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AstroSat observation of the newly discovered transient X-ray binary source: MAXI J0637-430

ATel #13296; N. T. Thomas, S. B. Gudennavar (CHRIST - Deemed to be University), R. Misra (IUCAA), S. G. Bubbly (CHRIST - Deemed to be University)
on 19 Nov 2019; 12:28 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dipankar Bhattacharya (dipankar@iucaa.in)

Subjects: X-ray, Binary, Black Hole, Transient

MAXI J0637−430 is a new transient X‐ray binary source discovered by MAXI/GSC on 2nd November, 2019 (ATEL #13256). Follow up observations of the source in different bands have been made by Swift (ATEL #13257, #13295), SOAR (ATEL #13260), NuSTAR (ATEL #13270), iTelescope.Net (ATEL #13276), ATCA (ATEL #13275) and others (ATEL #13291, #13292). Preliminary studies suggest that the source may be a black hole X‐ray binary.

We report the AstroSat observation of this newly discovered X‐ray binary with the LAXPC and SXT instruments. The source was observed for 40 ks by LAXPC and for ~22 ks by SXT, between 8th November, 2019, 02:19:18.72 UTC and 9th November, 2019, 03:08:35.82 UTC at RA(J2000): 99.098 and Dec(J2000): −42.867.

Spectral analysis in the 0.7 — 20 keV range was carried out by fitting its spectrum with the model combination: tbabs*simpl*diskbb. This resulted in an inner disk temperature of 0.64 +/− 0.04 keV, a photon index of 1.98 +/− 0.05 and a scattering fraction of 0.06 +0.04/−0.03 with an absorption column of NH = 2.57 +/− 0.7 × 1020 cm2 (90% confidence errors).

The photon index found is harder than that observed by NuSTAR (ATEL #13270) and Swift (#13257). This could be because the source is undergoing a state transition. However, this result may need to be verified with a more careful estimation of LAXPC background.

The total source flux and disk flux, in the 0.7 — 20.0 keV range, were calculated to be 6.82 +/− 0.05 × 10−9 erg/cm2/sec and 5.91 +/− 0.04 × 10−9 erg/cm2/sec respectively.

We examined the 0.1 sec binned lightcurve of the source in four energy bands from 3 — 30 keV, to study the rms variability. There appears to be a monotonic increase in the rms with energy as follows:

 
Energy band		R.M.S           Error 
3.0 — 6.0 keV		0.06836         +/- 0.27e-02 
6.0 — 12.0 keV		0.31297         +/- 0.60e-02 
12.0 — 20.0 keV		0.34616         +/- 0.19e-01 
20.0 — 30.0 keV		0.50805         +/- 0.32e-01 

Our preliminary analysis shows that the source appears to be undergoing a spectral state transition.

Follow up multiwavelength observations of MAXI J0637−430 is strongly encouraged.

Acknowledgment: We thank the AstroSat operations team for scheduling this ToO observation and the LAXPC/SXT POC for the provision of the data.