//////////////////////////////////////////////////////////////////////// ATEL #11037 ATEL #11037 Title: LIGO/Virgo GW170817: Chandra X-ray brightening of the counterpart 108 days since merger Author: R. Margutti, W. Fong (Northwestern), T. Eftekhari, K. Alexander, E. Berger (Harvard), R. Chornock (Ohio University) Queries: rafmargutti@gmail.com Posted: 7 Dec 2017; 18:00 UT Subjects:X-ray, Gravitational Waves 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22203 SUBJECT: LIGO/Virgo GW170817: Chandra X-ray brightening of the counterpart 108 days since merger DATE: 17/12/07 18:04:04 GMT FROM: Raffaella Margutti at Northwestern U 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.” //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22206 SUBJECT: LIGO/Virgo G298048: Brightening X-ray Emission from GW170817/GRB170817A/SSS17a DATE: 17/12/07 22:11:17 GMT FROM: Phil Evans at U of Leicester 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22207 SUBJECT: LIGO/Virgo GW170817: Further Hubble Space Telescope observations DATE: 17/12/07 22:47:21 GMT FROM: Andrew Levan at U.of Leicester 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." //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22211 SUBJECT: LIGO/Virgo GW170817: X-ray observations confirm prediction made from radio data DATE: 17/12/08 21:30:16 GMT FROM: Kunal Mooley at Oxford U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22372 SUBJECT: GW170817/GRB170817A: Preliminary results of Chandra monitoring DATE: 18/01/29 20:42:49 GMT FROM: Eleonora Troja at GSFC E. Troja (UMD/GSFC) and L. Piro (INAF/IAPS) report on behalf of a larger collaboration: The Chandra X-ray Observatory re-observed the field of GW170817 starting on January 17th, 2018, and performed five short exposures observations as part of its on-going monitoring program (PI: Wilkes). Only three of these exposures (ObsID: 20936, 20938, 20939) are currently archived and available to the public. Here we report the preliminary findings from these observations. The X-ray afterglow is detected with high significance in all the exposures at an average count rate of 0.0016 cts/s in the 0.5-8.0 keV energy band. A preliminary inspection of the hardness ratio does not show any significant spectral variation. Therefore, we perform a spectral analysis using an absorbed power-law model with absorption column fixed at the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.575 as derived from our broadband analysis (Troja et al. 2018, arXiv:1801.06516). We derive an unabsorbed X-ray flux of (3.2 +/- 0.3)E-14 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 the value measured by Chandra at ~110 days (~2.5E-14 erg/cm2/s, Troja et al. 2018), and higher than the value measured by XMM-Newton at ~135 days (D'Avanzo et al., 2018). The latest measurement is consistent with a rising afterglow with F~t^0.8, although, within the errors, a slow turn-over of the X-ray light curve cannot be excluded. Further analysis is on-going. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22374 SUBJECT: GW170817/GRB170817A: Updated results from the full Chandra dataset DATE: 18/01/30 13:00:12 GMT FROM: Eleonora Troja at GSFC E. Troja (UMD/GSFC) and L. Piro (INAF/IAPS) report on behalf of a larger collaboration: We analyzed the full set of five observations of GW170817 performed by the Chandra X-ray Observatory between January 17th and January 28th, 2018, i.e. ~153 and ~164 days after the merger. A log of observations is reported below: ObsID   Exposure [ks]     0.5-8.0 keV count rate [cts/s] 20936     31.75           0.0018 +/- 0.0002 20937     20.77           0.0014 +/- 0.0003 20938     15.86           0.0019 +/- 0.0003 20939     22.25           0.0011 +/- 0.0002 20945     14.22           0.0010 +/- 0.0003 The average net count-rate is 0.00148 +/- 0.00011 cts/s, consistent with the value of 0.00145 +/- 0.00014 observed at 110 days (Troja et al. 2018, arXiv:1801.06516). The average spectrum, obtained by coadding the five exposures, is well described by an absorbed power-law model with N_H=7.5E20 cm^-2 and photon index Gamma=1.65+/-0.16 (68% c.l.), consistent with the value derived from the broadband spectrum at earlier times (Troja et al. 2018, arXiv:1801.06516). Based on this new analysis, we estimate an unabsorbed X-ray flux of (2.6 +/- 0.3)E-14 erg/cm2/s (68% c.l.) in the 0.3-10 keV band, consistent with the X-ray flux measured at 110 days. Our results do not support the claim of a decreasing X-ray flux, as suggested by D'Avanzo et al. (2018, arXiv:1801.06164), and are consistent either with a slowly rising afterglow or a slow turn-over of the X-ray light curve expected when the afterglow reaches its peak (e.g. Lazzati et al. 2017, arXiv:1712.03237; Troja et al. 2018, arXiv:1801.06516). We note that the X-ray afterglow displays a marginal level of variability on timescales of a few days, being the count-rate from the last two exposures (20939,20945) consistently lower. The spectrum from these two observations is characterized by a photon index Gamma= 1.9 +/- 0.3 (68% c. l.), slightly softer than the value measured in the first three exposures (20936,20937, and 20938) Gamma = 1.59+/-0.17  (68% c. l.), yet consistent within the large uncertainties. The lower count-rate and soft spectral shape could be indicative of the cooling frequency entering the X-ray band, although the limited statistics prevent us to draw any firm conclusion. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22692 SUBJECT: Chandra observations of GW170817 260 days since merger: first statistically significant evidence for an X-ray decay DATE: 18/05/08 01:44:24 GMT FROM: Kate Alexander at Harvard A. Hajela (Northwestern/CIERA), K. D. Alexander, T. Eftekhari (Harvard/CfA), R. Margutti, W. Fong (Northwestern/CIERA), E. Berger (Harvard/CfA) report "The Chandra X-ray Observatory started another round of follow-up observations of GW170817. The first observation (ID 21080) was taken on May 03, 2018 at 10:41:26 UT (t ~ 259 d after merger) for a total exposure time of 50.8 ks (PI Wilkes; program 19408644). We find that an X-ray source is detected at the location of GW170817 with a count-rate of (7.7 +/- 1.3)e-4 cts/s (0.5 - 8 keV) and 13.8-sigma significance. We modeled the spectrum with an absorbed simple power-law and find a best-fitting photon index, Gamma = 1.4 +/- 0.3 and no evidence for intrinsic absorption. The galactic neutral hydrogen column density in the direction of the transient is, N_H = 7.8E+20cm-2 (Kalberla et al., 2005). For these parameters, the unabsorbed flux is (1.3 +/- 0.3)e-14 ergs cm-2 s-1 (0.3 - 10 keV). A second observation (ID 21090) was acquired on May 05, 2018 at 01:25:30 UT (~ 261 days after merger) for a total exposure time of 46.0 ks. GW170817 is detected with 14.8-sigma significance and a count-rate of (8.3 +/- 1.4)e-4 cts/s (0.5 - 8 keV). The best-fitting photon-index is Gamma = 1.3 +/- 0.3. The corresponding unabsorbed flux is (1.5 +0.5/-0.3) e-14 ergs cm-2 s-1 (0.3 - 10 keV). The best fitting photon index from a joint spectral fit is Gamma = 1.43 +/- 0.15, consistent with the photon index measured at ~160 days. We conclude that there is no evidence for the passage of the synchrotron cooling frequency through the Chandra band. The total 0.5-8 keV count-rate inferred from the two observations is (8.0+/-0.9)e-4 cts/s. The X-ray source shows significant fading compared to the previous epoch at ~160 days (inferred rate of (1.50 +\-0.12)e-3 cts/s, Margutti et al., 2018). Applying a simple binomial test, we find that the detected fading is significant at a level > 4 sigma. We can thus reject the hypothesis of a random fluctuation of the X-ray count-rate, and conclude that these Chandra observations provide the first statistically significant evidence for a decaying X-ray emission from GW170817. We thank the entire Chandra team for making these observations possible." //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22693 SUBJECT: Chandra observations of GW170817 reveal a fading afterglow DATE: 18/05/08 03:59:58 GMT FROM: Eleonora Troja at GSFC E. Troja (UMD/GSFC), L. Piro (IAPS/INAF), G. Ryan (UMD) report on behalf of a larger collaboration: The Chandra X-ray Observatory re-observed the field of GW170817 starting on May 3rd, 2018 as part of its on-going monitoring program (PI: Wilkes). Observations were split into two exposures of 50.7 ks and 46 ks, respectively. The X-ray afterglow is significantly detected at an average level of (8.5 +/- 1.4)E-04 cts/s, well below the value (14.8 +/- 1.1)E-04 cts/s measured  at ~160 d after the merger (Troja & Piro, GCN Circ. 22374; Troja et al. 2018). The spectrum is well described by an absorbed power-law model with photon index 1.5 +/- 0.2, consistent with the value of 1.575 derived at earlier times from the broadband spectrum. By using this value we derive an unabsorbed X-ray flux of (1.4 +/- 0.2)E-14 erg/cm2/s at 260 days after the merger. A comparison with the afterglow models derived in Troja et al. (2018) is shown at this link: https://www.astro.umd.edu/~gsryan/GW170817/xray.png This new X-ray observation supports earlier claims (D'Avanzo et al. 2018; Dobie et al. 2018) that the afterglow entered its post-peak decay phase. However, no clear distinction between the competing models is possible. Further observations are critical to probe the structure of the relativistic outflow. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22763 SUBJECT: GW170817/GRB170817A: LBT optical detection DATE: 18/06/05 14:03:37 GMT FROM: Andrea Rossi at INAF A. Rossi (INAF-OAS), M. Cantiello (INAF-OA Abruzzo) V. Testa, D. Paris (INAF-OAR), A. Melandri, S. Covino, O. S. Salafia, P. D'Avanzo, S. Campana (INAF-OAB), L. Nicastro, E. Palazzi, F. Cusano (INAF-OAS), G. Stratta (Urbino University/INFN Firenze), R. Carini, S. Piranomonte, E. Brocato (INAF-OAR), V. D'Elia (ASDC), and M. Branchesi (GSSI) report on behalf of the GRAWITA collaboration and its partners: We observed the optical counterpart of GRB 170817A (Kienlin et al., GCN 21520) associated to GW 170817 (LVC GCN Circ. 21509, 21513) with the LBC imager mounted on the Large Binocular Telescope (Mt Graham, AZ, USA). Observations were performed in the r-sloan filter on 2018-01-23, i.e., ~160 days after the GW/GRB trigger. At the location of the optical transient (e.g., Coulter et al., GCN 21529; Adams et al., 21816) we detect the optical afterglow of GRB 170817A with magnitude r-sloan=26.2+-0.4, calibrated against Pan-STARRS field stars. Image analysis was performed after preliminary removal of an elliptical model of the underlying host galaxy from each single frame. However, some residual emission is left which contributes for ~0.2 mags to the uncertainty of the photometry. Our detection is the first one from a ground-based optical telescope. It is in agreement with a turnover/flattening in the optical light curve of GW 170817/GRB 170817A as inferred by Alexander at al. 2018 (arXiv:1805.02870) and with the overall flattening/declining temporal evolution observed in the X-ray and radio bands (D'Avanzo et al. 2018, A&A, 613 L1; Hajela et al. GCN Circ. 22692; Troja et al. GCN Circ. 22693; Dobie et al. arXiv:1803.06853; Alexander at al. 2018; arXiv:1805.02870). We acknowledge the excellent support from the LBT staff in obtaining these observations. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 23137 SUBJECT: Chandra X-ray observations of GW170817 at 1 year post-merger: increasingly rapid fading DATE: 18/08/13 01:52:40 GMT FROM: Daryl Haggard at McGill U Daryl Haggard, Melania Nynka, and J. J. Ruan (McGill/MSI) report: We analyze new Chandra X-ray observations of neutron star merger GW170817 at 358.6 days post-merger, which now reveals fading at a t^-1.6 rate. Chandra obtained a 67.16 ks observation of GW170817 (ObsID: 21371, PI: Troja) on 10 August 2018, at 358.6 days post-merger. GW170817 is still clearly detected, and we measure a 0.5-8 keV count rate of 4.9 (+0.9/-0.9) cts/s. We extract and fit the X-ray spectrum assuming an absorbed power-law spectral model, with fixed NH = 7.5e20 cm^-2. We measure an absorbed flux of f(0.3-8 keV) = 6.3 (+3.8/-2.1) x10^-15 erg/s/cm^2 (90% c.l.) and photon index of Gamma = 1.6 (+1.3/-0.9), which corresponds to an unabsorbed luminosity of L(0.3-10 keV) = 1.7 (+1.1/-0.5) x10^39 erg/s assuming a luminosity distance of 42.5 Mpc. The previous Chandra observation at 260.0 days showed an absorbed flux of f(0.3-8 keV) = 1.09 (+0.24/-0.20) x10^-14 erg/s/cm^2 (Nynka et al. 2018). A power-law fit of this previous 260.0 day flux to the new 358.6 day flux reveals fading at a t^-1.6 rate. This is steeper than the t^-1.3 fading observed between 159.7 and 260.0 days post-merger, and thus the X-ray afterglow light curve is now fading more rapidly. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 23139 SUBJECT: LIGO/Virgo GW170817: A steep decline in the radio light curve and prediction for the X-rays DATE: 18/08/13 18:38:03 GMT FROM: Tara Murphy at U of Sydney D. Dobie (University of Sydney), K. Mooley (Caltech), T. Murphy (University of Sydney), D. Kaplan (UWM), E. Lenc (CSIRO), A. Corsi (TTU), D. Frail (NRAO), report on behalf of a larger collaboration Our continued observations of GW170817 with the ATCA and the VLA up to 300 days post-merger (Mooley et al. in prep) confirm the t^(-2) decline in the radio light curve initially reported in Mooley et al. 2018 (arXiv:1806.09693). Such a slope rules out a cocoon-dominated outflow at late times, and is instead the classic signature of a relativistic jet, consistent with the VLBI result from Mooley et al. 2018. The t^(-2) decline is also expected in the X-ray light curve, and may be confirmed by the Chandra observation carried out on 2018 Aug 10. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 23140 SUBJECT: Correction: Chandra X-ray observations of GW170817 at 1 year post-merger: increasingly rapid fading DATE: 18/08/14 15:42:03 GMT FROM: Daryl Haggard at McGill U Daryl Haggard, Melania Nynka, and J. J. Ruan (McGill/MSI) write: We correct a typo in GCN 23137: The 0.5-8 keV count rate measured in the 10 August 2018 Chandra observation of GW170817 should have read 4.9 (+0.9/-0.9) e-04 ct/s. All fluxes, photon indices, luminosities, light curve slopes, etc., reported in GCN 23137 are correct, and were derived from this correct count rate. GCN OPS NOTE(28aug18): Per author's request, the Subject-line was corrected with the proper referencing Subject-line.] //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 25631 SUBJECT: Chandra observations of GW170817 ~740-743 days since merger DATE: 19/09/03 15:17:27 GMT FROM: Aprajita Hajela at Northwestern U A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U Bath), D. Coppejans, G. Terreran, W. Fong, K. D. Alexander, A. Baldeschi, K. Paterson (Northwestern U.), E. Berger, P. K. Blanchard, T. Eftekhari, G. Hosseinzadeh, S. Gomez, V. A. Villar, P. K. G. Williams (Harvard U), M. Nicholl (U Edinburgh), R. Chornock (Ohio U), P. S. Cowperthwaite (Carnegie Observatories), D. Giannios (Purdue U.), A. MacFadyen (NYU), A. Kathirgamaraju (UC Berkeley) report: We report on Chandra observations of GW170817 acquired ~2 yrs after the binary neutron star merger. The Chandra X-ray Observatory started observing GW170817 August 27, 2019 at 14:16:40 UT (t ~ 740 d after merger) for an exposure time of 40.0 ks (PI Troja; program 20500691, observation ID 21372). Another two epochs of Chandra observations were acquired on August 29, 2019 at 13:46:01 UT (ID 22736) and on August 30, 2019 at 11:46:44 UT (ID 22737) for an exposure time of 33.6 ks and 25.3 ks, respectively. A preliminary reduction shows that GW170817 is detected in the merged exposure with a ~3 sigma significance and a net count-rate of (1.03 +\- 0.33)e-4 c/s (0.5-8 keV). Assuming negligible intrinsic absorption and a Galactic neutral hydrogen column density N_h = 7.8E+20cm-2 (Kalberla et al., 2005), we infer a best-fit photon index Gamma=1.0+/-1.3 and an unabsorbed flux of ~3e-15 erg/s/cm2 (0.3-10 keV). The photon index is loosely constrained by the observations and we therefore adopt the best-fit photon index from the entire Chandra data set acquired in the first two years of observations of GW170817 (Gamma=1.6) for the spectral calibration. We find an unabsorbed flux of (2.9 +\- 1.0)e-15 erg/s/cm2. These results are consistent within the uncertainties with the extrapolation of the off-axis structured jet model by Wu & MacFadyen (ApJ, 869, 55) at the current epoch. We thank the entire Chandra team for making these observations possible. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 27357 SUBJECT: Chandra observations of GW170817 ~ 2.5 years since merger (epoch 1 of 4) DATE: 20/03/11 00:37:55 GMT FROM: Aprajita Hajela at Northwestern U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 27411 SUBJECT: GW170817: Continued X-ray emission detected with Chandra at 940 days post-merger DATE: 20/03/18 17:59:10 GMT FROM: Eleonora Troja at NASA/GSFC/UMD 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 27414 SUBJECT: Chandra observations of GW170817 at 2.5 years since merger (All epochs 4 of 4) DATE: 20/03/19 03:15:10 GMT FROM: Aprajita Hajela at Northwestern U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29019 SUBJECT: Chandra observations of GW170817 at ~3.3 years since merger (Observation 1 of 4) DATE: 20/12/09 22:38:37 GMT FROM: Aprajita Hajela at Northwestern U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29038 SUBJECT: Long-lasting X-ray emission from GW170817 detected by Chandra DATE: 20/12/14 12:29:41 GMT FROM: Eleonora Troja at NASA/GSFC/UMD 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29041 SUBJECT: Potential X-ray KN afterglow from Chandra observations of GW170817 ~3.3 years since merger (3 Epochs of 4) DATE: 20/12/14 17:55:09 GMT FROM: Aprajita Hajela at Northwestern U 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. Further analysis is on-going. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29053 SUBJECT: VLA observations of GW170817 at ~3.3 years post-merger DATE: 20/12/16 01:40:37 GMT FROM: Kate Alexander at Northwestern U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29055 SUBJECT: Chandra observations of GW170817 at ~3.3 years since merger (Full report) DATE: 20/12/16 13:28:30 GMT FROM: Aprajita Hajela at Northwestern U 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 29263 SUBJECT: GW170817: HST observations at 3.4 years from merger DATE: 21/01/05 19:06:43 GMT FROM: Charles Kilpatrick at UC Santa Cruz 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.