Chandra Localization of the X-ray Transient MAXI J1957+032

ATel #9591; Deepto Chakrabarty (MIT), Peter G. Jonker (SRON), Craig B Markwardt (NASA/GSFC)
on 3 Oct 2016; 23:48 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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As part of an ongoing Chandra X-ray Observatory program for precise localization of X-ray transients in low-mass X-ray binaries, we obtained two short Chandra observations of the X-ray transient MAXI J1957+032 during its previous outburst in 2015 October (ATEL #8143, #8146) and its current outburst in 2016 September/October (ATELs #9565, #9572).

Our first observation was made nearly a year ago with HRC-S on 2015 October 10, 01:22 TT with an exposure time of 1155 s, four days after the source outburst was detected by MAXI (ATEL #8143). We did not detect the source, setting a 95-percent-confidence upper limit of 4.5 x 10^-3 count/s on the count rate. Assuming the absorbed power-law X-ray spectrum with photon index Gamma=2.8 and N_H = 4.4 x 10²¹ cm^-2 measured by Swift on 2015 October 9 (ATEL #8146), this corresponds to an upper limit of 1.8 x 10^-13 erg/cm²/s for the 0.5-10 keV unabsorbed flux.

Our second observation was made this week with ACIS-S on 2016 October 3, 00:26 TT with an exposure time of 970 s, also four days after the source outburst was detected by MAXI (ATEL #9565). We used a (1/8)-subarray of the S3 chip with 0.4 s readout time in order to minimize photon pileup. The source was detected with a measured intensity of 0.54(2) count/s or 0.22(1) count/frame, uncorrected for pileup. We estimate a pileup fraction of less than 10 percent. The data are well fit by an absorbed power-law spectrum with Gamma=2.45(22) and N_H=2.5(7) x 10²¹ cm^-2, corresponding to an unabsorbed 0.5-10 keV flux of 7.3 x 10^-12 erg/cm²/s. This is a significantly softer spectrum than observed a few days ago on 2016 September 29 by Swift (Gamma=1.8; ATEL #9572), but it is not as soft as the Swift spectra measured last year on 2015 May 13 (Gamma=2.9; ATEL #7506) and 2015 October 9 (Gamma=2.8; ATEL #8146).

The best-fit X-ray source position from our 2016 Chandra observation is:

RA(J2000) = 19h 56m 39.108s
Dec(J2000) = +03d 26m 43.66s

with an uncertainty radius of 0.6 arcsec (90-percent confidence). This lies 4.9 arcsec from the original 3.5 arcsec Swift X-ray position (ATEL #7506) and 1.4 arcsec from the improved 2.2 arcsec Swift X-ray position (ATEL #8146). Our position is coincident with the optical counterpart suggested by Rau et al. (ATELs #7524,#8149,#8197), thus confirming that association.

It remains unclear whether the compact object in this system is a neutron star or a black hole.