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LIGO/Virgo GW170817: Chandra X-ray Emission Continues to Rise ~156 Days Post-Merger

ATel #11242; Daryl Haggard, Melania Nynka, John J. Ruan (McGill/MSI), Phil Evans (Leicester), Vicky Kalogera (Northwestern/CIERA)
on 29 Jan 2018; 19:38 UT
Credential Certification: Daryl Haggard (daryl.haggard@mcgill.ca)

Subjects: X-ray, Gamma-Ray Burst, Gravitational Waves, Neutron Star

Referred to by ATel #: 11243

We have obtained new X-ray observations of GW170817 via Chandra Director's Discretionary Time (PI: Wilkes, Program Number 19408607). The X-ray counterpart to GW170817/GRB170817A/SSS17a continues to be detected and the X-ray emission continues to brighten approximately 156 days after the neutron star merger. This contradicts previous claims of the X-ray fading of GW170817 in XMM-Newton observations at 135 days (D'Avanzo et al. 2018, arXiv: 1801.06164).

This new Chandra program acquired several exposures of GRB170817A:

ObsID, ExpTime, StartDate, Days Post-Burst
----- ------- --------- ---------------
20936 31.75 ks 2018-01-17 21:55:17 153.5 days
20938 15.86 ks 2018-01-21 13:45:18 157.1 days
20939 22.25 ks 2018-01-24 08:18:48 159.9 days

Since these new Chandra exposures are close in time and the X-ray emission of GRB170817A is not expected to vary significantly over <10 day timescales, we co-add these three data sets into one 69.86 ks exposure at 156.4 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) = 1.93(+0.39/-0.32)e-14 erg s^-1 cm^-2 (with Gamma ~ 1.67) at 156.4 days post-burst, which corresponds to an unabsorbed luminosity of L(0.3-10 keV) = 5.23(+1.30/-0.95)e39 erg s^-1 (assuming a luminosity distance of 42.5 Mpc).

This represents continued X-ray brightening compared to Chandra observations at 15.6 and 109.2 days post-burst, for which we find an absorbed flux of f(0.3-8 keV) = 0.36(+0.1/-0.07)e-14 erg s^-1 cm^-2 (with Gamma = 2.4 +/- 0.8, unabsorbed L(0.3-10 keV) = 10.4(+2.0/-1.6)e38 erg s^-1; Haggard et al. 2017) and f(0.3-8 keV) = 1.58(+0.14/-0.13)e-14 erg s^-1 cm^-2 (with Gamma = 1.6 +/- 0.3, unabsorbed L(0.3-10 keV) = 42.5(+3.7/-3.5)e38 erg s^-1; Ruan et al. 2018), respectively.

Our findings here contradict recent reports of dimming in the X-ray flux from XMM Newton at 135 days (D'Avanzo et al. 2018), which was reported to be 2.1(+0.7/-0.5)e-15 erg s^-1 cm^-2 (0.3-10 keV unabsorbed). However, this reported flux value is a typo, and should be 2.1(+0.7/-0.5)e-14 erg s^-1 cm^-2 (D'Avanzo 2018, private communication). Taking this corrected flux and rescaling to a 0.3-8 keV absorbed flux for comparison to the previous Chandra measurements above gives 1.67(+0.87/-0.64)e-14 erg s^-1 cm^-2. Thus, the recent X-ray data at 15.6 days (Chandra), 109.2 days (Chandra), 135 days (XMM), and 156.4 days (Chandra) are all consistent with continued X-ray brightening. We provide a light curve table summarizing these measurements:

Days, Telescope, Flux* (0.3-8 abs), Ref
---- --------- ---------------- ---
15.6 Chandra 0.36(+0.10/-0.07)e-14 Haggard et al. (2017)
109.2 Chandra 1.58(+0.14/-0.13)e-14 Ruan et al. (2018)
135 XMM 1.67(+0.87/-0.64)e-14 D'Avanzo et al. (2018)
156.4 Chandra 1.93(+0.39/-0.32)e-14 This work

*Flux units: erg s^-1 cm^-2; all uncertainties are 90% confidence interval

Current post-merger models suggest that the origin of the X-rays could be afterglow emission from either a mildly-relativistic cocoon or a structured jet. For a cocoon, the continued rise of X-ray emission suggest that the cocoon has not yet reached a deceleration phase. For a structured jet, the rising X-ray emission suggest emission from the jet core has not yet entered the observed line of sight. Continued monitoring of GW170817 will be critical for discriminating between these and other models.

NB: Another ~30 ks of Chandra observations are forthcoming.

We thank Belinda Wilkes and the Chandra scheduling, data processing, and archive teams for making these observations possible.