Gamma-ray Burst 170817A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 17 Aug 17 12:41:31 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      47
TRIGGER_NUM:     524666471
GRB_RA:          172.017d {+11h 28m 04s} (J2000),
                 172.233d {+11h 28m 56s} (current),
                 171.404d {+11h 25m 37s} (1950)
GRB_DEC:         -34.783d {-34d 46' 59"} (J2000),
                 -34.881d {-34d 52' 49"} (current),
                 -34.508d {-34d 30' 27"} (1950)
GRB_ERROR:       32.65 [deg radius, statistical plus systematic]
GRB_INTEN:       162 [cnts/sec]
DATA_SIGNIF:     6.20 [sigma]
INTEG_TIME:      0.512 [sec]
GRB_DATE:        17982 TJD;   229 DOY;   17/08/17
GRB_TIME:        45666.47 SOD {12:41:06.47} UT
GRB_PHI:          20.00 [deg]
GRB_THETA:        85.00 [deg]
DATA_TIME_SCALE: 0.5120 [sec]
HARD_RATIO:      0.79
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      97%  GRB
2nd_MOST_LIKELY:   2%  Generic Transient
DETECTORS:       0,1,1, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       147.09d {+09h 48m 22s}  +13.25d {+13d 15' 09"}
SUN_DIST:         53.72 [deg]   Sun_angle= -1.7 [hr] (East of Sun)
MOON_POSTN:       87.69d {+05h 50m 46s}  +19.08d {+19d 04' 40"}
MOON_DIST:        96.50 [deg]
MOON_ILLUM:      23 [%]
GAL_COORDS:      284.12, 25.02 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      188.55,-34.66 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2017/bn170817529/quicklook/glg_lc_medres34_bn170817529.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 321.53,3.90 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Thu 17 Aug 17 12:41:44 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     524666471
GRB_RA:          186.620d {+12h 26m 29s} (J2000),
                 186.859d {+12h 27m 26s} (current),
                 185.945d {+12h 23m 47s} (1950)
GRB_DEC:         -48.840d {-48d 50' 23"} (J2000),
                 -48.937d {-48d 56' 14"} (current),
                 -48.563d {-48d 33' 47"} (1950)
GRB_ERROR:       17.45 [deg radius, statistical only]
DATA_SIGNIF:     6.00 [sigma]
DATA_INTERVAL:   0.512 [sec]
GRB_DATE:        17982 TJD;   229 DOY;   17/08/17
GRB_TIME:        45666.47 SOD {12:41:06.47} UT
GRB_PHI:          25.00 [deg]
GRB_THETA:       102.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4153 (Gnd S/W Version number)
SUN_POSTN:       147.09d {+09h 48m 22s}  +13.25d {+13d 15' 09"}
SUN_DIST:         71.42 [deg]   Sun_angle= -2.7 [hr] (East of Sun)
MOON_POSTN:       87.69d {+05h 50m 47s}  +19.08d {+19d 04' 40"}
MOON_DIST:       110.20 [deg]
MOON_ILLUM:      23 [%]
GAL_COORDS:      298.71, 13.83 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      209.45,-41.34 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2017/bn170817529/quicklook/glg_lc_medres34_bn170817529.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_524666471.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN Circular #21520
A. von Kienlin (MPE), C. Meegan (UAH) and A. Goldstein (USRA) report on behalf of the Fermi GBM Team:
"At 12:41:06.47 UT on 17 August 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170817A (trigger 524666471 / 170817529).
The on-ground calculated location, using the GBM trigger data, is RA = 176.8, DEC = -39.8 (J2000 degrees, equivalent to 12 h 47 m, -39 d 48'), with an uncertainty of 11.6 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ).
The angle from the Fermi LAT boresight at the GBM trigger time is 91 degrees.
The GRB light curve shows a weak short pulse with a duration (T90) of about 2 s (50-300 keV). The time-averaged spectrum from T0-0.512 s to 2.048 s is well fit by a power law function with an exponential high-energy cutoff. The power law index is -0.89 +/- 0.5 and the cutoff energy, parameterized as Epeak, is 82 +/- 21 keV
The event fluence (10-1000 keV) in this time interval is (2.3 +/- 0.4)E-07 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0-0.32 s in the 8-1000 keV band is 1.9 +/- 0.2 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
Editorial comment
This short GRB was accurately localized through aLIGO/Virgo, and subsequently the kilonova of this binary neutron star merger.
All of the instantaneous GCNs were not publicly available, and thus do not appear here. Those can be found at GCN archive searching for "LIGO/Virgo G298048", or concatenated into one file at G298048.gcn3.
After the public announcement on 2017, Oct. 16, about 80+ papers were released - a nice summary of those can be found under Arcavi's compilation page.
GCN Circular #22201
E. Troja (UMD/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), H. van Eeerten (U. Bath), T. Sakamoto (AGU), and S. B. Cenko (NASA/GSFC) report:
The Chandra X-ray Observatory re-observed the field of GW170817/AT2017gfo (PI: B. Wilkes) starting on 2017-12-03 (~109 days after the LVC trigger) for a total exposure of 74 ks. A second shorter observation was performed on 2017-12-06 for a total exposure of 25 ks.
Our preliminary analysis of the data clearly detects the X-ray counterpart of AT2017gfo at a flux level of� ~1.5E-14 erg/cm^2/s (0.5-8.0 keV). The observed value is three times higher than the flux measured at earlier times (~5E-15 erg/cm^2/s at 15 days; Troja, E., et al. 2017, Nature, 551, 71) and implies a continued rise of the X-ray afterglow. We do not measure any significant spectral variation between these observations and our observations at earlier times.
This behavior is hard to reconcile with the simple top-hat jet model and suggests a more complex structure of the GRB jet.
We thank Belinda Wilkes, Andrea Prestwich and the entire Chandra for performing these observations.
GCN Circular #22203 / ATEL #11037
R. Margutti, W. Fong (Northwestern), T. Eftekhari, K. Alexander, E. Berger (Harvard), R. Chornock (Ohio University) report:
The Chandra X-ray Observatory (CXO) started observing GW170817 on 2017 December 3 at 01:38:45UT for a total of 74.09 ksec (obs ID 20860, PI Wilkes). At the position of the electromagnetic counterpart, an X-ray source is clearly detected with a significance of ~33-sigma (RA = 13:09:48.090, Dec = -23:22:52.87, J2000), with a net count-rate of 1.47e-3 cps (0.5-8 keV).
The CXO observed the field for an additional 24.74 ksec starting on 2017 December 6 at 10:43:31UT (obs ID 20861, PI Wilkes). The X-ray source is still detected with a significance of ~15-sigma with a net count-rate of 1.41e-3 cps (0.5-8 keV).
The joint spectrum can be fit with an absorbed power-law spectral model with photon index Gamma= 1.50 +\-0.16 (1 sigma c.l.). We find no evidence for intrinsic neutral hydrogen absorption in addition to the Galactic value (NH_MW=0.0784e22 cm-2, Kalberla et al., 2005) and place a 3 sigma upper limit of NHint<0.7e22 cm-2. These properties are consistent with the X-ray spectral properties of GW170817 at t<15 days inferred by Margutti et al., 2017 and Troja et al., 2017.
Based on our best fitting spectral parameters, the 0.3-10 keV unabsorbed flux is 2.3e-14 erg/s/cm^2, indicating a substantial brightening of the X-ray source during the last ~90 days, in agreement with the results from Troja et al., GCN 22201. A comparison with radio observations acquired 93 days since merger (Mooley et al., 2017) indicates a spectral slope beta~-0.6 of the Fnu ~nu^-beta spectrum, similar to the radio-to-Xray spectrum of GW170817 before Sun block (Alexander et al., 2017; Margutti et al., 2017; Troja et al., 2017; Haggard et al., 2017; Hallinan et al., 2017; Mooley et al., 2017). This result suggests negligible spectral evolution of GW170817 in the last 90 days, and that the radio and X-ray emission continue to represent the same emission component.
Based on this spectrum, we expect GW170817 to be detectable at optical-NIR wavelengths at approximately 26.5 AB magnitude.
We thank Belinda Wilkes and the entire CXO team for approving these DDT requests and making these observations possible.
GCN Circular #22206
Daryl Haggard, John J. Ruan, Melania Nynka (McGill/MSI), Vicky Kalogera (Northwestern/CIERA), and Phil Evans (Leicester) report:
We have performed a detailed analysis of new Chandra X-ray observations of GW170817 obtained via Chandra Director's Discretionary Time (PI: Wilkes, Program Number 18408601). The X-ray counterpart to GW170817/GRB170817A/SSS17a is clearly detected in the new observations.
This program acquired two exposures of GRB170817A: (1) a 74.09 ks exposure (ObsID 20860) beginning at 2017 December 2.08 UT, approximately 108 days post-burst, and (2) a 24.74 ks exposure (ObsID 20861) beginning at 2017 December 6.45 UT, approximately 111 days post-burst. Since the two new exposures are close in time and the X-ray emission of GRB170817A is not expected to vary significantly over ~4 day timescales, we co-add the two data sets into one 98.83 ks exposure at 109.2 days post-burst.
We perform spectral extractions assuming an absorbed power-law spectral model with fixed NH = 7.5e20 cm^�??2 and find that the X-ray flux of GRB170817A has an absorbed flux of f(0.3�??8 keV) = 1.58e�??14 +/- 0.14 erg s^�??1 cm^�??2 (Gamma = 1.62 +/- 0.27) at 109.2 days post-burst, which corresponds to an unabsorbed luminosity of L(0.3�??10 keV) = 42.5e38 +/- 3.7 erg s^�??1 (see also Troja et al. GCN 22201 and Margutti et al. GCN 22203). This represents significant X-ray brightening compared to Chandra observations at 15.6 days post-burst, for which we find an absorbed flux of f(0.3�??8 keV) = 0.36e�??14 +/- 0.1 erg s^�??1 cm^�??2 (Gamma = 2.4 +/- 0.8) and an unabsorbed luminosity of L(0.3�??10 keV) = 10.4e38 +/- 2.0 erg s^�??1 (Haggard et al. 2017).
We also examine the three previously-detected X-ray sources CXOU J130948, CXOU 130946, and the host galaxy NGC 4993. The fluxes of CXOU 130946 and the host-galaxy NGC 4993 are consistent with our previous deep Chandra observations, while CXOU J130948 appears to be variable in X-rays (Margutti et al. 2017; Haggard et al. 2017).
The origin of the X-ray emission from the NS-NS coalescence GW170817/GRB170817A is an important diagnostic for all post-merger interpretations, and different scenarios predict distinct evolution in its X-ray light curve. These observations support scenarios in which the X-ray and radio emission share a common origin, i.e., the X-ray light curve is consistent with outflow models which may be either a cocoon shocked by the jet or dynamical ejecta from the merger. Further deep X-ray monitoring can place powerful constraints on the physical parameters of these models. The X-ray brightening strengthens the argument that simple top-hat jet models are not consistent with the latest observations. However, more advanced models of structured jets with off-axis viewing angles should be pursued and cannot yet be ruled out.
We thank Belinda Wilkes and the Chandra scheduling, data processing, and archive teams for making these observations possible.
GCN Circular #22207
A.J. Levan, J.D. Lyman (U. Warwick), N.R. Tanvir (U. Leicester), I. Mandel (U. Birmingham), J. Hjorth (DARK/NBI), A.S. Fruchter, T. Kangas (STScI), B. Gompertz, K. Wiersema, D. Steeghs (U. Warwick), S. Rosswog (Stockholm) report on behalf of a larger collaboration:
"We obtained further observations of the counterpart of GW170817 with the Hubble Space Telescope. Observations took place on 6 Dec 2017 and utilised both optical (F606W, F814W) and infrared (F140W,F160W) filters.
At the location of the transient we recover the source in both optical filters, but do not detect it in the infrared, where the background from the galaxy is higher. The measured magnitudes of the source in the optical bands are broadly consistent with the extrapolation from the 93 day radio epoch (Mooley et al. 2017 arXiv 1711.11573) to the near contemporaneous observations with Chandra (Troja et al. 2017 GCN 22201, Margutti et al. 2017 GCN 22202).
We thank the staff of STScI for their excellent support with these observations."
GCN Circular #22211
K. P. Mooley (Oxford, NRAO/Caltech) et al. report
The X-ray flux of GW170817 on December 03-06 (reported by Troja et al. 2017, GCN 22201; Margutti et al. 2017, GCN 22203, Haggard et al. 2017, GCN 22206 based on the Chandra observations) is exactly as predicted last week by Mooley et al. 2017 ( http://arxiv.org/abs/1711.11573) based on radio spectral indices.
This suggests that the radio and X-ray emission both arise from a mildly relativistic wide-angle outflow, consistent with the cocoon model (see also Gottlieb et al. 2017, Hallinan et al. 2017, Kasliwal et al. 2017). A relativistic jet core, if it exists, is either too weak (having a sub-dominant contribution to the radio light curve early on) or too strong (such that its electromagnetic signatures will be observed in the future). See Mooley et al. 2017 for details.
GCN Circular #24000
A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), E. Berger, P. Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans (Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh (Harvard), G. Terreran (Northwestern/CIERA), A. Villar (Harvard) report:
We initiated a series of Chandra X-ray observations of GW170817 at t~581 days since merger, for a total exposure time of 100 ks. The results from the first observation with the exposure time of ~36 ks is reported in Hajela et al. (GCN 23987).
Here, we report on the remaining ~64ks of Chandra observations of GW170817 acquired starting on 22 March 2019 at 06:17 UT (obsID 22157, exposure time of ~38 ks) and 23 March 2019 at 23:52 UT (obsID 22158, exposure time of ~25 ks, PIs Margutti and Fong). X-ray emission from GW170817 is detected with a significance of >=3 sigma in each of the observations, with a total of ~6 and ~3 photons from the source in obsID 22157 and obsID 22158, respectively (0.5-8 keV energy range).
Assuming a simple power-law spectral model with photon index Gamma = 1.6 and NHint = 0, as found in previous Chandra observations (e.g., Margutti et al., 2018; Alexander et al., 2018), we find an unabsorbed flux of (2.7 +\- 1.2)e-15 erg/s/cm2 and (6.3+\- 3.5)e-15 erg/s/cm2 for ObsID 22157 and 22158, respectively (0.3-10 keV). The Galactic neutral hydrogen column density in the direction of the transient is NH = 7.8e+20cm-2 (Kalberla et al., 2005).
From a joint spectral fit of the three epochs of data (obsID 21322, 22157, and 22158) we find that the photon index is loosely constrained to Gamma=1.6 +/- 0.8 and there is no evidence for intrinsic absorption. For Gamma=1.6, and NHint=0, the joint fit of the entire 100ks of Chandra observations indicates an unabsorbed flux of (3.6 +/- 0.9)e-15 erg/s/cm2 (t~582 days since merger, 0.3-10 keV). We confirm that this flux measurement is consistent with the expectations from the fast decay of the emission from the jet (e.g., Margutti et al., 2018; Alexander et al., 2018; Wu et al., 2018). All fluxes reported are in the 0.3-10 keV energy range with 1-sigma uncertainties.
We thank the entire Chandra team for making these observations possible.
GCN Circular #27357
A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D. Alexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley), D. Giannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson, M. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G. Hosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams (Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P. S. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D. Metzger, L. Sironi (Columbia U.), D. Radice (Princeton) report:
We initiated a new round of observations of the NS merger GW170817 with the Chandra X-ray Observatory (CXO) to continue following its evolution at ~ 2.5 years after the merger. Four observations will be acquired between March 9 and March 15, for a total exposure time of 100 ks. Here we report on the first observation (ObsID 21323, PI Margutti; program 20500299), which was acquired on March 09, 2020 at 22:04:48 UT (~935 days since merger) with an exposure time of 24.29 ks.
X-ray emission is detected at the location of GW170817 with a significance of ~ 3-sigma (Gaussian equivalent) and a 0.5-8 keV count-rate of (1.2 +/- 0.7)e-04 c/s, and a total of ~ 2.8 net photons. Assuming a simple power-law spectrum with photon index Gamma=1.6 and no intrinsic absorption, as found in previous Chandra observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H), and following the same spectra-extraction procedure as in Hajela et al. 2019, the count-rate above translates into an unabsorbed flux of ~ (3.2 +/- 1.9)e-15 erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column density in the direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et al. 2005). This X-ray flux measurement is nominally above, yet statistically consistent with the off-axis jet model predictions given the large flux uncertainties. Further CXO observations comprising the remaining exposure time of ~75 ks are scheduled in the next few days and will help quantify the statistical evidence for the possible departure from the off-axis jet model (which might represent the onset of the kilonova afterglow).
An updated plot with the most recent results can be found at this link: https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home
We thank the entire Chandra team for making these observations possible.
GCN Circular #27410
E. Troja (UMD/NASA/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), H. van Eerten (Bath U.), B. Zhang (UNLV) ob behalf of a larger collaboration:
The Chandra X-ray Observatory re-observed the field of GW170817 between March 9th and March 15th, 2020,and performed four short exposures observations (ObsIDs: 21323, 23183, 23184, 23185) for a total of 96 ks, as part of its on-going monitoring program. At the afterglow position, an X-ray source is detected with significance >4.5 sigma at an average count rate of (8+/-0.4)e-05 cts/s in the 0.5-8.0 keV energy band. By using an absorbed power-law model with absorption column fixed at the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.585, as derived from our broadband analysis, we estimate an unabsorbed X-ray flux of (1.1 +/- 0.6)E-15 erg/cm2/s in the 0.3-10 keV energy band. The quoted error is at the 68% confidence level. This new measurement is higher than earlier predictions of a structured jet, based on a full year of data (Troja E., et al., 2019, MNRAS, 489, 1919), yet still consistent with this model within the uncertainties.
Additional components of emission, such as continued energy injection from a long-lived neutron star (Piro L., et al., 2019, MNRAS, 483, 1912) or afterglow from the sub-relativistic ejecta (Kathirgamaraju A., Giannios D., Beniamini P., 2019, MNRAS, 487, 3914), cannot be excluded at this time.
Further analysis is on-going.
GCN Circular #27411
E. Troja (UMD/NASA/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), H. van Eerten (Bath U.), B. Zhang (UNLV) ob behalf of a larger collaboration
The Chandra X-ray Observatory re-observed the field of GW170817 between March 9th and March 15th, 2020,and performed four short exposures observations (ObsIDs: 21323, 23183, 23184, 23185) for a total of 96 ks, as part of its on-going monitoring program. At the afterglow position, an X-ray source is detected with significance >4.5 sigma at an average count rate of (8+/-0.4)e-05 cts/s in the 0.5-8.0 keV energy band. By using an absorbed power-law model with absorption column fixed at the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.585, as derived from our broadband analysis, we estimate an unabsorbed X-ray flux of (1.1 +/- 0.6)E-15 erg/cm2/s in the 0.3-10 keV energy band. The quoted error is at the 68% confidence level. This new measurement is higher than earlier predictions of a structured jet, based on a full year of data (Troja E., et al., 2019, MNRAS, 489, 1919), yet still consistent with this model within the uncertainties.
Additional components of emission, such as continued energy injection from a long-lived neutron star (Piro L., et al., 2019, MNRAS, 483, 1912) or afterglow from the sub-relativistic ejecta (Kathirgamaraju A., Giannios D., Beniamini P., 2019, MNRAS, 487, 3914), cannot be excluded at this time.
Further analysis is on-going.
GCN Circular #27414
A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D. Alexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley), D. Giannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson, M. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G. Hosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams (Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P. S. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D. Metzger, L. Sironi (Columbia U.), D. Radice (PSU) report:
We carried out Chandra X-ray observations to follow the evolution of GW170817 at t ~ 935-941 days after the neutron star merger, for a total exposure time of ~96 ks. The observations were taken in 4 distinct exposures of ~ 24ks, ~16ks, ~19ks, and ~36 ks, respectively (ObsID 21323, 23183, 23184 and 23185; PI Margutti; program 20500299, for which we waived proprietary data rights). The observations were taken on March 09, 2020 through March 15, 2020. We reported the results from our first observation (ObsID 21323) in Hajela et al. (GCN 27357). Here we report on the results from our entire data set of 96ks, using the most recent data products distributed after a data-processing issue affecting ObsID 23185 was resolved by the CXC-DS's SAP team. An earlier report can be found in Troja et al., GCN 27411.
X-ray emission is detected at the location of GW170817 with a significance of ~5 sigma with a total of ~7 source photons above background in the merged file, which translates to a 0.5-8 keV count-rate of (7.2 +/- 2.7)e-5 cts/s. We perform a joint spectral analysis of all the observations using a simple power-law spectral model with photon index Gamma ~ 1.6 and NHint = 0 cm-2, as found from the homogeneous modeling of all previous Chandra observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H). We freeze the galactic NH = 7.8e+20 cm-2 (Kalberla et al., 2005) and derive an unabsorbed flux of (2.3 +/- 0.9)e-15 erg/cm2/s (0.3-10 keV energy range) with 1-sigma uncertainties. The flux measurement derived here is slightly in excess to, but consistent with the predictions from updated off-axis jet models that best fitted the multi-wavelength afterglow until t~743 days. A similar conclusion was initially reported by Hajela et al., GCN 27357, and later by Troja et al., GCN 27411. An updated light-curve plot is linked here: https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home
These observations might represent the last X-ray detection of the jet afterglow of GW170817. Future Chandra observations are planned to be acquired in December 2020 as part of our program.
We thank the entire Chandra team for making these observations possible.
GCN Circular #29019
A. Hajela, R. Margutti, K. D. Alexander (Northwestern U.), T. Laskar (U. of Bath), D. Giannios (Purdue U.), V. A. Villar (Columbia U.) report:
We observed GW170817 with the Chandra X-ray Observatory (CXO) at t =1209 days (~3.3 years) after the binary neutron star merger. Here we report the preliminary results from the analysis of the first epoch of data acquired so far, for a total exposure time of 29.7 ks (out of a total of ~100 ks to be completed over the next few days) corresponding to ObsID 22677 (PI Margutti; program 21510449).
No X-ray emission is detected at the location of GW170817 and we infer a 3-sigma 0.5-8 keV count-rate upper limit of 1.0e-4 cts/s. Assuming a simple power-law spectral model with a photon index Gamma =1.57 and no intrinsic absorption NHint = 0 cm-2 (as found from previous CXO observations of GW170817, e.g., Hajela et al. 2019), the corresponding 3-sigma absorbed flux limit is Fx<1.6e-15 erg/cm2/s (0.3-10 keV), which is Lx< 3.5e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018). The Galactic neutral hydrogen column density in the direction of GW170817 is NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005). As a comparison, the off-axis jet models that best fit the earlier multi-band evolution of the afterglow predict a luminosity of Lx~1.5e+38 erg/s (0.3-10 keV) at the present epoch, which is below the sensitivity of current CXO observations. Further Chandra observations planned in the next few days will probe deeper fluxes.
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #29038
E. Troja (UMD/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), B. O' Connor (UMD/GWU), H. van Eerten (U. Bath), T. Sakamoto (AGU) report:
We analyzed the latest Chandra observations of GW170817 carried out between Dec 09 and Dec 13, 2020 for a total exposure of about 65 ks (out of 100 ks approved for program 21510449; PI: Margutti) At the transient position we detect a total of 9 counts within a circular region of 1 arcsec radius, corresponding to a count rate (1.5 +/- 0.5)E-04 cts/s (0.5-8.0 keV). This is slightly higher than the value measured at 940 days (0.8 +/- 0.3)E-04 cts/s (Troja et al., 2020, MNRAS, 498, 5643) and consistent within the errors. By using a power-law spectrum with photon index Gamma=1.57, we derive an X-ray flux of (2.6 +/- 0.9)E-15 erg/cm2/s.
Further analysis is on-going.
GCN Circular #29041
A. Hajela, R. Margutti, K. D. Alexander, W. Fong (Northwestern U.), T. Laskar (Bath), D. L. Coppejans (Northwestern U.), V. A. Villar (Columbia U.), D. Giannios (Purdue U.), T. Eftekhari (Harvard U.), L. Sironi (Columbia), B. Margalit (Berkeley) report:
We observed GW170817 with the Chandra X-ray Observatory (CXO) at t ~ 1209-1212 days (~3.3 years) after the binary neutron star merger. We report here the preliminary results from the first three epochs acquired so far, for a total exposure time of ~ 64 ks (out of a total of 100 ks requested; we will report on the results from the complete data set in the next few days). The observations were taken in three distinct exposures of ~30, ~18, and ~17 ks, corresponding to ObsID 22677, 24888, and 24889, respectively (PI Margutti; program 21510449). Preliminary results on these data have been reported in Troja et al. (GCN 29038).
We find evidence for statistically significant X-ray emission at the location of GW170817 in the merged file with a statistical significance of >5 sigma (Gaussian equivalent) corresponding to a net count-rate of (1.2+\- 0.4)e-04 (0.5 - 8 keV). We perform a joint spectral fit of all three epochs with a simple power-law model using a photon index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al. 2019) and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005). With these spectral parameters we derive an unabsorbed flux of 4.4 (+1.78, -1.50) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties), corresponding to a luminosity of 8.7 (+3.5, -2.9)e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018).
These observations are consistent with a constant X-ray flux at t> 580 days, which deviates from the predictions of the off-axis jet afterglow model (e.g. Hajela et al. 2019). The derived flux at 3.3 yrs after the merger is in excess to the structured jet model that best described all the observations up to t ~ 743 days after the merger and might represent the first evidence for the emergence of the kilonova afterglow ( https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home ).
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #29053
K. D. Alexander, A. Hajela, R. Margutti, J. Bright (Northwestern U.), T. Eftekhari (Harvard U.), A. Kathirgamaraju (UC Berkeley), and E. Berger (Harvard U.) report on behalf of a larger collaboration:
We observed GW170817 with the NSF’s Karl G. Jansky Very Large Array (the VLA) at t =1216 days (~3.3 years) after the binary neutron star merger (joint program SL0449, PI: Margutti). In our preliminary analysis, no radio emission is detected at the location of GW170817; we infer a 3-sigma upper limit of ~13 microJy at a central frequency of 3 GHz. Assuming the X-ray flux density recently reported by Hajela et al. (GCN 29041) and a simple power-law spectrum, this is fully consistent with the F_nu ~ nu^-0.575 spectrum observed throughout GW170817’s earlier evolution (e.g. Hajela et al. 2019).
We thank the VLA staff for scheduling and executing these observations.
GCN Circular #29055
A. Hajela, R. Margutti, K. D. Alexander, W. Fong (Northwestern U.), E. Berger (Harvard), T. Laskar (Bath), T. Eftekhari (Harvard), D. Giannios (Purdue U.), P. K. Blanchard, R. Chornock, D. L. Coppejans (Northwestern U.), V. A. Villar (Columbia U.), P. Cowperthwaite (Carnegie), L. Sironi (Columbia), B. Margalit (Berkeley) report:
We report on the complete Chandra data set of observations of GW170817 acquired at t=1209-1213 days since merger (IDs 22677, 24887, 24888, and 24889; PI Margutti; program 21510449) for a total exposure time of ~91 ks. Preliminary results from a subset of these observations have been reported by Hajela et al., GCN 29041, GCN 29019; Troja et al., GCN 29038.
We confirm the presence of significant X-ray emission from GW170817 (significance of detection > 5 sigma, Gaussian equivalent) from the merged observations, with a source count rate of (0.9 +/- 0.3) e-4 cps (0.5-8 keV). From a joint spectral fit with a simple power-law spectral model using a photon index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al. 2019), and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005), we estimate an unabsorbed flux of 3.46(+1.06 -1.31) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties), corresponding to a luminosity of 6.9 (+2.1 -2.6) e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018). The observed emission is a factor of ~2 brighter than the first X-ray non-detection of GW170817 at ~2 days after merger.
As previously reported in Hajela et al., GCN 29041, the inferred flux is in excess to the extrapolation of the off-axis jet model that best fits the broad-band afterglow observations at t<580 days (e. g., Hajela et al. 2019) and might represent the emergence of a new component of emission. An updated plot of the X-ray evolution of GW170817 can be found here: https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home
We thank the entire Chandra team for scheduling and executing these observations.
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #29263
C. D. Kilpatrick, W. Fong, A. Hajela, K. D. Alexander (Northwestern), E. Berger (Harvard), P. K. Blanchard, R. Chornock, R. Margutti, K. Paterson, J. Rastinejad (Northwestern) report:
We report on new Hubble Space Telescope (HST) observations of the near-infrared counterpart to GW170817 at a mid-time of 4.98 Jan 2021 UT, 1236.45 days from merger (GO-15886, PI Fong). These observations consist of 3 orbits observed in WFC3/IR F140W for a total of 7.8 ks on-source exposure time.
Compared to previous F606W HST imaging of the optical and near-infrared afterglow to GW170817 (Lyman et al. 2018, Nature Astronomy, 2, 751; Lamb et al. 2019, ApJL, 870, L15; Fong et al. 2019, ApJL, 883, L1), we do not visually detect a counterpart at the site of GW170817 in the latest observations. We injected artificial sources at the afterglow site and recover a 3-sigma limiting magnitude of >26.3 mag (AB and without correcting for Galactic extinction).
Based on recent Chandra detections of the X-ray counterpart to GW170817 at 1209-1213 days after merger (Hajela et al., GCN 29041, GCN 29019; Troja et al., GCN 29038) as well as radio non-detections with the VLA at 1216 days post-merger (Alexander et al., GCN 29053), we consider whether this limit is constraining in the context of the detection discussed in Hajela et al., GCN 29055. Assuming the counterpart had an unabsorbed flux of 3.46e-15 erg/cm2/s (0.3-10 keV) at that time and a Gamma~1.6 photon index, we infer that the source would have f_F140W = 1.5e-8 Jy or m_F140W = 28.5 mag (AB; unabsorbed) or 28.6 mag (AB; accounting for Galactic extinction) in the latest observations. Similarly, the radio observations yielded a 3-sigma limiting flux of 13 muJy at a central frequency of 3 GHz, which imply that m_F140W > 28 mag for the same photon index, significantly fainter than our limit.
Thus the latest observations are not constraining for this model. However, additional HST observations are planned in F814W, F110W, and F160W between now and 2021 March 1 with comparable exposure times in each filter, enabling deeper constraints on the optical and near-infrared counterpart to GW170817 over a broader range of its spectral energy distribution.
We thank the STScI staff for facilitating these observations, especially our program coordinator Shelly Meyett.
GCN Circular #29375
A. Hajela, R. Margutti, J. Bright, K. D. Alexander, W. Fong (Northwestern U.), E. Berger (Harvard), R. Chornock, D. L. Coppejans, P. Blanchard (Northwestern U.), V. A. Villar (Columbia U.), T. Eftekhari (Harvard U), A. Kathirgamaraju (UC Berkeley), D. Giannios (Purdue U.), T. Laskar (Bath), J. Zrake (Clemson), A. MacFadyen (NYU), K. Paterson (Northwestern U.) report:
We acquired 3 distinct exposures (ObsID 23870, 24923, and 24924) of GW170817 with the Chandra X-ray Observatory (CXO) between January 18 and January 27, 2021, under our program 22510329 (PI Margutti), for a total exposure time of ~98 ks. We initiated these observations after we inferred a potential excess of X-ray emission compared to our off-axis jet model in our previous round of observations (ObsID 22677, 24887, 24888, and 24889; results reported in Hajela et al., GCN 29055; also see Troja et al. GCN 27411). The 3 new observations combined with our previous round span 1209-1258 days post-merger and have a mean epoch of 1234 days.
We merge all 7 observations and find significant X-ray emission at the location of GW170817 at the level of ~5 sigma (Gaussian equivalent). From a joint spectral fit with a simple power-law spectral model using a photon index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al. 2019), and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005), we estimate an unabsorbed flux of 2.4(+0.53 -0.82) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties), corresponding to a luminosity of 4.8 (+1.1 -1.6) e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018). Based on the expectations from the best-fitting off-axis jet model from Hajela+2019, the chance probability to obtain at ~1234 days after the merger an X-ray count-rate at least as large as the measured count-rate is P=0.0014 (corresponding to 3.2 sigma, Gaussian equivalent). Radio observations to date have not shown the same excess (Alexander et al. GCN 29053), but more detailed analysis is in progress.
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #31187
A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander (Northwestern U.), T. Laskar (Radboud U.), R. Chornock (UC Berkeley), E. Berger (Harvard), V. Ashley Villar (PSU) report:
We have acquired X-ray observations of GW170817 with the Chandra X-ray Observatory (CXO) at t ~ 1573 days (~ 4.3 years) after the binary neutron star merger. We report here the preliminary results from the first epoch (ObsID 23869; PI Margutti; program 22510329), for a total exposure time of 28.5 ks (out of a total of 100 ks requested; we will report on the results from the complete data set in the next few days).
After aligning the images to a common astrometric solution as described in Hajela et al., 2021, we find evidence for X-ray emission at the location of GW170817 with a statistical significance of 3.8 sigma (Gaussian equivalent) corresponding to a net count-rate of (1.31 +/- 0.72)e-04 c/s (0.5 - 8 keV). Assuming a simple absorbed power-law spectral model with a photon index Gamma ~ 1.6, no intrinsic absorption (NH,int = 0 cm-2 as found in e.g. Hajela et al. 2019) and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005), we derive an unabsorbed flux of ~ 1.7e-15 erg/cm2/s (0.3-10 keV), corresponding to a luminosity of ~3.4e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018).
Based on the extrapolation of the jet afterglow models calculated with JetFit (gammaB=12) and the universal post jet break models with start time tstart=196 days as calculated in Hajela et al., 2021, and the observed background, the expected number of photons within a 1” region at the location of GW170817 is ~0.55 (0.5-8 keV, exposure time of 28.5 ks). The observed number of counts after Xspec filtering corresponds to a ~10% chance fluctuation probability for universal post jet break models (~12% for JetFit). For this choice of tstart and gammaB, the total probability of observing at least as many counts as those collected for GW170817 at t>900 days based on jet afterglow models as a result of a chance fluctuation is ~6e-5 (~ 4.0 sigma Gaussian equivalent, for the universal post jet break model) and ~3e-5 (~ 4.2 sigma Gaussian equivalent for JetFit). The remaining ~75 ks of planned CXO observations will further constrain the statistical evidence of an excess of X-ray emission in GW170817 with respect to jet afterglow models as reported in Hajela et al., 2021.
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #31231
A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander (Northwestern U.), J. S. Bright (UC Berkeley), T. Laskar (Radboud U.), E. Berger (Harvard U.) report:
Chandra X-ray Observatory (CXO) observed GW170817 at t ~ 1575 days (~ 4.3 years) after the binary neutron star merger in four distinct exposures (ObsID 23869, 26223, 24336, 24337; PI Margutti; program 22510329). We reported the preliminary results from the first epoch (ObsID 23869) in GCN 31187 (Hajela et al.). Here, we report results from all four observations acquired for a total exposure time of ~ 98ks.
We merged all four observations after reprocessing and correcting for the absolute astrometry (as described in Hajela et al. 2021), and found 5 total photons in 0.5-8 keV energy range in an 1” region at the position of GW170817. This corresponds to a net 0.5-8 keV count-rate of ~4.3e-5 c/s, which translates to a detection of X-ray emission at the location of GW170817 with a statistical significance of ~3.2 sigma (Gaussian equivalent). We joint-fitted all the four observations with an absorbed power law model with photon index fixed to ~ 1.6, no intrinsic absorption (NH,int = 0 cm-2 as found in e.g. Hajela et al. 2019), and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005). The derived unabsorbed flux is ~1.3e-15 erg/cm2/s (0.3-10 keV).
From a preliminary analysis we find that this measurement is in slight excess to, but statistically consistent with, the expectations from the off-axis jets models of Hajela et al., 2021. However, we also note that, when compared to the Chandra observations of GW170817 obtained in March 2020 (t=939.31 days since merger) and Dec 2020+Jan2021 (t=1234.11 days since merger), and using a Binomial test for variability in the count phase space, we find no statistically significant evidence of fading of the X-ray source associated with GW170817 in the last ~650 days.
We thank the entire Chandra team for scheduling and executing these observations.
GCN Circular #32065
B. O'Connor (GWU/UMD), E. Troja (Tor Vergata), on behalf of a larger collaboration:
The Chandra X-ray Observatory (CXO) carried out observations of GW170817 starting at 1671 d post-merger (ObsIDs: 25733, 25734; PI: Troja) for 59.4 ks, an additional epoch was performed starting at 1734 d (ObsID: 25527) for 36.6 ks. The data were re-processed and the astrometry corrected following Troja et al. 2020, MNRAS, 4, 5643. In the 0.5-8 keV energy range we measure 2 counts within a 1" aperture centered on the X-ray position. A background contribution of about 0.85 counts was estimated from adjacent source-free regions.
Following Kraft, Burrows, and Nousek (1991), we derive a 95% upper limit of <5.5 source counts, yielding a rate of <6E-5 cts/s in the combined 95.8 ks. This corresponds to a 0.3-10 keV flux limit of 1.3E-15 erg/cm2/s for a power-law with photon index of 1.585. This limit is comparable to the value measured at 1234 d (1.6E-15 erg/cm2/s; Troja et al. 2022, MNRAS, 510, 1902), and does not show evidence for a rebrightening of the X-ray emission.
By combining this new epoch with the CXO observations acquired in December 2021 at 1575 d (Hajela et al. 2021, GCN 31231), we obtain a weak (3-sigma) detection with an average count-rate of 3E-05 cts/s, and a 0.3-10 keV flux of ~0.6E-15 erg/cm2/s.
An updated light curve can be found at this link: https://nora.users.roma2.infn.it/GW170817_May2022.jpg
We thank the Chandra staff for scheduling these observations.
GCN Circular #32094
R. Ricci (INRiM/INAF-IRA), B. O' Connor (GWU/UMD), E. Troja (UTV) on behalf of a larger collaboration
We report on the Karl G. Jansky Very Large Array latest observations of GW170817 as part of our ongoing X-ray and radio monitoring. The observations were performed in S-band between May 2nd and May 11th 2022 in eight execution blocks for a total observing time of 16 hours. The data were calibrated, concatenated and imaged in CASA. A preliminary analysis of the cleaned map reveals no source at the target position. We derive a 3-sigma flux density upper limit of 6 microJy at 3 GHz, consistent with the behavior observed at X-ray energies (O'Connor et al., GCN 32065).
Further joint Chandra/VLA observations are planned in 2023.
We thank the VLA staff for executing these observations.

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Jochen Greiner, last update: 18-Mar-2020 [Disclaimer]