SRGA J144459.2-604207: ATLAS non-detection of an optical counterpart before and after the first X-ray detection

ATel #16477; F. J. Cowie, J. H. Gillanders, L. Rhodes (Oxford), S. J. Smartt (Oxford/QUB), I. Heywood (Oxford/Rhodes), R. P. Fender (Oxford)
on 23 Feb 2024; 18:11 UT
Credential Certification: Lauren Rhodes (lauren.rhodes@physics.ox.ac.uk)

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ATel #16464 reports the discovery of a neutron star X-ray binary SRGA J144459.2-604207. MAXI/GSC subsequently reported an earlier trigger from the system on February 17 2024 (ATel #16469). The detection of millisecond X-ray pulses from SRGA J144459.2-604207 identify it as a millisecond pulsar low mass X-ray binary (ATel #16474). Previously discovered objects of this type have been found to have optical brightening before the X-ray outburst (e.g., ATel #12964). ATel #16476 reported no optical counterpart post-outburst (February 22 2024) brighter than V-band magnitude of 15.5. Here we report optical observations of SRGA J144459.2-604207 obtained during normal survey operations with ATLAS.

ATLAS is a quadruple 0.5-m telescope system with two units in Hawaii, one in Chile and one in South Africa (see Tonry et al. 2018, PASP, 130:064505), routinely surveying the visible sky on a daily basis. We promptly process all data with our transient science server (Smith et al. 2020, PASP, 132:085002).

The ATLAS system observed the field of SRGA J144459.2-604207 in normal survey mode on February 10, 18 and 20, 2024 (MJD 60349.99, 60358.05, 60360.31). We obtained forced photometry at the coordinates determined by Swift/XRT (ATel #16471) of RA/Dec(J2000) = 221.248375, -60.697972. We find no evidence for significant emission in any of the three epochs within the Swift/XRT 3.5” error region. These epochs range from approximately seven days before the first detection of the X-ray outburst from MAXI/GSC (ATel #16469) to approximately three days after.

Our forced photometry at the coordinates of SRGA J144459.2-604207 results in the following non-detections in the ATLAS o-band (a composite of r and i bands). On MJD 60349.99, we extract a 3-sigma upper limit of 155 uJy (m_o >18.4 AB mag), from a stack of three 30-s frames. On MJD 60358.05, we obtain a 3-sigma upper limit of 76 uJy (m_o >19.2 AB mag) from stacking four 30-s frames, and on MJD 60360.31, we obtain a 3-sigma upper limit of 96 uJy (m_o >18.9 AB mag), from stacking three 30-s frames. None of these measurements are corrected for Galactic dust extinction.