Archival X-ray observations of SRGA J144459.2-604207 and a candidate 2MASS counterpart

ATel #16470; Lara Sidoli (INAF-IASF Milano, Italy), Vito Sguera (INAF-OAS Bologna, Italy)
on 22 Feb 2024; 13:32 UT
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Credential Certification: Lara Sidoli (sidoli@lambrate.inaf.it)

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Following the discovery of a new X-ray transient during observations by the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory (SRGA J144459.2-604207; Atel #16464; also observed by MAXI, ATel #16469), we have searched the NASA HEASARC for soft X-ray observations covering its sky position. Only two Swift/XRT observations were found, performed in December 2010 (ObsID 00042436001 and 00042443001, part of the Swift Galactic Survey), with the source located at an off-axis position of about 9 arcmin. Summing the 0.3-10 keV Swift/XRT images (and exposure maps; net exposure time of 1160 s), we have estimated (with SOSTA tool in XIMAGE) a 3 sigma upper limit to the Swift/XRT/PC count rate of 1.23E-2 count/s (0.3-10 keV). Using WebPIMMS and assuming a power law model (with a photon index of 2) with a column density of NH=1.7E22 cm-2 (the total Galactic value towards the source, according to HI4PI Collaboration), we have calculated the following upper limits to the source fluxes (corrected for the absorption): UF<1.6E-12 erg/cm2/s (0.3-10 keV) and UF< 4.9E-13 erg/cm2/s (4-12 keV). This latter flux, compared with the outburst observed by ART-XC (Atel #16464) in the same energy band, implies a dynamic range of about 2000. The closest and brightest 2MASS source (2.76 arcsec away) within 10 arcsec from the X-ray position is 2MASS 14445957-6042066 (J=11.936+/-0.040 mag; H=10.299+/-0.038 mag; K=9.584+/-0.031 mag). It is located at a distance of 1.9 kpc (in-between 1.5 kpc and 2.5 kpc; Gaia EDR3 value, Bailer-Jones et al. 2021). The near-infrared photometry implies a reddening-free factor Q=0.4 (Negueruela & Schurch 2007) which is more consistent with a K or M star, rather than a high mass, early type star. The Gaia EDR3 distance to this star implies an X-ray luminosity in outburst of about 8e35 erg/s (4-12 keV). If this 2MASS source is the true counterpart, a Galactic low mass X-ray binary is favored.