Optical counterpart of MAXI J1733-222 / 4U 1730-22

ATel #14693; David M. Russell, Maria Cristina Baglio, Payaswini Saikia, Moaz Abdelmaguid (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 9 Jun 2021; 20:16 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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We report on optical observations of the field of MAXI J1733-222, an X-ray transient discovered by the MAXI satellite on June 7th, 2021 (ATel #14683), confirmed by Swift XRT with a refined X-ray position (ATel #14686, ATel #14688). The transient is likely to be associated with historical X-ray source 4U 1730-22 (ATel #14686), discovered by Uhuru in 1972 (Forman et al. 1978, Cominsky et al. 1978). The most recent XRT observation (ATel #14688) has led to confirmation of the transient to be associated with a Chandra source in its quiescent state, CXOU J173357.5-220156 (Tomsick, Gelino & Kaaret 2007). The Chandra spectrum was well described by a neutron star atmosphere model, so this source is likely to be a neutron star low-mass X-ray binary, and this is the first outburst of the source detected in 50 years.

We observed the location of the transient with the Las Cumbres Observatory (LCO) 1-m robotic telescopes on June 9th, 2021, at 00:45-00:54 UT (MJD 59374.03; LCO node at the South African Astronomical Observatory; SAAO, Sutherland, South Africa) and at 06:30-06:36 UT (MJD 59374.27; LCO node at McDonald Observatory in Texas, USA). Two exposures in the SDSS r'-band filter and one in SDSS i'-band were taken from SAAO, and one each in g', r' and i' from McDonald (100-sec each). The two r'-band images from SAAO were aligned and combined in IRAF. All fluxes have been calibrated against a group of isolated field stars in the Pan-STARRS catalogue (DR2).

We find a single, faint source within the updated XRT error region detected significantly in the combined r'-band image from SAAO, and only marginally significant in i'-band. Its position is also consistent with the location of the Chandra source. The star lies 2" from a brighter star that is in the Pan-STARRS catalogue (r' = 18.95 +- 0.02). Aperture photometry was performed using PHOT in IRAF using a small aperture to minimise contamination from the nearby brighter star. The magnitude of this possible counterpart is r' = 20.01 +- 0.19. There is no Pan-STARRS object at these coordinates; other stars in the region detected by Pan-STARRS are as faint as r' ~ 22.3. Tomsick et al. (2007) also noted there was no optical star detected at the position of the Chandra counterpart, deriving an upper limit of R > 22.1. The detection in the LCO image therefore represents a brightening of this object by more than two magnitudes, so this is very likely the optical counterpart to the X-ray source during this outburst.

By comparing to Pan-STARRS star positions we estimate the coordinates of the source, RA = 17:33:57.57, Dec = -22:01:56.3 (J2000), with estimated error radius of ~0.5". The position error could be reduced with deeper or higher resolution images, or if the source brightens, improving the S/N in future images. Below we provide a finding chart. The seeing conditions at McDonald were less favourable but the source is visible, with estimated magnitudes of g' = 20.34 +- 0.30, r' = 19.48 +- 0.15, i' = 18.90 +- 0.16, although the detection in g'-band is marginal (note there may be some contamination from the nearby brighter star; these magnitudes are less reliable than the SAAO estimate). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical finding chart for MAXI J1733-222