity (gas) component rises to a maximum of 5.1 c/s at 16,000 s after impact, in an 18 arcsec (11,700 km) radius aperture. After this, the count rate falls because gas molecules have travelled beyond the limits set by the aperture. If we assume, guided by Fig. 1, that 90% of this count rate is due to OH (i.e. allowing for ∼10% contamination by CS) we can translate the maximum count rate into the number of OH molecules present at this time, resulting from photodissociation of H2 O. The count rate attributable to OH is thus 4.6 per second, which corresponds to a total energy flux of 3.1 × 10−12 erg cm−2 s−1 assuming that the line photons are emitted in a region centered at about 3000 Å (Fig. 1). Adopting a fluorescence efficiency at 1 AU of 1.49 × 10−15 erg s−1 molecule−1 (Schleicher and A’Hearn, 1988) and scaling to a heliocentric distance of 1.5 AU for the comet, we derive a total number of OH molecules of 1.1 × 1031 , 16,000 s after impact, at a distance of 0.53 AU from Earth. We estimate systematic un-
certainties in this value due to the model that separates the gas and dust components in our data, and uncertainties in the possible contamination of the gas component by molecules other than OH, at the 15% level. Applying the Küppers et al. (2005) photodissociation model, we derive an estimate of the total number of water molecules released by the impact of (1.4 ± 0.2) × 1032 molecules. This is very consistent with the data of Küppers et al. (2005) and lies within their uncertainty band. We can also estimate the total scattering area of dust ejected by the impact. Our modelling of the photometric data indicates a maximum count rate due to the slower velocity (dust) component of 2.3 counts per second in an 18 arcsec radius aperture. Assuming this derives from a scattered solar spectrum, and that the scattering albedo is 0.1, we can compute the total size of the scattering target at the location of the comet that is required to give the observed count rate through the UVW1 filter effective area curve (Fig. 1). We find a value of 190 ± 20 km2 . This is somewhat lower than the value of 330 km2 derived by Küppers et al. (2005), which was based on data taken at 650 nm. Further comparison of the UV and longer wavelength data may shed light on the post-impact size distribution of the dust. Our data support the conclusion by Küppers et al. that the overall dust/gas ratio in Comet Tempel is high. Acknowledgments The Swift programme is supported by NASA, PPARC and ASI (Contract no. I/R/039/04). G.H.J. was partially supported by the Max Planck Gesellschaft. We thank the anonymous referees for their constructive and helpful comments. References A’Hearn, M.F., and 31 colleagues, 2005. Deep Impact: Excavating Comet Tempel 1. Science 310, 258–264. Barthelmy, S., and 14 colleagues, 2005. The Swift burst alert Telescope. Space Sci. Rev. 120, 143–164. Burrows, D., and 23 colleagues, 2005. The Swift X-ray telescope. Space Sci. Rev. 120, 165–195. Feldman, P.D., Fournier, K.B., Grinin, V.P., Zvereva, A.M., 1993. The abundance of ammonia in Comet P/Halley derived from ultraviolet spectrophotometry of NH by ASTRON and IUE. Astrophys. J. 404, 348–355. Gehrels, N., and 70 colleagues, 2004. The Swift Gamma-Ray Burst Mission. Astrophys. J. 611, 1005–1020. Jehin, E., Manfroid, J., Hutsemkers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Deep Impact: Highresolution optical spectroscopy with the ESO VLT and the Keck I telescope. Astrophys. J. 641, L145–L148. Keller, H.U., and 11 colleagues, 2005. Deep Impact observations by OSIRIS onboard the Rosetta Spacecraft. Science 310, 281–283. Küppers, M., and 40 colleagues, 2005. A large dust/ice ratio in the nucleus of Comet 9P/Tempel 1. Nature 437, 987–990. Meech, K.J., and 207 colleagues, 2005. Deep Impact: Observations from a worldwide Earth-based campaign. Science 310, 265–269. Roming, P., and 24 colleagues, 2005. The Swift ultraviolet and optical telescope. Space Sci. Rev. 120, 95–142. Schleicher, D.G., A’Hearn, M.A., 1988. The fluorescence of cometary OH. Astrophys. J. 331, 1058–1077.
Swift UV observations of the Deep Impact encounter
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Schulz, R., Owens, A., Rodriguez-Pascual, P.M., Lumb, D., Erd, C., Stuwe, J.A., 2006. Detection of water ice grains after the Deep Impact onto Comet 9P/Tempel 1. Astron. Astrophys. 448, L53–L56. Sugita, S., and 22 colleagues, 2006. Subaru telescope observations of Deep Impact. Science 310, 274–278. Willingale, R., and 16 colleagues, 2006. Swift XRT observations of the Deep Impact collision. Astrophys. J. 649, 541–552.
Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1 K.O. Mason a,b,∗ , M. Chester c , A. Cucchiara c , C. Gronwall c , D. Grupe c , S. Hunsberger c , G.H. Jones d , S. Koch c , J. Nousek c , P.T. O’Brien e , J. Racusin c , P. Roming c , P. Smith a , A. Wells c,e , R. Willingale e , G. Branduardi-Raymont a , N. Gehrels f a Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK b Particle Physics & Astronomy Research Council, Polaris House, North Star Ave., Swindon, Wilts SN2 1SZ, UK c Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA d Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e Department of Physics & Astronomy, University of Leicester, University Rd., Leicester, UK f NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA
Received 31 March 2006; revised 14 September 2006 Available online 22 November 2006
Abstract We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29–2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant rebrightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4 ± 0.2) × 1032 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190 ± 20 km2 . © 2006 Elsevier Inc. All rights reserved. Keywords: Comets; Comet Tempel-1; Comets, composition; Comets, coma; Comets, dust
1. Introduction The Swift observatory (Gehrels et al., 2004) flies in a lowEarth orbit and is designed to localise and respond rapidly to Gamma-ray bursts (GRB). As well as a wide-field hard X-ray/ Gamma-ray camera (the Burst Alert Telescope; Barthelmy et al., 2005), Swift carries two narrow field telescopes that are sensitive in the optical/UV and X-ray bands, respectively. These are the Optical/UV Telescope (UVOT; Roming et al., * Corresponding author.
E-mail address: [email protected] (K.O. Mason). 0019-1035/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.09.021
2005) and the X-ray Telescope (XRT; Burrows et al., 2005). Designed to provide multiwavelength follow-up of GRB afterglows, these telescopes provide a multiwavelength observing facility on a flexible and rapidly manoeuvrable spacecraft. We have used this capability to make extended observations of Comet 9P/Tempel 1 during and after the collision between it and the Deep Impact probe (A’Hearn et al., 2005). Willingale et al. (2006) describe the analysis of XRT Comet Tempel observations. We concentrate in this letter on data from the UVOT, which we used for imaging and timeresolved photometry of the comet in a passband centered on 250 nm.
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Fig. 1. Effective area as a function of wavelength for the UVW1 filter on Swift UVOT. Also shown, on a relative scale, is the photon spectrum of Comet 73P/Schwassmann–Wachmann 3-C taken with the UVOT in May 2006 (lighter curve, courtesy of Wayne Landsman), multiplied by the UVW1 effective area at each wavelength. This illustrates the actual response of the filter to a cometary spectrum. The dotted curve is the solar spectrum normalised to the continuum flux of Comet 73P/3-C.
2. Observations and analysis UVOT has a 0.3 m diameter primary mirror and utilises an intensified CCD detector to record individual photon events with a precision of 11 ms within a maximum field of view of 17 × 17 arcmin. The spatial pixel size on the sky is 0.5 arcsec, and the point spread function of the instrument has a FWHM of about 2 arcsec. Data on Comet Tempel were collected over a field of view of 8 arcmin on a side, chosen to restrict the telemetry flow to the ground to manageable levels. A series of broadband filters and grisms can be inserted into the optical path of UVOT by rotating a filter wheel. For the observations of Comet 9P/Tempel 1, we used an ultraviolet passband filter (designated UVW1). The wavelength response of the telescope with this filter is shown in Fig. 1. The sensitivity peaks at about 250 nm, with a full width at 10% response of about 130 nm. The UVW1 filter includes emission bands of the species CS, and OH, thus providing coverage of one of the direct dissociation products of H2 O (e.g., Feldman et al., 1993; Smith et al., 1980). We do not have a spectrum of Comet 9P/Tempel 1 in the waveband covered by the UVW1 filter, but we did obtain a spectrum of Comet 73P/3-C with the UVOT UV grism in May 2006. We show the photon spectrum from Comet 73P/3-C in Fig. 1 for illustration, multiplied by the response of the filter. Broad OH emission and a weaker CS feature are distinguished above a solar-like continuum due to dust scattering. We can anticipate a similar response to Comet 9P/Tempel 1. An initial observation of the Comet 9P/Tempel 1 was made on 2005 June 29 to test out the observing procedure used during the encounter and to assess the signal from the comet. This lasted for about 35 min. We started observing the comet in earnest on 2005 July 4 03:50 UT, approximately one Swift orbit before the Deep Impact collision at 05:52:02 UT seen from
Earth (A’Hearn et al., 2005). The Swift observations consist of continuous coverage for intervals of between a few minutes and about 40 min long every Swift orbit, interrupted by periods when the comet was occulted by the Earth, or the Swift satellite was in regions of high background. We observed the comet on every available Swift orbit for about 2 days following the impact, and then at a decreasing frequency interspersed with observations of GRB and other Swift targets. Occasionally the observations were also interrupted by bright stars that entered the UVOT field of view as it tracked the comet across the sky. Such stars have the potential to damage the sensitive UVOT detector, so observations were not scheduled during these times. The apparent track of the comet exhibits significant periodic effects (∼20 arcsec) due to parallax across the baseline of Swift’s orbit about Earth. This was dealt with empirically during data analysis by forming an image of the comet every 200 s and, from that, determining the centroid of the comet on the sky. We then interpolated between these time-resolved values to yield a comet track that was used to correct the position of individual photon events within an image, referenced to the start of the observation. This took out the effects both of Earth-orbit parallax and the relative motion of the comet and the Earth in their solar orbits. A consequence of this process is that the background stars appear trailed in the resulting comet images. An example of the UVOT imaging data is shown in Fig. 2. 2.1. Impact Swift was monitoring the comet at the time of the collision with the Deep Impact probe. The Swift observing window in question started at about 2005 July 4 05:28 UT and continued until just after 06:00 UT, some 8 min after the Deep Impact probe crashed into the comet nucleus, allowing for light travel delay as viewed from Earth. We have formed a light curve from the UVOT data for this Swift orbit by collecting counts within 2 arcsec radius of the comet nucleus, and these are presented at 20 s time resolution in Fig. 3. The physical scale is 649 km/arcsec at the time of the encounter. The background count rate has been subtracted from the data. This is computed from an annular region with inner and outer radii 18 arcsec and 36 arcsec, respectively, centered on the nucleus. This region contains some residual emission from the outer coma, but was chosen as a compromise between minimising the level of coma contamination and minimising the probability of contamination of the background signal by bright field stars. The data of Fig. 3 are consistent with a constant count rate prior to the moment of impact, and a gradual rise thereafter. The best fit such model is shown in the figure and has an acceptable χ 2 of 1.3 per degree of freedom (d.o.f.); without the rise in counts towards the end of the observation, the χ 2 is 2.1 per d.o.f. There is no evidence for a flash of light at the time of the impact. Specifically, the time bin that contains the impact does not exhibit an excess count with respect to its neighbours. To place an upper limit on such a flash, we have computed the standard deviation of the data about the best fit
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Fig. 2. Example of an image of Comet 9P/Tempel 1 taken with the Swift UVOT in the UVW1 filter. These data were taken about one day after collision with the Deep Impact probe. Logarithmic intensity scaling is used and the image is 5 arcmin on a side, with North up and East to the left. The image is formed from the time-resolved UVOT event data by compensating for the apparent motion of the comet on the siderial sky (see text). Thus the background stars appear as non-linear trails.
model, and this is 0.18 counts per second. The translation from counts to flux depends on the spectrum of the emission. For a solar-type (G5) spectrum, each count corresponds to 1.45 × 10−16 erg cm−2 Å−1 at an effective wavelength of about 3000 Å. For a hotter spectrum, corresponding to an emission temperature of about 20,000 K, each count is equivalent to 3.5 × 10−16 erg cm−2 Å−1 at an effective wavelength of about 2500 Å. Thus the 5-sigma upper limit on a possible flash is 1.3 × 10−16 erg cm−2 Å−1 for a cool spectrum like the Sun, increasing to 3.2 × 10−16 erg cm−2 Å−1 for a hot spectrum. We have also checked for a flash in a larger aperture, 6 arcsec radius, allowing for the possibility that the impact site was partially obscured from Earth at the time of collision, but that light generated in the collision might be scattered within the inner coma. The count rate from a 6 arcsec aperture is significantly higher, allowing us to reduce the time bin to 10 s. The standard deviation of the data is 0.56 c/s, which yields a 5-sigma upper limit of 4.1 × 10−16 erg cm−2 Å−1 for a solar spectrum, and 9.8 × 10−16 erg cm−2 Å−1 for a hot spectrum.
2.2. Light curve evolution To examine how the coma of the comet evolves following the collision between the nucleus and the Deep Impact probe, we examine the signal in a 6 arcsec radius aperture (3891 km at the comet) centred on the nucleus. The background-subtracted light curve from this region is shown in Fig. 4. The main portion of the figure shows the overall light curve between June 29 and July 22. Each data point in this plot represents the average signal over a Swift orbit. The inset shows detail around the time of the Deep Impact probe collision with the comet, at 100 s time resolution. The count rate data were corrected for the change in Earth–Comet and Sun–Comet distance during the course of the observations. We use the values for July 1 as the fiducial point. As noted previously, the count rate from the nuclear region of the comet begins to rise immediately after the time of collision as seen from Earth, 05:52:02 UT (A’Hearn et al., 2005). Coverage is interrupted at about 06:00 UT when Swift slewed to another target as the comet entered a constrained viewing region. When Swift observations resume, at about 07:00 UT, the
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Fig. 3. Light curve taken from a 2 arcsec radius aperture centered on the nucleus of Comet 9P/Tempel 1 during the Swift orbit that included the encounter with the Deep Impact probe. The time resolution is 20 s. The background count rate has been determined from an annulus with inner radius 18 arcsec and outer radius 36 arcsec centered on the nucleus, and has been subtracted from the data. The time of impact seen from Earth is marked. The solid line is a best fit model which has a constant count-rate prior to impact, and a linear increase beyond.
Fig. 4. The light curve measured in the UVW1 (250 nm) filter of UVOT within 6 arcsec of the nucleus of Comet 9P/Tempel 1. The main plot shows the data between 2005 June 29 and 2005 July 22. The data points are the signal averaged over each interval of continuous Swift visibility (usually one every Earth-orbit during which observations were made). The upward arrows are spaced with a period of 40.8 h, and aligned to coincide with the peak of the initial outburst. The horizontal dashed line at 2 c/s marks the approximate pre-impact count rate for reference. The inset shows the data from the time immediately around the Deep Impact collision on an expanded time scale. The time of impact as seen from Earth is marked. The resolution of the data in this case is 100 s.
count rate has increased from about 2 per second prior to collision to over 5 counts per second, peaking in the next Swift orbit at about 09:00 UT, i.e. ∼3 h post-impact. Thereafter the count rate decays slowly, reaching 50% of peak excess above the pre-collision level about 9 h after the collision. The decay of the UV emission can be approximated by a power-law form F ∝ t α with a decay index of α = −0.55. There is a significant
re-brightening of the signal, with an amplitude of about 0.3 c/s, peaking about 40 h after the initial outburst. This is consistent with the reported 40.83 ± 0.33 h spin period of the comet nucleus (A’Hearn et al., 2005) and suggests enhanced emission of material as the impact site, or a nearby active site, comes back into sunlight. The upward arrows in Fig. 4 are spaced every 40.8 h and aligned with the peak of the initial outburst.
Swift UV observations of the Deep Impact encounter
Fig. 5. Data in the interval between about 2005 July 6 and 2005 July 12 folded modulo a 40.8 h period after subtraction of a mean power-law decay function. Data from four separate cycles are distinguished by different symbols.
To investigate the re-brightening further, we have subtracted a mean power-law decay function from the UVOT data, and fold the residual data on the 40.8 h period. Fig. 5 shows the result for four cycles of data covering the interval between about July 6 and July 12. Though the data become increasingly noisy and poorly sampled with time, as the comet fades and the density of Swift coverage decreases, there is plausible periodic variability on the spin period beyond the initial re-brightening, close in phase to the initial impact site. 2.3. Ejecta expansion: Photometric study To investigate how the material ejected by the collision with the Deep Impact probe propagates through the coma, we analyse the data in two independent ways: a purely photometric analysis, and the search for morphological changes in the UVOT images. First, we consider the outburst light curves at different radial distances from the cometary nucleus. Fig. 6 shows the background-subtracted light curve from the central 6 arcsec region, and from annuli 6–12 arcsec (3891–7782 km) and 12– 18 arcsec (7782–11673 km) from the nucleus. The latter show a delayed rise and a broader peak compared to the central aperture, consistent with expectations for an expanding cloud. We have constructed a simple model for the ejection of material due to the collision with the Deep Impact probe and its subsequent expansion into the coma. This allows us to estimate the ejection velocity. We assume that the volume of ejecta emitted from the cometary nucleus reaches a peak rapidly, within 1000 s of the collision, and then decays with time with a Gaussian form and 1/e width σd . We assume that the resulting material expands spherically away from the nucleus with a characteristic velocity v and a Gaussian spread about this mean of σv . This is incorporated into a Monte Carlo code which computes the footprint of the cloud on the sky as a function of time from the impact. The model parameters are then varied to yield a best fit to the data using a gradient-expansion non-linear least squares algorithm.
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A model with a single characteristic velocity is a poor fit to the data, yielding a χ 2 /d.o.f. of 12, and a characteristic velocity of v = 370 m/s with a Gaussian width to the velocity distribution of σv = 150 m/s. This model does not reproduce the breadth of the light curve in the two outer annuli. A much better fit is obtained if we allow for two characteristic velocities in the ejecta, each with a Gaussian distribution about the mean. This yields χ 2 /d.o.f. of 7, still statistically unacceptable, but a substantial improvement over the single velocity fit. It reproduces the main qualitative characteristics of the data, including the evolution in the breadth of the curve as a function of radius. The model has a distinct double-peaked form in the 12–18 arcsec annulus, which is a direct consequence of the dual velocity systems. The best fit model is shown in Fig. 6. The lower velocity ejecta system has v1 = 190 m/s and σ1 = 32 m/s, whereas the higher velocity system has v2 = 550 m/s and σ2 = 180 m/s. The fraction of the total light contributed by the lower velocity system is about 30%. The characteristic decay time in the rate of ejecta emission from the nucleus, σd = 10,000 s, i.e. 2.75 h. It should be noted that the model fit above also includes an effective opacity term which is significant within the central region close to the nucleus. Without this, the predicted central light curve is systematically about 20% brighter relative to those from the outer annuli. Physically, this could be ascribed to a combination of simple optical depth effects, the finite timescale for photodissociation of molecular species, and the release of dust grains embedded in icy material as the ice ablates. We model this using a simple radial dependant opacity term. Physically we would expect the ‘optical depth’ of the cloud to vary with time as well as radius. However our simple modelling is insensitive to these subtleties. 2.4. Ejecta expansion: Imaging of morphology changes We have also studied the expansion of the impact ejecta directly by means of time resolved imaging. A separate image of the comet was constructed for each observing period. Changes in the morphology of the comet’s coma were searched for using several image enhancement techniques. The most successful employed the subtraction from each image of an azimuthallysymmetric radial coma profile derived from a pre-impact image. The pre-impact image was normalised to the peak brightness prior to subtraction. Fig. 7 shows the resulting enhanced images. Immediately after impact, the image of the comet becomes more point-like as the region immediately around the nucleus brightens, leading to a dip in the surrounding brightness relative to the reference (pre-impact) image. At 4.6 h after the impact we begin to resolve a knot of excess emission at a position angle of about 190◦ . Thereafter the plume of emission is seen to expand in the hemisphere of the nucleus facing the impactor’s direction of motion. The plume’s projected leading edge velocity is measured to be 190 ± 30 m/s, consistent with one of the velocity systems identified in the photometric modelling results presented above, and with other observations of the dust plume (Meech et al., 2005; Schleicher et al., 2006), including by the Deep Impact flyby spacecraft itself (A’Hearn et al., 2005). We find that the
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Fig. 6. Outburst light curves for the Deep Impact collision with Comet 9P/Tempel 1 for three different radial intervals on the sky with respect to the comet nucleus. The data are the average signal over each interval of continuous visibility from Swift. Red points are taken from within 6 arcsec of the nucleus, blue points from an annulus with an inner radius 6 arcsec and an outer radius 12 arcsec, and green points from an annulus with an inner radius 12 arcsec and an outer radius 18 arcsec. The continuous curves are model fits of an expanding cloud to the data (see text).
Fig. 7. Montage of inner coma UVOT images obtained on 2005 July 4. Each frame measures 60 arcsec across, i.e. 38,910 km at the comet, at a pixel resolution of 324 km and FWHM of ∼1300 km. Celestial north is upwards, and east to the left. The direction towards the Sun, and the arrival direction of the impactor spacecraft are shown. The Sun–comet–Swift phase angle was 40.9◦ –41.0◦ during this sequence. All frames have been enhanced by the subtraction of a synthetic pre-impact coma image, which was normalised to the peak flux of each image. These are ‘negative’ images, i.e. positive excursions with respect to the reference profile are dark, and negative excursions are light. The mean epoch of each image measured in hours since the impact is indicated.
plume covers position angles 150–300◦ , compared to the impact direction at position angle 241◦ . To check that our results are self-consistent, we repeat the photometric analysis of the previous section, but this time dis-
tinguish the data according to the position angle on the sky with respect to the nucleus. The results are contained in Fig. 8, which shows the light curves from the 12–18 arcsec (7800–11,700 km) photometric annulus in the four position angle quadrants, PA =
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Fig. 8. The lightcurve of Comet 9P/Tempel 1 measured with the Swift UVOT UVW1 filter in an annulus between 12 and 18 arcsec (7800–11,700 km) from the nucleus, as a function of quadrant on the sky and labelled by position angle. The data for quadrants PA = 0◦ –90◦ (filled symbols) and PA = 180◦ –270◦ are plotted as a histogram for clarity of comparison.
0 − 90◦ , 90◦ –180◦ , 180◦ –270◦ and 270◦ –360◦ . The opposing quadrants PA = 0–90◦ and PA = 180–270◦ are highlighted by being plotted as histograms. We find that the relative strength of the signal from the lower velocity (190 m/s) component, which in Fig. 6 dominates the emission in this annulus later than about 0.5 days after the impact, is much less pronounced in the PA = 0–90◦ quadrant than elsewhere. Conversely it is most pronounced in the opposite quadrant, PA = 180–270◦ . This is consistent with our imaging analysis which showed a plume of relatively slowly moving material centered roughly on the direction of the Deep Impact probe impact, at PA 241◦ . The evolution of the plume recorded with UVOT is qualitatively consistent with that observed by Sugita et al. (2006) using the mid-infrared camera on the Subaru telescope. They report three images of the comet taken between 07:09 UT and 09:17 UT on July 4, i.e. between about 1 and 3 h after impact. The measured size of the plume is about 2 arcsec, i.e. the resolution limit of the UVOT, in the middle Subaru image taken at 08:04 UT, about 2 h after impact. This is consistent with the fact that we first clearly resolve the plume with UVOT about 4.6 h after impact. The Subaru spectroscopy demonstrates that the plume was dominated by dust (silicate) emission at this time. 3. Discussion and conclusions The UVOT instrument aboard Swift has provided information on the near-UV development of Tempel 1’s coma following the Deep Impact collision. The lack of a positive fireball detection at the moment of impact, even with the high time resolution afforded by UVOT, agrees with the negative results of others (e.g. Meech et al., 2005; Schleicher et al., 2006). The increase in brightness in the minutes after impact was however clearly captured. Although the noise and error levels in the data are too high for a definitive comparison, we note that at 05:55–
05:56 UT, there is a suggestion that the post-impact growth in brightness did temporarily halt (Figs. 3 and 4). This is consistent with the observations by the OSIRIS camera aboard Rosetta, which also captured a similar feature (Keller et al., 2005). The latter was suggested to reflect the return of an ejecta curtain to the nucleus surface, in the case of a gravity-controlled impact. Schulz et al. (2006), utilising the Optical Monitor (OM) aboard XMM-Newton, reported the disappearance of small grains imaged by the instrument’s ∼230 nm filter around 2 h after impact. Although the Swift UVOT filter includes this bandpass, we cannot confirm that this was also seen in our observations. We find evidence for a periodic modulation of the UVOT signal consistent with the reported 40.8 h rotation period of the comet nucleus (A’Hearn et al., 2005), approximately aligned to the initial outburst phase. There is some indication from our data that the first peak of the modulation after the main outburst is more distinct than subsequent maxima, which might suggest decaying activity triggered by the collision. However, we only have good coverage after the impact, so we cannot be certain from our data whether this is connected to the impact event. Jehin et al. (2006) have studied the emission from CN and NH as a function of time, including before, during and after the Deep Impact collision. They conclude that there is a persistent modulation in the signal at the rotation period of the nucleus, which can be modelled by three active regions on the nucleus, the strongest of which is at a similar phase to the Deep Impact probe collision site. Though the density of coverage is not high, they find no evidence that this modulation pattern is disturbed by the impact. Küppers et al. (2005) show a light curve obtained in a dust-dominated filter at 648 nm. This also shows evidence for modulation consistent with the 40.8 h period prior to the
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collision, with a similar phasing. Thus the modulation observed using UVOT might well be the result of pre-existing active sites on the nucleus. The coincidence in phasing will make it difficult to establish whether the activity has been enhanced by the impact. Two velocity systems were required to model our ejecta photometry: 190 m/s, σ = 32 m/s, and 550 m/s, σ = 180 m/s. The former value agrees well with the dust expansion velocity derived both from analysis of the UVOT observations of the coma morphology, and others’ results (Meech et al., 2005; Küppers et al., 2005; Schleicher et al., 2006). The 500 m/s value is that expected for ejected gas (e.g., Schulz et al., 2006). It therefore appears that UVOT captured the ejection of both solid grains and gas components. Support for this interpretation comes from the relative strength of the two components as determined by the modelling. This is consistent with the ratio of emission line flux (gas) to reflected solar continuum (dust) in the spectrum of Comet 73P/Schwassmann–Wachmann 3-C in the UVW1 filter passband (Fig. 1). There is no evidence of the higher velocity material in the differential imaging data, which was designed to enhance non-axisymmetric structure. This may suggest that the higher velocity material (gas) emerged more spherically symmetrically from the nucleus. Küppers et al. (2005) have discussed a model for the time evolution in the number of OH molecules in the coma resulting from the outburst. They balance the creation of OH by photodissociation of the parent H2 O molecules with the photodissociation of the OH molecules themselves, and compare the results with their time-resolved measurements of OH derived from the OSIRIS wide angle camera on the ESA Rosetta spacecraft. The photodissociation timescale for H2 O is 7.67 × 104 s and for OH is 1.32 × 105 s at 1 AU. In this way they estimate that (1.5 ± 0.5) × 1032 water molecules were produced by the impact. We can apply the same methodology to our data to more accurately constrain the OH content at early times, and in turn derive a more precise estimate of the total number of water molecules by application of the Küppers et al. (2005) model. Our modelling of the UVOT time resolved photometry indicates that the count rate due to the high velocity (gas) component rises to a maximum of 5.1 c/s at 16,000 s after impact, in an 18 arcsec (11,700 km) radius aperture. After this, the count rate falls because gas molecules have travelled beyond the limits set by the aperture. If we assume, guided by Fig. 1, that 90% of this count rate is due to OH (i.e. allowing for ∼10% contamination by CS) we can translate the maximum count rate into the number of OH molecules present at this time, resulting from photodissociation of H2 O. The count rate attributable to OH is thus 4.6 per second, which corresponds to a total energy flux of 3.1 × 10−12 erg cm−2 s−1 assuming that the line photons are emitted in a region centered at about 3000 Å (Fig. 1). Adopting a fluorescence efficiency at 1 AU of 1.49 × 10−15 erg s−1 molecule−1 (Schleicher and A’Hearn, 1988) and scaling to a heliocentric distance of 1.5 AU for the comet, we derive a total number of OH molecules of 1.1 × 1031 , 16,000 s after impact, at a distance of 0.53 AU from Earth. We estimate systematic un-
certainties in this value due to the model that separates the gas and dust components in our data, and uncertainties in the possible contamination of the gas component by molecules other than OH, at the 15% level. Applying the Küppers et al. (2005) photodissociation model, we derive an estimate of the total number of water molecules released by the impact of (1.4 ± 0.2) × 1032 molecules. This is very consistent with the data of Küppers et al. (2005) and lies within their uncertainty band. We can also estimate the total scattering area of dust ejected by the impact. Our modelling of the photometric data indicates a maximum count rate due to the slower velocity (dust) component of 2.3 counts per second in an 18 arcsec radius aperture. Assuming this derives from a scattered solar spectrum, and that the scattering albedo is 0.1, we can compute the total size of the scattering target at the location of the comet that is required to give the observed count rate through the UVW1 filter effective area curve (Fig. 1). We find a value of 190 ± 20 km2 . This is somewhat lower than the value of 330 km2 derived by Küppers et al. (2005), which was based on data taken at 650 nm. Further comparison of the UV and longer wavelength data may shed light on the post-impact size distribution of the dust. Our data support the conclusion by Küppers et al. that the overall dust/gas ratio in Comet Tempel is high. Acknowledgments The Swift programme is supported by NASA, PPARC and ASI (Contract no. I/R/039/04). G.H.J. was partially supported by the Max Planck Gesellschaft. We thank the anonymous referees for their constructive and helpful comments. References A’Hearn, M.F., and 31 colleagues, 2005. Deep Impact: Excavating Comet Tempel 1. Science 310, 258–264. Barthelmy, S., and 14 colleagues, 2005. The Swift burst alert Telescope. Space Sci. Rev. 120, 143–164. Burrows, D., and 23 colleagues, 2005. The Swift X-ray telescope. Space Sci. Rev. 120, 165–195. Feldman, P.D., Fournier, K.B., Grinin, V.P., Zvereva, A.M., 1993. The abundance of ammonia in Comet P/Halley derived from ultraviolet spectrophotometry of NH by ASTRON and IUE. Astrophys. J. 404, 348–355. Gehrels, N., and 70 colleagues, 2004. The Swift Gamma-Ray Burst Mission. Astrophys. J. 611, 1005–1020. Jehin, E., Manfroid, J., Hutsemkers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Deep Impact: Highresolution optical spectroscopy with the ESO VLT and the Keck I telescope. Astrophys. J. 641, L145–L148. Keller, H.U., and 11 colleagues, 2005. Deep Impact observations by OSIRIS onboard the Rosetta Spacecraft. Science 310, 281–283. Küppers, M., and 40 colleagues, 2005. A large dust/ice ratio in the nucleus of Comet 9P/Tempel 1. Nature 437, 987–990. Meech, K.J., and 207 colleagues, 2005. Deep Impact: Observations from a worldwide Earth-based campaign. Science 310, 265–269. Roming, P., and 24 colleagues, 2005. The Swift ultraviolet and optical telescope. Space Sci. Rev. 120, 95–142. Schleicher, D.G., A’Hearn, M.A., 1988. The fluorescence of cometary OH. Astrophys. J. 331, 1058–1077.
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Schulz, R., Owens, A., Rodriguez-Pascual, P.M., Lumb, D., Erd, C., Stuwe, J.A., 2006. Detection of water ice grains after the Deep Impact onto Comet 9P/Tempel 1. Astron. Astrophys. 448, L53–L56. Sugita, S., and 22 colleagues, 2006. Subaru telescope observations of Deep Impact. Science 310, 274–278. Willingale, R., and 16 colleagues, 2006. Swift XRT observations of the Deep Impact collision. Astrophys. J. 649, 541–552.