[vsnet-grb-info 20275] LIGO/Virgo GW170817: Chandra X-ray Emission Continues to Rise ~156 Days Post-Merger

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22371
SUBJECT: LIGO/Virgo GW170817: Chandra X-ray Emission Continues to Rise ~156 Days Post-Merger
DATE:    18/01/29 19:21:22 GMT
FROM:    Daryl Haggard at McGill U  <daryl.haggard at mcgill.ca>

Daryl Haggard, Melania Nynka, John J. Ruan (McGill/MSI), Phil Evans 
(Leicester), and Vicky Kalogera (Northwestern/CIERA) report:

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.

Note that another ~30 ks of Chandra observations during this same time 
interval are forthcoming.

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