- Email: [email protected]
Observing Gamma Ray Bursts with the RHESSI satellite
Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334 www.elsevierphysics.com
Observing Gamma Ray Bursts with the RHESSI satellite C. Wiggera∗ , W. Hajdasa , D.M. Smithb , M. G¨ udela , K. Hurleyb , A. Mchedlishvilia , A. Zehndera a
Laboratory for Astrophysics, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
b
UC Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, U.S.A.
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched successfully on the 5th of February 2002 into a low Earth orbit. It is a NASA Small Explorer satellite designed to study hard X-rays and gamma-rays from solar flares. In addition, its lightly shielded array of 9 germanium detectors can see photons from high-energy sources throughout the Universe, in particular also from Gamma Ray Bursts (GRBs). With its wide field of view, RHESSI observes about one GRB per week, the sensitive energy band ranging from about 30 keV to 15 MeV. By presenting preliminary lightcurves and raw spectra from three very strong GRBs observed with RHESSI we demonstrate its high time and energy resolution. Since the arrival time and energy of each photon is recorded, combined time/energy studies, e.g. time dependent hardness ratios, can be studied.
1. INTRODUCTION The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, [1]) was launched successfully on the 5th of February 2002 into a low Earth orbit. It is a NASA Small Explorer satellite mainly built at the University of California, Berkeley, the NASA Goddard Space Flight Center, the Paul Scherrer Institut in Switzerland and Spectrum Astro, Inc.. RHESSI’s primary science goals are imaging spectroscopy and highresolution nuclear spectroscopy of solar flares. The instrument consists of an imaging system, a spectrometer, and the instrument data processing unit. The spectrometer has 9 large, coaxial germanium detectors (cooled to 75 K by a mechanical cooler) that cover an energy range of 3 keV to about 17 MeV. The arrangement of the detectors in the cryostat is shown in Fig. 1. The imaging system consists of a so called Rotation Modulation Collimator, the aspect and the roll angle system, for details see [2],[3]. Each germanium detector is a cylinder with a diameter of 7.1 cm and a height of 8.5 cm. The detectors are electronically segmented into a thin front segment and a thick rear segment; signals are read out separately from each segment. For ∗ e-mail
address: [email protected]
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.04.060
9 3 5
2
8
7
1
6
4
Figure 1. RHESSI Spectrometer with its 9 germanium detectors in the cryostat. RHESSI spins around its axis at 15 rpm.
more details, see [4]. The rear segments view nearly half the sky through side walls of only 4 mm of aluminum, which have significant transmission above 25 keV. The front segments shield the rear segments from solar photons below 100 keV. Above about 100 keV, the detectors have a significant response to photons from any direction in the sky. RHESSI’s
2. LIGHTCURVES With its wide field of view, RHESSI observes on the average one GRB per week. A public database of GRBs observed with RHESSI is being maintained on the World Wide Web at http://grb.web.psi.ch/. 2.1. High resolution lightcurve The lightcurves of three very strong GRBs observed with RHESSI are shown in Fig. 2, for observed energies between 20 and 1000 keV. Fig. 3 shows a detail of the lightcurve of GRB021206, marked in Fig. 2 (middle) by the two vertical dotted lines. The many sub-peaks are separated by roughly 100 ms. The time resolution is only limited by counting statistics. 2.2. Hardness Ratio The background subtracted lightcurves of the first peak of GRB030329 are plotted in Fig. 4 for two energy bands (top and middle). The ratio of
4000 3000
GRB021008
2000 1000 0 07:00:50 07:01:00 07:01:10 07:01:20 Start Time (08-Oct-02 07:00:45)
counts / 0.2 s
lack of shielding around the detectors (to minimize weight) makes it an effective wide-fieldof-view hard X-ray/gamma-ray all-sky monitor. RHESSI is in particular a good Gamma Ray Burst (GRB) detector. The rear segments, which are mainly used for GRB analysis, are read out from 20 keV to 17 MeV and have an energy resolution of about 4 keV FWHM at 2 MeV when summed together. RHESSI data are telemetered one photon at a time with full energy information (bins 1/3 keV wide below 2.7 keV and 2.1 keV wide above) and full time resolution (in units of microseconds). Absolute time calibration is known to a few milliseconds. To conserve the onboard memory and downlink capacity, event storing is turned off while the spacecraft is going through the South Atlantic Anomaly, and the front segments are turned off during spacecraft night. On some occasions, also to save memory, events in the rear segments below a threshold energy (usually about 400 keV) are ”decimated” so that only 1 out of N are stored, where N is typically 4. This decimation is active mostly at high magnetic latitude (the orbit inclination being 38 degrees).
counts / 0.2 s
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
counts / 0.2 s
332
5000 4000 3000 2000 1000 0 22:49:10
GRB021206
22:49:20 22:49:15 Start Time (06-Dec-02 22:49:10)
3000
GRB030329
2000 1000 0
11:38:00 11:37:40 11:37:20 Start Time (29-Mar-03 11:37:10)
Figure 2. Lightcurves of three very strong GRBs observed with RHESSI, 20-1000 keV band.
the two curves, that is the hardness ratio HR HR =
counts in 200-1000 keV band counts in 120-200 keV band
is plotted in Fig. 4 (bottom). There might be a spectral evolution even within the substructure of the lightcurve. 2.3. RHESSI and the IPN RHESSI is part of the 3rd Interplanetary network (IPN) of GRB detectors. The IPN consists of gamma-ray detectors on six missions throughout the solar system. When a GRB occurs, its lightcurves from each instrument are crosscorrelated to find the relative arrival times at each spacecraft and to determine the position of the GRB. The accuracy of the burst position can be as good as a few square arcminutes, depending on the brightness, the number of spacecraft observing and the burst direction. Speed is critical, since these error boxes are used to trigger searches
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
333
200 150 50 0 17.0 16.5 16.0 Time (sec) since 06-Dec-2002 22:49
Figure 3. High resolution lightcurve (detail) of GRB021206, 20-1000 keV band.
2.4. Position estimation with RHESSI RHESSI can also provide a crude position measurement for a burst by itself. As the satellite rotates at 15 rpm, the nine germanium detectors occult each other, so that a GRB signal coming in from the side is modulated in the single detectors. This is demonstrated in Fig. 5, where the countrate of each detector normalized by the total count rate is plotted for GRB021008 (see also Fig. 2, top). The azimuth angle can be obtained from the phase of the modulation. The polar angle can be obtained from the following three considerations: 1. For a GRB coming from the side (90◦ from the spacecraft axis), the amplitude of the modulation is maximal, while for a GRB from the back or the front direction, the signal shows no modulation at all. 2. Similarly, a GRB near 90◦ gives more counts in the outer six detectors (5,3,9,8,4,6, see Fig. 1) and fewer in the inner three (1,2,7), whereas for a burst near the axis (front or back) the count rates will be nearly the same.
120-200keV
400 300 200 100 0
200-1000keV
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
HR = high / low
for rapidly fading X-ray and optical transients associated with the burst. RHESSI data are stored on board and downloaded during passes over the Berkeley Ground Station. The delay is about 1272 hours, which is often fast enough to be useful for the IPN. About 55 GCN messages have been sent out to date with a RHESSI contribution.
400 300 200 100 0
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
100 counts/0.138s
counts / 10 ms
300 250
counts/0.138s
GRB030329
Detail of GRB021206
1.2 1.0 0.8 0.6 0.4
hardness ratio
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
Figure 4. Top and middle: lightcurve of GRB030329 for two energy bands, after background subtraction. bottom: ratio (higher energy band)/(lower energy band) as a function of time. The vertical lines should guide the eye.
3. The ambiguity between bursts from the front and behind can be resolved by looking at the ratio between the total counts in the front and rear segments of the detectors. 3. SPECTRA To obtain the spectrum of a burst, the background has to be subtracted carefully. The background is estimated from a time interval just before and after the event. The observed spectra (after background subtraction) of the three GRBs from Fig. 2 are shown in Fig. 6, integrated over the time range indicated in Fig. 2 by the vertical broken lines. For comparison, a typical background (integrated from 07:04 to 07:34 UTC on 08-Oct-2002) is also shown in
07:01:08 07:01:04 Time (08-Oct-2002)
0.3 0.2 0.1 0.0 07:01:00
Det. 9 Det. 8
07:01:08 07:01:04 Time (08-Oct-2002)
0.3 0.2 0.1 0.0 07:01:00
Det. 4 Det. 6
counts/s/keV
0.1 0.0 07:01:00
Det. 5 Det. 3
100.000 GRB021008 10.000 1.000 0.100 0.010 typical background 0.001 10000 1000 100 energy / keV
100.000 10.000 1.000 0.100 0.010 0.001
GRB021206
100 07:01:08 07:01:04 Time (08-Oct-2002)
Figure 5. Modulation of GRB signal caused by detector shadowing. GRB021008 comes in at a polar angle of 50◦ . The position of the detectors and the rotation direction is sketched in Fig. 1. Data in the 50-1500 keV band were used.
counts/s/keV
Det. i / Sum
Det. i / Sum
0.3 0.2
counts/s/keV
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
Det. i / Sum
334
100.000 10.000 1.000 0.100 0.010 0.001
4. CONCLUSIONS With its excellent time and energy resolution and its wide field of view, RHESSI is also a very good GRB detector. It gives us an excellent chance for finding narrow emission or absorption lines in GRB spectra. Analyzing software is still under development. All RHESSI data are public, and scientists interested in participating should contact one of the authors.
10000
GRB030329
100
the top plot of Fig. 6. Visible in the background spectrum are many narrow lines, the dominant feature being the 511 keV annihilation line. The GRB spectra show no lines (apart from the 511 keV line that is GRB induced in the spacecraft structure). The decline below about 100 keV is due to the detector response. Routines are under development to model this effect and to obtain the incoming spectrum.
1000 energy / keV
1000 energy / keV
10000
Figure 6. Spectra (background subtracted) of the three GRB shown in Fig. 2. The dashed lines in the lower plot indicate the energy bands used for Fig. 4.
REFERENCES 1. R.P. Lin et al., Solar Physics 210: 3-32, 2002. 2. A. Zehnder et al., SPIE Proc. 4853, Waikoloa, Hawaii, 2002. 3. G.J. Hurford et al., Solar Physics 210:61-86, 2002. 4. D.M. Smith et al., Solar Physics 210:33-60, 2002.
Observing Gamma Ray Bursts with the RHESSI satellite C. Wiggera∗ , W. Hajdasa , D.M. Smithb , M. G¨ udela , K. Hurleyb , A. Mchedlishvilia , A. Zehndera a
Laboratory for Astrophysics, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
b
UC Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, U.S.A.
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched successfully on the 5th of February 2002 into a low Earth orbit. It is a NASA Small Explorer satellite designed to study hard X-rays and gamma-rays from solar flares. In addition, its lightly shielded array of 9 germanium detectors can see photons from high-energy sources throughout the Universe, in particular also from Gamma Ray Bursts (GRBs). With its wide field of view, RHESSI observes about one GRB per week, the sensitive energy band ranging from about 30 keV to 15 MeV. By presenting preliminary lightcurves and raw spectra from three very strong GRBs observed with RHESSI we demonstrate its high time and energy resolution. Since the arrival time and energy of each photon is recorded, combined time/energy studies, e.g. time dependent hardness ratios, can be studied.
1. INTRODUCTION The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI, [1]) was launched successfully on the 5th of February 2002 into a low Earth orbit. It is a NASA Small Explorer satellite mainly built at the University of California, Berkeley, the NASA Goddard Space Flight Center, the Paul Scherrer Institut in Switzerland and Spectrum Astro, Inc.. RHESSI’s primary science goals are imaging spectroscopy and highresolution nuclear spectroscopy of solar flares. The instrument consists of an imaging system, a spectrometer, and the instrument data processing unit. The spectrometer has 9 large, coaxial germanium detectors (cooled to 75 K by a mechanical cooler) that cover an energy range of 3 keV to about 17 MeV. The arrangement of the detectors in the cryostat is shown in Fig. 1. The imaging system consists of a so called Rotation Modulation Collimator, the aspect and the roll angle system, for details see [2],[3]. Each germanium detector is a cylinder with a diameter of 7.1 cm and a height of 8.5 cm. The detectors are electronically segmented into a thin front segment and a thick rear segment; signals are read out separately from each segment. For ∗ e-mail
address: [email protected]
0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2004.04.060
9 3 5
2
8
7
1
6
4
Figure 1. RHESSI Spectrometer with its 9 germanium detectors in the cryostat. RHESSI spins around its axis at 15 rpm.
more details, see [4]. The rear segments view nearly half the sky through side walls of only 4 mm of aluminum, which have significant transmission above 25 keV. The front segments shield the rear segments from solar photons below 100 keV. Above about 100 keV, the detectors have a significant response to photons from any direction in the sky. RHESSI’s
2. LIGHTCURVES With its wide field of view, RHESSI observes on the average one GRB per week. A public database of GRBs observed with RHESSI is being maintained on the World Wide Web at http://grb.web.psi.ch/. 2.1. High resolution lightcurve The lightcurves of three very strong GRBs observed with RHESSI are shown in Fig. 2, for observed energies between 20 and 1000 keV. Fig. 3 shows a detail of the lightcurve of GRB021206, marked in Fig. 2 (middle) by the two vertical dotted lines. The many sub-peaks are separated by roughly 100 ms. The time resolution is only limited by counting statistics. 2.2. Hardness Ratio The background subtracted lightcurves of the first peak of GRB030329 are plotted in Fig. 4 for two energy bands (top and middle). The ratio of
4000 3000
GRB021008
2000 1000 0 07:00:50 07:01:00 07:01:10 07:01:20 Start Time (08-Oct-02 07:00:45)
counts / 0.2 s
lack of shielding around the detectors (to minimize weight) makes it an effective wide-fieldof-view hard X-ray/gamma-ray all-sky monitor. RHESSI is in particular a good Gamma Ray Burst (GRB) detector. The rear segments, which are mainly used for GRB analysis, are read out from 20 keV to 17 MeV and have an energy resolution of about 4 keV FWHM at 2 MeV when summed together. RHESSI data are telemetered one photon at a time with full energy information (bins 1/3 keV wide below 2.7 keV and 2.1 keV wide above) and full time resolution (in units of microseconds). Absolute time calibration is known to a few milliseconds. To conserve the onboard memory and downlink capacity, event storing is turned off while the spacecraft is going through the South Atlantic Anomaly, and the front segments are turned off during spacecraft night. On some occasions, also to save memory, events in the rear segments below a threshold energy (usually about 400 keV) are ”decimated” so that only 1 out of N are stored, where N is typically 4. This decimation is active mostly at high magnetic latitude (the orbit inclination being 38 degrees).
counts / 0.2 s
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
counts / 0.2 s
332
5000 4000 3000 2000 1000 0 22:49:10
GRB021206
22:49:20 22:49:15 Start Time (06-Dec-02 22:49:10)
3000
GRB030329
2000 1000 0
11:38:00 11:37:40 11:37:20 Start Time (29-Mar-03 11:37:10)
Figure 2. Lightcurves of three very strong GRBs observed with RHESSI, 20-1000 keV band.
the two curves, that is the hardness ratio HR HR =
counts in 200-1000 keV band counts in 120-200 keV band
is plotted in Fig. 4 (bottom). There might be a spectral evolution even within the substructure of the lightcurve. 2.3. RHESSI and the IPN RHESSI is part of the 3rd Interplanetary network (IPN) of GRB detectors. The IPN consists of gamma-ray detectors on six missions throughout the solar system. When a GRB occurs, its lightcurves from each instrument are crosscorrelated to find the relative arrival times at each spacecraft and to determine the position of the GRB. The accuracy of the burst position can be as good as a few square arcminutes, depending on the brightness, the number of spacecraft observing and the burst direction. Speed is critical, since these error boxes are used to trigger searches
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
333
200 150 50 0 17.0 16.5 16.0 Time (sec) since 06-Dec-2002 22:49
Figure 3. High resolution lightcurve (detail) of GRB021206, 20-1000 keV band.
2.4. Position estimation with RHESSI RHESSI can also provide a crude position measurement for a burst by itself. As the satellite rotates at 15 rpm, the nine germanium detectors occult each other, so that a GRB signal coming in from the side is modulated in the single detectors. This is demonstrated in Fig. 5, where the countrate of each detector normalized by the total count rate is plotted for GRB021008 (see also Fig. 2, top). The azimuth angle can be obtained from the phase of the modulation. The polar angle can be obtained from the following three considerations: 1. For a GRB coming from the side (90◦ from the spacecraft axis), the amplitude of the modulation is maximal, while for a GRB from the back or the front direction, the signal shows no modulation at all. 2. Similarly, a GRB near 90◦ gives more counts in the outer six detectors (5,3,9,8,4,6, see Fig. 1) and fewer in the inner three (1,2,7), whereas for a burst near the axis (front or back) the count rates will be nearly the same.
120-200keV
400 300 200 100 0
200-1000keV
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
HR = high / low
for rapidly fading X-ray and optical transients associated with the burst. RHESSI data are stored on board and downloaded during passes over the Berkeley Ground Station. The delay is about 1272 hours, which is often fast enough to be useful for the IPN. About 55 GCN messages have been sent out to date with a RHESSI contribution.
400 300 200 100 0
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
100 counts/0.138s
counts / 10 ms
300 250
counts/0.138s
GRB030329
Detail of GRB021206
1.2 1.0 0.8 0.6 0.4
hardness ratio
36 34 32 30 28 26 24 Time (sec) since 29-Mar-2003 11:37
Figure 4. Top and middle: lightcurve of GRB030329 for two energy bands, after background subtraction. bottom: ratio (higher energy band)/(lower energy band) as a function of time. The vertical lines should guide the eye.
3. The ambiguity between bursts from the front and behind can be resolved by looking at the ratio between the total counts in the front and rear segments of the detectors. 3. SPECTRA To obtain the spectrum of a burst, the background has to be subtracted carefully. The background is estimated from a time interval just before and after the event. The observed spectra (after background subtraction) of the three GRBs from Fig. 2 are shown in Fig. 6, integrated over the time range indicated in Fig. 2 by the vertical broken lines. For comparison, a typical background (integrated from 07:04 to 07:34 UTC on 08-Oct-2002) is also shown in
07:01:08 07:01:04 Time (08-Oct-2002)
0.3 0.2 0.1 0.0 07:01:00
Det. 9 Det. 8
07:01:08 07:01:04 Time (08-Oct-2002)
0.3 0.2 0.1 0.0 07:01:00
Det. 4 Det. 6
counts/s/keV
0.1 0.0 07:01:00
Det. 5 Det. 3
100.000 GRB021008 10.000 1.000 0.100 0.010 typical background 0.001 10000 1000 100 energy / keV
100.000 10.000 1.000 0.100 0.010 0.001
GRB021206
100 07:01:08 07:01:04 Time (08-Oct-2002)
Figure 5. Modulation of GRB signal caused by detector shadowing. GRB021008 comes in at a polar angle of 50◦ . The position of the detectors and the rotation direction is sketched in Fig. 1. Data in the 50-1500 keV band were used.
counts/s/keV
Det. i / Sum
Det. i / Sum
0.3 0.2
counts/s/keV
C. Wigger et al. / Nuclear Physics B (Proc. Suppl.) 132 (2004) 331–334
Det. i / Sum
334
100.000 10.000 1.000 0.100 0.010 0.001
4. CONCLUSIONS With its excellent time and energy resolution and its wide field of view, RHESSI is also a very good GRB detector. It gives us an excellent chance for finding narrow emission or absorption lines in GRB spectra. Analyzing software is still under development. All RHESSI data are public, and scientists interested in participating should contact one of the authors.
10000
GRB030329
100
the top plot of Fig. 6. Visible in the background spectrum are many narrow lines, the dominant feature being the 511 keV annihilation line. The GRB spectra show no lines (apart from the 511 keV line that is GRB induced in the spacecraft structure). The decline below about 100 keV is due to the detector response. Routines are under development to model this effect and to obtain the incoming spectrum.
1000 energy / keV
1000 energy / keV
10000
Figure 6. Spectra (background subtracted) of the three GRB shown in Fig. 2. The dashed lines in the lower plot indicate the energy bands used for Fig. 4.
REFERENCES 1. R.P. Lin et al., Solar Physics 210: 3-32, 2002. 2. A. Zehnder et al., SPIE Proc. 4853, Waikoloa, Hawaii, 2002. 3. G.J. Hurford et al., Solar Physics 210:61-86, 2002. 4. D.M. Smith et al., Solar Physics 210:33-60, 2002.