Optical upper limits and near infrared candidate counterparts to the newly discovered accreting millisecond pulsar SRGA J144459.2-604207

ATel #16487; M. C. Baglio (INAF-OAB), D. M. Russell, P. Saikia (NYUAD), S. Motta, I. Mariani (INAF-OAB), K. Alabarta (NYUAD), S. Campana, S. Covino, P. D'Avanzo (INAF-OAB), P. Goldoni (APC/IRF), N. Masetti (INAF-OAS), T. Munoz-Darias (IAC), S. Rout (NYUAD)
on 27 Feb 2024; 11:54 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The newly discovered X-ray binary SRGA J144459.2-604207 was first detected by the SRG / Mikhail Pavlinsky ART-XC telescope on February 21, 2024 (MJD 60361; ATel #16464). The outburst was then confirmed in the X-rays by MAXI (Atel#16469) and Swift/XRT observations, thanks to which a refined position was also proposed (RA, DEC (J2000) = 14:44:59.61, -60:41:52.7; ATel #16471). The detection of Type-I X-ray bursts by NICER (ATel#16474), Swift/XRT (ATel#16475) and INTEGRAL (ATel#16485), and the observation of X-ray pulsations at 447.8 Hz (ATel #16474) allowed the source to be classified as an accreting millisecond X-ray pulsar (AMXP). MeerKAT observations (~1.28 GHz) failed to detect a counterpart down to 170 microJy (ATel#16475). Similarly, no optical counterpart was detected with a 0.25m wide-field optical telescope located at Tarija, Bolivia (ATel #16476) and with ATLAS (ATel#16477). A proposed 2MASS infrared counterpart within the SRG error region lies 14'' from the XRT position (ATel#16470); being the field particularly crowded, this is unlikely to be the actual counterpart to SRGS J144459.2-604207.

We performed observations at optical (g', r', i' bands) and near-infrared (NIR; H-band) frequencies using the Robotic Eye Mount (REM) 60cm telescope in La Silla (Chile) on February 25, 2024 (MJD 60365.2). Two sets of strictly simultaneous 150-second exposure images were taken in the optical filters, while two sets of five 30-second exposure images were obtained in the H-band and averaged five by five to subtract the sky contribution. After averaging the optical images to increase the signal-to-noise ratio, no source is detected within the XRT error region, with the following 3 sigma upper limits: g'>18.30, r'>18.44, i'>18.56.

In the XRT position error circle (3.5 arc-seconds; ATel#16471), two different sources (Star 1 and Star 2) can be observed in the H-band images, both catalogued in 2MASS. Star 1 (RA, DEC (J2000) = 14:44:59.770, -60:41:50.28) and Star 2 (RA, DEC (J2000) = 14:44:59.523, -60:41:57.23) have 2MASS tabulated magnitudes of H=13.972+/-0.035 and H= 13.244+/-0.036, respectively. We note that these magnitudes are significantly brighter with respect to the typical magnitudes of quiescent AMXPs (see, e.g. D'Avanzo et al. 2009).
We performed aperture photometry on the field and we obtained the following magnitudes for the two stars in the two images:

Star 1 -> H =13.65+/-0.19 and H=13.76+/-0.21
Star 2 -> H=12.61+/-0.09 and H=12.61+/-0.12

The brightness of the two sources has slightly increased with respect to the magnitudes tabulated in the 2MASS catalog, by up to 0.32 +/- 0.19 mag (Star 1) and 0.63 +/- 0.10 (Star 2). The increase, however, is still too low to indicate the onset of a new outburst in a transient source. We note however that it is possible that the choice of different aperture sizes to perform photometry of the 2MASS and REM images in this crowded region of the sky is influencing the results.
The actual NIR counterpart of SRGA J144459.2-604207 might be fainter, located behind the two stars, or might be blended with one of the two candidates, contributing to the observed marginal flux increase, but being too faint to be directly detected with REM.

We encourage multi-wavelength observations of this new AMXP.

H-band finding chart