SALT observations of the optical counterpart of MAXI J1957+032

ATel #9649; D. A.H. Buckley, M. M. Kotze (SAAO), P. A. Charles (U. Southampton, IAC), D. Mata Sanchez, T. Munoz-Darias (IAC), G. Israel (INAF/OA-Roma), E. J. Marchesini (UNLP, Argentina) N. Masetti (INAF/IASF-Bologna), P. Jonker (SRON)
on 19 Oct 2016; 11:31 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

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Optical imaging and spectroscopic observations with SALT of the X-ray transient MAXI J1957+032 (=IGR J19566+0326) were obtained on 2016-10-01.764, 2.4 d after the recent X-ray outburst (ATel #9565, #9572). A 60 s r' band image with SALTICAM revealed the previously identified optical counterpart (ATel #8149, #8197) to be at a magnitude of r' = 18.5 +/- 0.15. This is comparable to the brightness reported 4.2 d after the October 2015 outburst, namely R = 18.27 (ATel #8149), and brighter than the r' = 20.03 measured 3.7 d after the 2015 May outburst (ATel #7524).

Immediately following the SALTICAM imaging observation, a 1000 s exposure was taken with the SALT Robert Stobie Spectrograph with the 300 l/mm grating giving a mean resolving power of 320, using a 1.5 arcsec slit and a PSF of 1.3 arcsec FWHM. The spectrum of the optical counterpart, covering 360-770 nm, was essentially featureless, other than for telluric lines. This contrasts to the spectrum obtained with ALFOSC at a similar resolution on the NOT 8.2 d after the 2015 October outburst (ATel #8197), which reported absorption features typical of A-F stars.

We have re-analyzed the ALFOSC 2015 October data, and from a 250 s image taken immediately after the spectrum, we obtained a brightness for the optical counterpart of R = 21.4 +/- 0.2. The SALT and NOT images, with the optical counterpart marked, are available in the link below.

We subsequently discovered that the reported NOT spectrum was not of the optical counterpart, but rather of the brighter neighboring star, immediately to the north-east of the MAXI optical counterpart. This star was measured to be r' = 18.0 and R = 18.3, respectively, from the SALTICAM and ALFOSC images taken a year apart, and is consistent with the fluxes measured in the NOT spectrum. All of the photometry reported here used the same reference star, USNO B1.0 0934-0560330, as in ATel #8149.

The featureless SALT spectrum of MAXI J1957+032 is reminiscent of another high latitude (b = 12.2 deg) X-ray transient, namely Swift J1753.5-0127 (Cadolle Bel et al., 2007, ApJ, 659, 549), although the decline from outburst for MAXI J1957+032 is very much faster, measured in days, compared to the over 40 d decline period for Swift J1753.5-0127.

Based on observations made with the Southern African Large Telescope under proposal 2016-1-MLT-010.

MAXI J1957+032 optical counterpart