Search for the optical counterpart of the accreting millisecond pulsar SRGA J144459.2-604207

ATel #16489; Payaswini Saikia, D. M. Russell (NYUAD), M. C. Baglio (INAF-OAB), Kevin Alabarta, S. Rout (NYUAD), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 28 Feb 2024; 07:19 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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SRGA J144459.2-604207 is a newly discovered accreting millisecond X-ray pulsar first detected during an outburst by the SRG / Mikhail Pavlinsky ART-XC telescope on 2024 February 21 (MJD 60361; ATel #16464), and later confirmed in X-ray by MAXI (ATel #16469), Swift/XRT (ATel #16471), NICER (ATel #16474) and INTEGRAL (ATel #16485). There are claims for its possible infrared counterpart with 2MASS (ATel #16470) and the Robotic Eye Mount (REM) telescope (ATel #16487), but no radio counterpart has been detected till now (ATel #16475). The search for an optical counterpart with the 0.25m wide-field optical telescope of Bolivia (ATel #16476) and ATLAS (ATel #16477) has not resulted in any detection. Upper limits of g'>18.30, r'>18.44, i'>18.56 (within the Swift/XRT error region) have been proposed with REM (ATel #16487).

Here, we report daily optical observations of the field of SRGA J144459.2-604207 with the Las Cumbres Observatory (LCO) network. Images were taken in the i'-band with the 1-m nodes at the South African Astronomical Observatory, Sutherland and the Cerro Tololo Interamerican Observatory, Chile on 2024 February 21, 22, 23, 24 and 25. We found five optical sources within two times the size of the Swift/XRT position error circle (radius of 3.5 arcsec, ATel #16471), all of which are cataloged in GAIA. Of these five, the stars A and D were previously suspected as the two possible infrared counterparts with REM (stars 1 and 2; ATel #16487). We performed aperture photometry on the field of SRGA J144459.2-604207 for five days continuously to check for any variability in these stars. The GAIA coordinates, historical GAIA magnitudes, the average LCO magnitudes as well as the standard deviations for all the five sources are tabulated below :

Star A -> Position : 221.2490926684,-60.69732796289; Gaia G mag : 20.6718+/-0.0077; LCO i' mag : 19.645; LCO std dev : 0.155

Star B -> Position : 221.2475944469,-60.69756458780; Gaia G mag : 20.4393+/-0.0067; LCO i' mag : 19.445; LCO std dev : 0.091

Star C -> Position : 221.2464295374,-60.69833785302; Gaia G mag : 20.2987+/-0.0063; LCO i' mag : 19.356; LCO std dev : 0.081

Star D -> Position : 221.2480304745,-60.69923318779; Gaia G mag : 19.5895+/-0.0038; LCO i' mag : 19.136; LCO std dev : 0.107

Star E -> Position : 221.2523778903,-60.69805034916; Gaia G mag : 18.4378+/-0.0016; LCO i' mag : 17.706; LCO std dev : 0.118

We find a marginal increase in the brightness of the sources in the LCO i'-band with respect to the magnitudes tabulated in the GAIA catalog for G-band. The G - i' color is fairly constant for all the five sources (values ranging from 0.43 to 0.98), suggesting that none of them has gotten magnitudes brighter. Therefore, it is unlikely that any of these five sources is the optical counterpart of SRGA J144459.2-604207.

We will continue to observe the field at optical wavelengths, to search for a variable source as the outburst evolves and decays, and encourage multi-wavelength observations of this new accreting millisecond X-ray pulsar.

Optical finding chart and light-curve