- Email: [email protected]
XRT in 2015
CHINESE ASTRONOMY AND ASTROPHYSICS Chinese Astronomy and Astrophysics 41 (2017) 198–207
The Outburst Observations of Black Hole Binary System V404 Cyg by Swift/XRT in 2015† FU Bo-wen1,2
CHEN Yu-peng2
ZHANG Shu2
JI Long2
LI Jian3 1 2
School of Physics, Nanjing University, Nanjing 210093
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 3
Institute of Space Science, Cerdanyola del Vall`es, Spain 08193
Abstract After a quiescence of 26 years, the black-hole binary system V404 Cyg underwent another outburst in June 2015. During the later phase of this outburst, Swift/XRT (X-ray Telescope) detected for the first time a peculiar structure of a series of concentric rings under the photon counting (PC) mode. In this paper, by using the public Swift/XRT data, we analyzed the energy spectrum and time variation of this ring structure. Our results show that the energy spectrum of the ring structure is highly softened with respect to that of the central source, and the spectral intensity varies as the rings move outward. These results were explained with the model of a cloud which is located between the source and the observer at a distance of roughly 6842 light years (about 2.1 kpc). And some constraints for the relationship between the characteristics of the rings and their burst sources were presented. Key words stars: black holes—X-rays: binaries—X-rays: ISM—X-rays: bursts—ISM: clouds 1.
INTRODUCTION
The black-hole binary V404 Cyg (also known as GS2023+338) was discovered by Ginga in 1989, when it was in an outburst phase with a flux of 6.5×10−7 erg · cm−2 (about 27 Crab)[1] . †
National Natural Science Foundation (11473027, 11133002, 11103020), Strategic Pilot Projects in
Space Science of Chinese Academy of Sciences (XDA04060604, XDB09000000) Received 2015–09–16; revised version 2015–10–12
A translation of Acta Astron. Sin. Vol. 57, No. 2, pp. 156–164, 2016 fbw [email protected]
[email protected]
[email protected]
0275-1062/17/$-see front matter © 2017 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2017.04.003
Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
199
The orbital period of V404 is (6.473 ± 0.001) d, and the companion of V404 is a K3 III type star with a mass of about 0.7 M [2] . In 1992, Casares et al. estimated the minimum mass of the compact object in the system is (6.2 ± 0.31) M by use of the mass function of the system, and confirmed the black hole nature of V404[3] . In 2009, Miller-Jones et al. performed an accurate parallax measurement and calculated the exact value of the distance between V404 and the Earth to be (2.39 ± 0.14) kpc[4] . The outburst of V404 is different from the ordinary black hole binary by the higher brightness, the strong absorption feature in the spectrum, and the violent variation in the column density of photoelectric absorption. All these features make it a very interesting sample for the study of outburst evolution of black-hole X-ray binary system (X-ray binary, XRB). A renewed outburst of V404 was detected by Swift/XRT on 15th June 2015 after a quiescent period of 26 years. Additionally, a concentric ring structure and the evolution of this structure were observed in this radiation source, which triggered a global joint observations of high-energy telescopes on this source. The known results are: The analysis of INTEGRAL (International Gamma-Ray Astrophysics Laboratory) data shows that the spectra of V404 in the visible and soft γ-ray bands varied violently in the period from 15:50 UTC on 20th June to 4:05 UTC on 25th June 2015. 18 bursts with fluxes exceeding 6 Crab were detected in the time span of 3 days, and the minimum peak-to-peak interval of these bursts was as short as ∼ 20 minutes. Further analysis shows that the bursts with fluxes >6 Crab have a relatively hard energy spectrum. And the analysis of the burst and non-burst spectra in the 10∼400 keV band shows that the variation of line intensity depends on only the variation of the cut-off power law component[5] . A narrow Fe Kα line was found by analyzing the Chandra data, and its width may exceed 1 keV some times, this means that the detector has not directly observed the central source. The obscuration of the central source, and the narrow but strong emission line both indicate that the outer disk may be radiant, and its structure may have affected the violent variation of the V404’s energy spectrum. While a strong P-Cygni profile and the strong disk wind with very high kinetic energy were observed at the time of peak flux. All the above results are obtained by analyzing the energy spectrum and time variation of the burst source[6] , the concentric ring structure and its evolution which detected by Swift/XRT are not concerned. In this paper, we use the data of XRT to study the characteristics of the ring structure and its evolution. 2.
OBSERVATION AND DATA ANALYSIS
The X-ray astronomical satellite Swift was launched in 2004, equipped with an X-ray imaging telescope (XRT) and a wide-field large-area coded-aperture telescope (Burst Alert Telescope, BAT), in which BAT is mainly used to find and locate the cosmic Gamma-ray burst events. Though BAT has made simultaneously the observations in the hard X-ray band during the outburst of V404, but the imaging capability of BAT is poor and the angular resolution
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Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
of BAT is only in the order of arc minute, which is much larger than the size of the ring structure found by XRT. Hence, we only analyze the observational data of XRT in the data analysis. The incident X-rays are collected at the focal plane after grazing twice through the XRT mirrors, to give at the same time the information about the position, spectrum and temporal variation of the observed object. At present, the effective energy range of XRT is 0.2∼10 keV, the spatial angular resolution is about 3 , the effective area is ∼ 135 cm2 (1.5 keV), and the observing sensitivity is about 2 × 10−14 erg · cm−2 · s−1 (PC mode). For the observation of strong sources, XRT has also the so-called photon accumulation (pile-up) problem. Therefore, in order to analyze the energy spectrum and temporal variation of a strong source, a window timing (WT) mode is designed for the XRT, in which the speed of data recording is faster at the expense of imaging capability. Usually, XRT adopts the PC mode for imaging. In our data analysis, we mainly analyze the observational data of XRT under the PC mode. The ring structure of V404 has been detected in 7 observations from all the PC mode observations of XRT during the outburst period from 15th June to 5th July 2015. We mainly use these data to investigate the ring structure; meanwhile, in order to obtain the long-term variability, we also analyze all the data of XRT under the WT mode. The software Heasoft 16.6 is used in the data analyzing process, the images and spectra are extracted according to the user manual of XRT. The total number of XRT observations of the outburst of V404 from MJD 57188.77 to MJD 57208.38 is 51, the typical timescale of which is 1400 s. In this period there are only 12 observations of the PC mode, in which five observations are made in the early stage of the outburst, the corresponding total exposure time of the PC mode is about 19 ks. We extract first the images of V404 observed by XRT, and get the structure of three concentric rings. The PC mode observation (ID 00031403071) at MJD 57203.45 shows the most clear ring structure (see Fig.1). From this figure, we can find clearly the three concentric rings around the central point source, which are denoted as the inner, middle and outer rings, respectively. The images of other six observations in the PC mode are shown in Fig.2, and it is apparent that the 3 concentric rings are expanding outwards. In order to make analysis of energy spectra, using the annular region outside the outer ring as the background, we extract the energy spectra of the central source and the surrounding three rings. Then we fit the spectra with the model of single-photon power-law spectrum multiplied by the photoelectric absorption of Wisconsin cross-section (wabs × powerlaw). And in order to analyze the time variation of the ring structure, for the different observations, we extract the energy spectrum of the inner ring which has the highest visibility, the results are shown in Table 1. In which, NH is the column density of neutral hydrogen in unit of 1022 cm−2 ; norm is the photon number at 1 keV, in unit of keV−1 · cm−2 · s−1 ; PhoIndex is the index of the power-law spectrum. We find that the flux of the inner ring is decreasing gradually in the time span of 5 days, and the energy spectrum and column
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density are approximately constant within the 2σ range. The flux is reduced by nearly 50% from the observation of ID 00031403071 to the observation of ID 0031403072 within 2 days, and remains constant in the following days within the 2σ range.
Fig. 1 The image of the No.00031403071 observation
Table 1 Fitting results of inner rings in different observations with the wabs×powerlaw model Observation Number
Time (MJD)
NH / (1022 cm−2 )
norm/ PhoIndex
(keV−1 · cm−2 · s−1 )
Reduced Flux/(erg·
Energy
χ2
cm−2 · s−1 )
range of the
0.5–5.5
flux/keV
00031403071 57203.45857
1.146+0.096 −0.088
6.105+0.373 −0.335
37.0+10.3 −7.6
1.145
4.280+0.178 −0.205
00033861006 57205.46200
1.103+0.096 −0.089
6.122+0.396 −0.348
+5.8 20.3−4.2
1.070
2.507+0.104 −0.124
0.5–6.1
+0.461 00031403072 57205.79964 0.883+0.116 −0.102 5.196−0.402
+4.2 11.3−2.9
0.907
3.439+0.126 −0.172
0.5–4.3 0.5–3.7
00031403074 57206.66506
1.014+0.161 −0.139
5.622+0.641 −0.545
+7.1 13.7−4.3
0.967
+0.123 2.221−0.170
00033861007 57207.38524
1.127+0.154 −0.136
6.086+0.614 −0.519
+7.1 14.8−4.4
1.266
1.765+0.109 −0.129
0.5–4.2
+0.679 00031403076 57207.52753 1.042+0.181 −0.162 5.802−0.587
+7.3 12.9−4.3
1.309
1.858+0.108 −0.174
0.5–3.4
Since the outer rings in other observations are indistinct and the counting rates are not enough for fitting, we only select the observations of ID 00031403071 and ID 00033861006 to make comparison. The results are shown in Table 2. The time interval between the two observations is 2 d. We find that the absorption column densities of the central point source
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and the three concentric rings are approximately the same (within the 2σ range), but the intensities and spectral shapes of them are very different. The power-law spectral indices of the central point source and the three rings are 1.7 and ∼6, respectively; and there is no significant flux variation for the central point source, but the fluxes of the three rings are reduced by about 50% in 2 days.
Fig. 2 Other images
We find that the three rings are moving outwards gradually. Since the counting rates of the middle and outer rings are not enough for fitting (except for the observation of ID 00031403071), we mainly use the data of the inner ring to study the spatial evolution of the ring structure, and do not analyze the data of the middle and outer rings except for the observation of ID 00031403071. Firstly, since the amount of observational data is small, we
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extend the data point in the image of the ring with a Gaussian distribution function, thus the discrete point distribution of the image becomes a continuous statistical distribution, and the brightness at the different places on the ring is converted into statistical average values. Then, assuming that the intensity distribution of the ring is gaussian and has a maximum, we use the data at the place of 1/2 maximum intensity to fit the radius of the ring, the results are shown in Table 3. Finally, by “root”, we fit the model parameters from the radii at different times. Table 2 Fitting results of different rings in the same observation with the wabs×powerlaw model Observation Number
Ring
central source 00031403071
inner ring middle ring outer ring central source
00033861006
inner ring middle ring outer ring
NH /
norm/
(1022 cm−2 )
PhoIndex
1.645+0.565 −0.483 1.146+0.096 −0.088 1.286+0.068 −0.064 1.152+0.062 −0.058 1.512+0.359 −0.303 1.103+0.096 −0.089 1.271+0.100 −0.092 1.180+0.078 −0.072
1.663+0.353 −0.327 6.105+0.373 −0.335 6.477+0.273 −0.252 6.105+0.244 −0.226 1.692+0.362 −0.243 6.122+0.396 −0.348 6.790+0.396 −0.357 6.611+0.348 −0.316
(keV−1 ·
Reduced Flux/(erg·
cm−2 · s−1 ) +0.7 1.1−0.4 37.0+10.3 −7.6 +22.9 119.2−18.3 +15.4 89.1−12.6 +0.4 0.8−0.2 +5.8 20.3−4.2 +13.9 47.9−10.1 44.9+10.4 −8.0
Energy
χ2
cm−2 · s−1 )
range of the
0.926
3.397+0.221 −0.488 +0.178 4.280−0.205 9.917+0.105 −0.276 10.23+0.32 −0.26 +0.221 2.844−0.242 +0.104 2.507−0.124 +0.124 3.732−0.193 4.255+0.124 −0.170
0.5–7.5
1.145 1.376 1.102 0.972 1.070 0.932 1.036
flux/keV
0.5–5.5 0.5–4.5 0.5–9.5 0.5–9.0 0.5–6.1 0.5–5.5 0.5–4.7
We also present the outburst light curve of V404 observed by XRT, in which the every observation ID is expressed as a data point, to study the relationship between the evolutions of the outburst and rings of V404. For those PC-mode observations with the ring structure detected, the rings are also included in the counting range. We also add in the observations of Swift BAT at 15∼50 keV for comparison1 , as shown by Figs.3 and 4. It is found that in the period of MJD 57190∼ MJD 57200, V404 experienced a series of burst activities in the soft X-ray band, and the strongest burst occurred near MJD 57200. In this period of time, the BAT observations of V404 show that there were 2 peaks in the hard X-ray band, and the strongest peak occurred before MJD 57200, this may correspond to the precedent low-hardness state of a normal XRB process.
1
http://swift.gsfc.nasa.gov/results/transients/BAT detected.html
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Fig. 3 Light curves of XRT (top) and BAT (bottom)
Fig. 4 Light curves of XRT in different energy ranges. From top to bottom: the light curves in the 2∼4 keV, 4∼6 keV, 6∼8 keV, and 8∼10 keV ranges
Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
205
Table 3 Fitting results of radii of rings Observation Number
Time(MJD)
inner ring 00031403071
middle ring
Radius/degree 35.159±0.082
57203.45857057
outer ring
70.786±0.042 93.437±0.070
00031403072
57205.79964017
45.812±0.053
00031403074
57206.66506057
46.381±0.158
00033861007
57207.38523995
50.657±0.053
00031403076
57207.52752712
50.482±0.038
00033861008
57208.38133268
52.939±0.105
3.
DISCUSSION
After the two-week observations in the WT mode, Swift/XRT observed an interesting ring structure under the PC mode on 30th June 2015; NASA described this phenomenon as the X-ray “echoes” generated by the reflection of dust layers in the news report of its website on 10th July. We use the exact value (2.39 ± 0.14) kpc given by Miller-Jones et al.4 as the distance of V404, if assuming that the rings are emitted from a light source at a distance of (2.39 ± 0.14) kpc from the detector, then the radii of the rings will be in the order of several hundred light years, and the expansion speed of these rings will exceed the light speed to reach several ten light years per day, thus the generation of the ring structure can not be related to the outburst of V404. Viewing from the energy spectra, there is a drastic decay at the high-energy band in the energy spectra of the rings, significantly different from the energy spectrum of the central source, thus it can be concluded that the emission mechanisms of the three rings are similar and apparently distinct from that of the central source. Based on the available data, we deduce that there is a cloud existed between V404 and us, as shown in Fig.5. The scattering of X-ray photons (assuming that the X-rays produced by the outburst are isotropic) from the same burst by the particles in the cloud results in the different arrival times of the X-ray photons since the light paths to the detector are different. Based on this model, the photons on the different rings at the same time should come from the different bursts. Assuming that the angles between the light paths l2 , l1 from the Earth to the cloud and the light path l0 are θ1 and θ2 , respectively. The distances of the Earth from the cloud and V404 are respectively r and d, and the burst time is t0 , then the photons through the light path l2 l4 arrive at the detector at the time t1 = rsec θ1 + (rtan θ1 )2 + (d − r)2 , and through the light path l1 l3 arrive at the detector at the time t2 = rsec θ2 + (rtan θ2 )2 + (d − r)2 . Thus, according to the time variation
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Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
of the radius of the inner ring, we can make fitting for the cloud position r and the burst time t0 , and therefore derive the times when the photons produced by the bursts of the three rings arrive at the detector through the optical path l0 . By fitting (see Fig.6), the distance from the cloud to the Earth is r = 6842ly (about 2.10 kpc), and the times when the burst photons of the three rings arrive at the detector through the light path l0 are MJD 57199.473, 57187.810 and 57176.194, respectively. From the light curves, it can be seen that there is a burst with a relatively long duration between MJD 57199 and MJD 57200; since there is no observational data before MJD 57189, further verification of this assumption is impossible. Moreover, it is found from the light curves of different energy bands that the counting rate at the high-energy band is significantly lower than that of the low-energy band in the period from MJD 57199 to MJD 57200, the maximum counting rate at the 8∼10 keV band (61cts· s−1 ) is about 1/10 of that at the 2∼4 keV band (577 cts) only; and according to the fitting results of energy spectra, a rough calculation shows that the differential flux (in unit of cts·s−1 ·keV−1 ) at 9 keV is only ∼ 1/700 of the value at 3 keV, which is much smaller than the ratio of the counting rates derived from the light curves. This phenomenon can be explained by the scattering of photons by the particles of the cloud. The higher the incident photon energy, the smaller the scattering amplitude in the Compton scattering. Thus, for high-energy photons, the large-amplitude angular variation is more difficult to be caused by the particle scattering, and the energy spectra of the rings decay significantly in the high-energy band. Furthermore, the photons will lose a part of energy in scattering, which makes the energy spectrum soften. Considering that the cloud has a certain structure, the optical depth and temperature may differ with the position, especially the cloud structure may cause a structure of scattering optical depth, and further the evolution of ring intensity with the time. To understand in detail the intensity variations of the rings needs more information about the cloud structure, we will not further discuss it here.
Fig. 5 A simple model of the cloud. E, B, and the ellipse represent the Earth, V404, and the cloud, respectively. The photons that travel along the central optical path l0 took the shortest time to arrive at the detector, while those along the paths l2 l4 and l1 l3 took a longer time (the larger the angle between the path and l0 is, the longer time it took). The angle and distance ratios in this figure don’t represent the real values.
Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
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Fig. 6 Fitting result of time-radius relation
4.
CONCLUSION
This outburst of V404 is very different from before, it is the first time that the XRT made the imaging observations when V404 happened an outburst, which show clearly a ring structure. These observations are helpful for us to understand the outburst evolution of the bright black-hole binary V404. We find that the existence of a cloud at a distance of about 2.10 kpc from us can explain the observed evolution of the rings, and explain the structure of the energy spectra qualitatively, for the quantitative analysis of energy spectra further study is needed. Furthermore, the data available is not enough to confirm the exact burst times of the middle and outer rings, and the assumption of the cloud may be tested by further observations if the ring structure of V404 is still observable in the future. References 1
Zycki P. T., Done C., Smith D. A., MNRAS, 1999, 309, 561
2
Khargharia J., Froning C. S., Robinson E. L., ApJ, 2010, 716, 1105
3
Casares J., Charles P. A., Naylor T., Nature, 1992, 355, 614
4
Miller-Jones J. C. A., Jonker P. G., Dhawan V. E., et al., ApJL, 2009, 706, L230
5
Rodriguez J., Cadolle Bel M., Alfonso-Garz´ onJ., arXiv:1507.06659, 2015
6
King A. L., Miller J. M., Raymond J., et al., arXiv:1508.01181, 2015
The Outburst Observations of Black Hole Binary System V404 Cyg by Swift/XRT in 2015† FU Bo-wen1,2
CHEN Yu-peng2
ZHANG Shu2
JI Long2
LI Jian3 1 2
School of Physics, Nanjing University, Nanjing 210093
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 3
Institute of Space Science, Cerdanyola del Vall`es, Spain 08193
Abstract After a quiescence of 26 years, the black-hole binary system V404 Cyg underwent another outburst in June 2015. During the later phase of this outburst, Swift/XRT (X-ray Telescope) detected for the first time a peculiar structure of a series of concentric rings under the photon counting (PC) mode. In this paper, by using the public Swift/XRT data, we analyzed the energy spectrum and time variation of this ring structure. Our results show that the energy spectrum of the ring structure is highly softened with respect to that of the central source, and the spectral intensity varies as the rings move outward. These results were explained with the model of a cloud which is located between the source and the observer at a distance of roughly 6842 light years (about 2.1 kpc). And some constraints for the relationship between the characteristics of the rings and their burst sources were presented. Key words stars: black holes—X-rays: binaries—X-rays: ISM—X-rays: bursts—ISM: clouds 1.
INTRODUCTION
The black-hole binary V404 Cyg (also known as GS2023+338) was discovered by Ginga in 1989, when it was in an outburst phase with a flux of 6.5×10−7 erg · cm−2 (about 27 Crab)[1] . †
National Natural Science Foundation (11473027, 11133002, 11103020), Strategic Pilot Projects in
Space Science of Chinese Academy of Sciences (XDA04060604, XDB09000000) Received 2015–09–16; revised version 2015–10–12
A translation of Acta Astron. Sin. Vol. 57, No. 2, pp. 156–164, 2016 fbw [email protected]
[email protected]
[email protected]
0275-1062/17/$-see front matter © 2017 Elsevier B.V. All rights reserved. doi:10.1016/j.chinastron.2017.04.003
Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
199
The orbital period of V404 is (6.473 ± 0.001) d, and the companion of V404 is a K3 III type star with a mass of about 0.7 M [2] . In 1992, Casares et al. estimated the minimum mass of the compact object in the system is (6.2 ± 0.31) M by use of the mass function of the system, and confirmed the black hole nature of V404[3] . In 2009, Miller-Jones et al. performed an accurate parallax measurement and calculated the exact value of the distance between V404 and the Earth to be (2.39 ± 0.14) kpc[4] . The outburst of V404 is different from the ordinary black hole binary by the higher brightness, the strong absorption feature in the spectrum, and the violent variation in the column density of photoelectric absorption. All these features make it a very interesting sample for the study of outburst evolution of black-hole X-ray binary system (X-ray binary, XRB). A renewed outburst of V404 was detected by Swift/XRT on 15th June 2015 after a quiescent period of 26 years. Additionally, a concentric ring structure and the evolution of this structure were observed in this radiation source, which triggered a global joint observations of high-energy telescopes on this source. The known results are: The analysis of INTEGRAL (International Gamma-Ray Astrophysics Laboratory) data shows that the spectra of V404 in the visible and soft γ-ray bands varied violently in the period from 15:50 UTC on 20th June to 4:05 UTC on 25th June 2015. 18 bursts with fluxes exceeding 6 Crab were detected in the time span of 3 days, and the minimum peak-to-peak interval of these bursts was as short as ∼ 20 minutes. Further analysis shows that the bursts with fluxes >6 Crab have a relatively hard energy spectrum. And the analysis of the burst and non-burst spectra in the 10∼400 keV band shows that the variation of line intensity depends on only the variation of the cut-off power law component[5] . A narrow Fe Kα line was found by analyzing the Chandra data, and its width may exceed 1 keV some times, this means that the detector has not directly observed the central source. The obscuration of the central source, and the narrow but strong emission line both indicate that the outer disk may be radiant, and its structure may have affected the violent variation of the V404’s energy spectrum. While a strong P-Cygni profile and the strong disk wind with very high kinetic energy were observed at the time of peak flux. All the above results are obtained by analyzing the energy spectrum and time variation of the burst source[6] , the concentric ring structure and its evolution which detected by Swift/XRT are not concerned. In this paper, we use the data of XRT to study the characteristics of the ring structure and its evolution. 2.
OBSERVATION AND DATA ANALYSIS
The X-ray astronomical satellite Swift was launched in 2004, equipped with an X-ray imaging telescope (XRT) and a wide-field large-area coded-aperture telescope (Burst Alert Telescope, BAT), in which BAT is mainly used to find and locate the cosmic Gamma-ray burst events. Though BAT has made simultaneously the observations in the hard X-ray band during the outburst of V404, but the imaging capability of BAT is poor and the angular resolution
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Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
of BAT is only in the order of arc minute, which is much larger than the size of the ring structure found by XRT. Hence, we only analyze the observational data of XRT in the data analysis. The incident X-rays are collected at the focal plane after grazing twice through the XRT mirrors, to give at the same time the information about the position, spectrum and temporal variation of the observed object. At present, the effective energy range of XRT is 0.2∼10 keV, the spatial angular resolution is about 3 , the effective area is ∼ 135 cm2 (1.5 keV), and the observing sensitivity is about 2 × 10−14 erg · cm−2 · s−1 (PC mode). For the observation of strong sources, XRT has also the so-called photon accumulation (pile-up) problem. Therefore, in order to analyze the energy spectrum and temporal variation of a strong source, a window timing (WT) mode is designed for the XRT, in which the speed of data recording is faster at the expense of imaging capability. Usually, XRT adopts the PC mode for imaging. In our data analysis, we mainly analyze the observational data of XRT under the PC mode. The ring structure of V404 has been detected in 7 observations from all the PC mode observations of XRT during the outburst period from 15th June to 5th July 2015. We mainly use these data to investigate the ring structure; meanwhile, in order to obtain the long-term variability, we also analyze all the data of XRT under the WT mode. The software Heasoft 16.6 is used in the data analyzing process, the images and spectra are extracted according to the user manual of XRT. The total number of XRT observations of the outburst of V404 from MJD 57188.77 to MJD 57208.38 is 51, the typical timescale of which is 1400 s. In this period there are only 12 observations of the PC mode, in which five observations are made in the early stage of the outburst, the corresponding total exposure time of the PC mode is about 19 ks. We extract first the images of V404 observed by XRT, and get the structure of three concentric rings. The PC mode observation (ID 00031403071) at MJD 57203.45 shows the most clear ring structure (see Fig.1). From this figure, we can find clearly the three concentric rings around the central point source, which are denoted as the inner, middle and outer rings, respectively. The images of other six observations in the PC mode are shown in Fig.2, and it is apparent that the 3 concentric rings are expanding outwards. In order to make analysis of energy spectra, using the annular region outside the outer ring as the background, we extract the energy spectra of the central source and the surrounding three rings. Then we fit the spectra with the model of single-photon power-law spectrum multiplied by the photoelectric absorption of Wisconsin cross-section (wabs × powerlaw). And in order to analyze the time variation of the ring structure, for the different observations, we extract the energy spectrum of the inner ring which has the highest visibility, the results are shown in Table 1. In which, NH is the column density of neutral hydrogen in unit of 1022 cm−2 ; norm is the photon number at 1 keV, in unit of keV−1 · cm−2 · s−1 ; PhoIndex is the index of the power-law spectrum. We find that the flux of the inner ring is decreasing gradually in the time span of 5 days, and the energy spectrum and column
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Fu Bo-wen et al. / Chinese Astronomy and Astrophysics 41 (2017) 198–207
density are approximately constant within the 2σ range. The flux is reduced by nearly 50% from the observation of ID 00031403071 to the observation of ID 0031403072 within 2 days, and remains constant in the following days within the 2σ range.
Fig. 1 The image of the No.00031403071 observation
Table 1 Fitting results of inner rings in different observations with the wabs×powerlaw model Observation Number
Time (MJD)
NH / (1022 cm−2 )
norm/ PhoIndex
(keV−1 · cm−2 · s−1 )
Reduced Flux/(erg·
Energy
χ2
cm−2 · s−1 )
range of the
0.5–5.5
flux/keV
00031403071 57203.45857
1.146+0.096 −0.088
6.105+0.373 −0.335
37.0+10.3 −7.6
1.145
4.280+0.178 −0.205
00033861006 57205.46200
1.103+0.096 −0.089
6.122+0.396 −0.348
+5.8 20.3−4.2
1.070
2.507+0.104 −0.124
0.5–6.1
+0.461 00031403072 57205.79964 0.883+0.116 −0.102 5.196−0.402
+4.2 11.3−2.9
0.907
3.439+0.126 −0.172
0.5–4.3 0.5–3.7
00031403074 57206.66506
1.014+0.161 −0.139
5.622+0.641 −0.545
+7.1 13.7−4.3
0.967
+0.123 2.221−0.170
00033861007 57207.38524
1.127+0.154 −0.136
6.086+0.614 −0.519
+7.1 14.8−4.4
1.266
1.765+0.109 −0.129
0.5–4.2
+0.679 00031403076 57207.52753 1.042+0.181 −0.162 5.802−0.587
+7.3 12.9−4.3
1.309
1.858+0.108 −0.174
0.5–3.4
Since the outer rings in other observations are indistinct and the counting rates are not enough for fitting, we only select the observations of ID 00031403071 and ID 00033861006 to make comparison. The results are shown in Table 2. The time interval between the two observations is 2 d. We find that the absorption column densities of the central point source
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and the three concentric rings are approximately the same (within the 2σ range), but the intensities and spectral shapes of them are very different. The power-law spectral indices of the central point source and the three rings are 1.7 and ∼6, respectively; and there is no significant flux variation for the central point source, but the fluxes of the three rings are reduced by about 50% in 2 days.
Fig. 2 Other images
We find that the three rings are moving outwards gradually. Since the counting rates of the middle and outer rings are not enough for fitting (except for the observation of ID 00031403071), we mainly use the data of the inner ring to study the spatial evolution of the ring structure, and do not analyze the data of the middle and outer rings except for the observation of ID 00031403071. Firstly, since the amount of observational data is small, we
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extend the data point in the image of the ring with a Gaussian distribution function, thus the discrete point distribution of the image becomes a continuous statistical distribution, and the brightness at the different places on the ring is converted into statistical average values. Then, assuming that the intensity distribution of the ring is gaussian and has a maximum, we use the data at the place of 1/2 maximum intensity to fit the radius of the ring, the results are shown in Table 3. Finally, by “root”, we fit the model parameters from the radii at different times. Table 2 Fitting results of different rings in the same observation with the wabs×powerlaw model Observation Number
Ring
central source 00031403071
inner ring middle ring outer ring central source
00033861006
inner ring middle ring outer ring
NH /
norm/
(1022 cm−2 )
PhoIndex
1.645+0.565 −0.483 1.146+0.096 −0.088 1.286+0.068 −0.064 1.152+0.062 −0.058 1.512+0.359 −0.303 1.103+0.096 −0.089 1.271+0.100 −0.092 1.180+0.078 −0.072
1.663+0.353 −0.327 6.105+0.373 −0.335 6.477+0.273 −0.252 6.105+0.244 −0.226 1.692+0.362 −0.243 6.122+0.396 −0.348 6.790+0.396 −0.357 6.611+0.348 −0.316
(keV−1 ·
Reduced Flux/(erg·
cm−2 · s−1 ) +0.7 1.1−0.4 37.0+10.3 −7.6 +22.9 119.2−18.3 +15.4 89.1−12.6 +0.4 0.8−0.2 +5.8 20.3−4.2 +13.9 47.9−10.1 44.9+10.4 −8.0
Energy
χ2
cm−2 · s−1 )
range of the
0.926
3.397+0.221 −0.488 +0.178 4.280−0.205 9.917+0.105 −0.276 10.23+0.32 −0.26 +0.221 2.844−0.242 +0.104 2.507−0.124 +0.124 3.732−0.193 4.255+0.124 −0.170
0.5–7.5
1.145 1.376 1.102 0.972 1.070 0.932 1.036
flux/keV
0.5–5.5 0.5–4.5 0.5–9.5 0.5–9.0 0.5–6.1 0.5–5.5 0.5–4.7
We also present the outburst light curve of V404 observed by XRT, in which the every observation ID is expressed as a data point, to study the relationship between the evolutions of the outburst and rings of V404. For those PC-mode observations with the ring structure detected, the rings are also included in the counting range. We also add in the observations of Swift BAT at 15∼50 keV for comparison1 , as shown by Figs.3 and 4. It is found that in the period of MJD 57190∼ MJD 57200, V404 experienced a series of burst activities in the soft X-ray band, and the strongest burst occurred near MJD 57200. In this period of time, the BAT observations of V404 show that there were 2 peaks in the hard X-ray band, and the strongest peak occurred before MJD 57200, this may correspond to the precedent low-hardness state of a normal XRB process.
1
http://swift.gsfc.nasa.gov/results/transients/BAT detected.html
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Fig. 3 Light curves of XRT (top) and BAT (bottom)
Fig. 4 Light curves of XRT in different energy ranges. From top to bottom: the light curves in the 2∼4 keV, 4∼6 keV, 6∼8 keV, and 8∼10 keV ranges
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Table 3 Fitting results of radii of rings Observation Number
Time(MJD)
inner ring 00031403071
middle ring
Radius/degree 35.159±0.082
57203.45857057
outer ring
70.786±0.042 93.437±0.070
00031403072
57205.79964017
45.812±0.053
00031403074
57206.66506057
46.381±0.158
00033861007
57207.38523995
50.657±0.053
00031403076
57207.52752712
50.482±0.038
00033861008
57208.38133268
52.939±0.105
3.
DISCUSSION
After the two-week observations in the WT mode, Swift/XRT observed an interesting ring structure under the PC mode on 30th June 2015; NASA described this phenomenon as the X-ray “echoes” generated by the reflection of dust layers in the news report of its website on 10th July. We use the exact value (2.39 ± 0.14) kpc given by Miller-Jones et al.4 as the distance of V404, if assuming that the rings are emitted from a light source at a distance of (2.39 ± 0.14) kpc from the detector, then the radii of the rings will be in the order of several hundred light years, and the expansion speed of these rings will exceed the light speed to reach several ten light years per day, thus the generation of the ring structure can not be related to the outburst of V404. Viewing from the energy spectra, there is a drastic decay at the high-energy band in the energy spectra of the rings, significantly different from the energy spectrum of the central source, thus it can be concluded that the emission mechanisms of the three rings are similar and apparently distinct from that of the central source. Based on the available data, we deduce that there is a cloud existed between V404 and us, as shown in Fig.5. The scattering of X-ray photons (assuming that the X-rays produced by the outburst are isotropic) from the same burst by the particles in the cloud results in the different arrival times of the X-ray photons since the light paths to the detector are different. Based on this model, the photons on the different rings at the same time should come from the different bursts. Assuming that the angles between the light paths l2 , l1 from the Earth to the cloud and the light path l0 are θ1 and θ2 , respectively. The distances of the Earth from the cloud and V404 are respectively r and d, and the burst time is t0 , then the photons through the light path l2 l4 arrive at the detector at the time t1 = rsec θ1 + (rtan θ1 )2 + (d − r)2 , and through the light path l1 l3 arrive at the detector at the time t2 = rsec θ2 + (rtan θ2 )2 + (d − r)2 . Thus, according to the time variation
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of the radius of the inner ring, we can make fitting for the cloud position r and the burst time t0 , and therefore derive the times when the photons produced by the bursts of the three rings arrive at the detector through the optical path l0 . By fitting (see Fig.6), the distance from the cloud to the Earth is r = 6842ly (about 2.10 kpc), and the times when the burst photons of the three rings arrive at the detector through the light path l0 are MJD 57199.473, 57187.810 and 57176.194, respectively. From the light curves, it can be seen that there is a burst with a relatively long duration between MJD 57199 and MJD 57200; since there is no observational data before MJD 57189, further verification of this assumption is impossible. Moreover, it is found from the light curves of different energy bands that the counting rate at the high-energy band is significantly lower than that of the low-energy band in the period from MJD 57199 to MJD 57200, the maximum counting rate at the 8∼10 keV band (61cts· s−1 ) is about 1/10 of that at the 2∼4 keV band (577 cts) only; and according to the fitting results of energy spectra, a rough calculation shows that the differential flux (in unit of cts·s−1 ·keV−1 ) at 9 keV is only ∼ 1/700 of the value at 3 keV, which is much smaller than the ratio of the counting rates derived from the light curves. This phenomenon can be explained by the scattering of photons by the particles of the cloud. The higher the incident photon energy, the smaller the scattering amplitude in the Compton scattering. Thus, for high-energy photons, the large-amplitude angular variation is more difficult to be caused by the particle scattering, and the energy spectra of the rings decay significantly in the high-energy band. Furthermore, the photons will lose a part of energy in scattering, which makes the energy spectrum soften. Considering that the cloud has a certain structure, the optical depth and temperature may differ with the position, especially the cloud structure may cause a structure of scattering optical depth, and further the evolution of ring intensity with the time. To understand in detail the intensity variations of the rings needs more information about the cloud structure, we will not further discuss it here.
Fig. 5 A simple model of the cloud. E, B, and the ellipse represent the Earth, V404, and the cloud, respectively. The photons that travel along the central optical path l0 took the shortest time to arrive at the detector, while those along the paths l2 l4 and l1 l3 took a longer time (the larger the angle between the path and l0 is, the longer time it took). The angle and distance ratios in this figure don’t represent the real values.
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Fig. 6 Fitting result of time-radius relation
4.
CONCLUSION
This outburst of V404 is very different from before, it is the first time that the XRT made the imaging observations when V404 happened an outburst, which show clearly a ring structure. These observations are helpful for us to understand the outburst evolution of the bright black-hole binary V404. We find that the existence of a cloud at a distance of about 2.10 kpc from us can explain the observed evolution of the rings, and explain the structure of the energy spectra qualitatively, for the quantitative analysis of energy spectra further study is needed. Furthermore, the data available is not enough to confirm the exact burst times of the middle and outer rings, and the assumption of the cloud may be tested by further observations if the ring structure of V404 is still observable in the future. References 1
Zycki P. T., Done C., Smith D. A., MNRAS, 1999, 309, 561
2
Khargharia J., Froning C. S., Robinson E. L., ApJ, 2010, 716, 1105
3
Casares J., Charles P. A., Naylor T., Nature, 1992, 355, 614
4
Miller-Jones J. C. A., Jonker P. G., Dhawan V. E., et al., ApJL, 2009, 706, L230
5
Rodriguez J., Cadolle Bel M., Alfonso-Garz´ onJ., arXiv:1507.06659, 2015
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King A. L., Miller J. M., Raymond J., et al., arXiv:1508.01181, 2015