Searching for an optical counterpart of MAXI J1834-021

ATel #15940; Payaswini Saikia, David M. Russell, Kevin Alabarta, Maria Cristina Baglio (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 9 Mar 2023; 18:12 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

Tweet

We report on optical observations of the field of the new X-ray transient MAXI J1834-021, discovered by MAXI/GSC on around 5 February 2023 (MJD 59980, ATel #15929). The source coordinates were constrained by Swift/XRT with an error radius of 3.5'' (ATel #15932). It was not detected at radio frequencies (15.5 GHz), with a 3-sigma upper limit of 200 uJy/beam (ATel #15939). We observed the location of the transient with the Las Cumbres Observatory (LCO) 1-m and (Faulkes) 2-m robotic telescopes at optical wavelengths (in the SDSS g', r' and i' bands with exposure times of 200 sec in each filter) on 7-8 March 2023 (MJD 60010-60011), using the Cerro Tololo Inter-American Observatory in Chile, the McDonald Observatory in Texas, and the Siding Spring Observatory, Australia.

We detect a bright source, and hint of a very faint source (the latter is not significantly detected in most of the filters) in the Swift/XRT error region of our optical images. The coordinates of the two stars, from the PanStarrs catalog, are as follows:

Bright star : RA = 279.036453300, Dec = -02.286732850

Faint star : RA = 279.036122820, Dec = -02.287883630

We use IRAF to perform aperture photometry on both the possible counterparts, and calibrate the magnitudes obtained using several isolated and unsaturated field stars in the PansSTARRS DR1 catalog. The southern faint star is not significantly detected in most of our images. The flux calibrated magnitude of the bright source in the region can be found below for two images taken on the 7th March (MJD 60010) in all the three filters, and another two images taken on the 8th March (MJD 60011) in two filters, respectively :

g' = 20.70 +- 0.31, 21.43 +- 0.17

r' = 18.02 +- 0.05, 18.02 +- 0.04, 17.97 +- 0.05, 18.11 +- 0.22

i' = 15.24 +- 0.02, 15.22 +- 0.03, 15.21 +- 0.03, 15.17 +- 0.07

The spectral energy distribution at optical wavelengths hints at a highly reddened star. The magnitudes are almost comparable (in fact slightly fainter) to the previous (prior to the outburst) optical magnitudes of the source obtained with PanSTARRS (g = 20.420 +- 0.030, r = 17.825 +- 0.108, i = 15.276 +- 0.010). The only other pre-outburst optical observation of the field with LCO, taken with the Cerro Tololo Inter-American Observatory in Chile on 19 May 2021 (MJD 59353), is also consistent with the PanSTARRS magnitudes, with g' = 20.09 +- 0.39.

It could be that either the optical counterpart of MAXI J1834-021 is too faint to be detected with LCO, or it is one of the two candidate counterparts that we have detected. We speculate that if it is the bright star, the transient could be a high mass X-ray binary where the accretion disk does not have any significant contribution over the bright optical companion, and hence the magnitude has not changed after the outburst. In order to confirm the association of this optical source with the X-ray transient, we either need radio detections to have better-constrained coordinates of the system, or optical monitoring of the system to look for variability, or classification from a spectrum.

The monitoring with LCO at optical wavelengths is ongoing. Multiwavelength observations are encouraged to better constrain the location of the system. The Faulkes Telescope / LCO observations are part of an on-going monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory (LCO).

Optical finding chart of MAXI J1834-021