Chandra Observations of MAXI J1848-015 prior to its outburst and a possible NIR counterpart

ATel #14499; Jeremy Hare (NASA/GSFC), Hui Yang (GWU), Oleg Kargaltsev (GWU), Blagoy Rangelov (TSU), Sean N. Pike (Caltech/SRL), John Tomsick (UC Berkeley/SSL)
on 31 Mar 2021; 19:55 UT
Credential Certification: Jeremy Hare (harej10@gmail.com)

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MAXI J1848-015 is a transient X-ray source that underwent an outburst on 2020 December 20 (ATEL #14282) and is coincident on the sky with the massive stellar cluster Glimpse-C01 (ATEL #14420, ATEL #14424). We obtained six observations of this cluster with Chandra X-ray Observatory (CXO) in an unrelated observing program carried out between 2019 June 23 and 2020 November 15. The summed scientific exposure of all six observations is about 180 ks. In an attempt to detect the source prior to its outburst, we stacked all six observations after correcting the CXO astrometry to the CXO image reported in Hare et al., 2018 (ApJ, 865, 33), which was matched to 2MASS.

This allowed us to search for the X-ray counterpart at the source's radio position (ATEL #14432). We find five photons in a r=0.5'' circular region centered on the radio source position and corresponding to a ~50% encircled energy fraction. The proximity of MAXI J1848-015 to source X5 (see Pooley et al. 2007 arXiv:0708.3365; Hare et al.) makes it difficult to use a larger extraction region. The background contribution was estimated from two source free regions located within the cluster as there is evidence of faint diffuse emission across the cluster (Hare et al.). We estimate a source detection significance of 3.2 sigma following Weisskopf et al. 2007 (ApJ, 657, 1026). The source count rate, corrected for the finite extraction aperture, is ~5e-05 cts/s. Assuming an absorption of 4e22 cm^-2, a power-law photon index of 1, and a distance of 3.3 kpc (Hare et al.), this count rate corresponds to an unabsorbed luminosity of ~3.3e30 erg/s in the 0.5-10 keV energy band. We note that due to heavy crowding in the cluster, the X-ray emission may be coming from a nearby unrelated source, but in this case the reported luminosity should be treated as an upper-limit on the quiescent luminosity of MAXI J1848-015.

In the HST WFC3/IR/F153M image of Glimpse-C01 there is a relatively bright star (12.7 Vega mag in the F153M filter; R.A.=282.20761875, Decl=-1.49717731; Hare et al.), which is located ~0.17 arcsec away from MAXI J1848-015's radio position but within the 3 sigma uncertainty of the radio position. The potential NIR counterpart lies on the red giant branch of the CMD (see Fig. 9 in Hare et al.), suggesting an evolved donor star, if it is indeed the true counterpart. We caution that the cluster's center is very crowded and the spurious coincidence probability is large (>50%).