MAXI X-ray observation of the NS-NS merger GW 170817

October 16, 2017
MAXI team

At 12:41 August 17, 2017 the Advanced LIGO and the Advanced Virgo detected an gravitational wave event GW 170817. MAXI onboard the International Space Station (ISS) made the earliest X-ray observation of GW 170817. The upper limit at 4 hour 40 minutes after the event was 0.37 Crab. It was even earlier than the discovery of the optical counter part (11 hours after the GW). MAXI scans the 85 % of the whole sky in one ISS orbit (= 92 minutes). Very unfortunately, the GW position was not observable for about 5 hours.
The GW170817 was considered to be a NS(neutron star)-NS merger by the GW waveform. A short gannma-ray burst (sGRB) was observed from GW 170817. But it was very (1/1000) weak compared to the standard sGRB. The scientists rather consider that it was not a standard sGRB but another kind of GRB. Several theories are proposed, such as slant viewing of the relativistic jet, emission from a shock wave, and radiation from radioactive isotopes.

Fig 1　MAXI X-ray map around GW170817 region

Green contours are the estimated region of the GW 170817. Cross is the position of optical counter part SSS17a which was discovered 11 hours later. Orange dots are X-ray photons which MAXI detected. There is no point source. (upper left) MAXI happened to scan over the SSS17a 1 minute after the GW event. But MAXI was off at the time very unfortunately. (upper right) MAXI observed the GW 170817 region 4 hours 30 minutes after the event. There was no X-ray from SSS17a. (lower left) Since 6th orbit MAXI continued scan over GW 170817, but there was no X-ray emission.

Fig 2 MAXI upper limits of GW170817 plotted over the afterglow of standard short GRBs

Blue arrow points are upper limits by MAXI at the scan time. Gray lines are light curves of previous X-ray afterglows of standard sGRB placed at the GW 170817 distance. MAXI upper limits cannot tell the absence of the afterglow. Red upper limit is the level that MAXI could have given in the first orbit if it had been on.

Fig 3 Scenario of a NS-NS merger and EM emission