NuSTAR observations and a refined position estimate of MAXI J1848-015

ATel #14290; Sean N. Pike, Fiona A. Harrison, Karl Forster, Brian Grefenstette, Hiromasa Miyasaka, Yanjun Xu (Caltech/SRL), John Tomsick (UC Berkeley/SSL)
on 25 Dec 2020; 07:06 UT
Credential Certification: Sean Pike (spike@caltech.edu)

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We report the results of follow-up observations of the X-ray transient MAXI J1848-015 performed by NuSTAR. The source was first observed at a 4-10 keV flux of 63 mCrab by MAXI/GSC at 05:04 UTC on 2020 December 20 (ATel #14282), and NuSTAR follow-up was performed on December 23 between 02:50 and 07:20 UTC. Due to the proximity of the source to the Sun, pointing was unstable during the NuSTAR observation, producing multiple peaks in the resulting image. Additionally, the source fell on the gap between NuSTAR detectors. Nonetheless, we are able to report a refined source position of: (R.A., Dec) = (282.192, -1.503) = (18 48 46, -01 30 10) ; (J2000) We report a circular 90% C.L. error region with radius 66 arcseconds, and an additional ~30 arcsecond systematic uncertainty is introduced due to unstable pointing. This position estimate does not rule out the possibility that the transient is associated with the previously known X-ray source AX J1848.8-0129, which is offset from our position estimate by 71 arcseconds (Sugizaki et al., 2001). We have also performed a preliminary analysis of the source spectrum. We find that it is described well by an absorbed (nH ~ 6x10^22) multi-temperature disk blackbody (diskbb) plus a power law component, which is typical of accreting black holes in the soft state. Residuals around 6.5 keV also indicate the presence of Fe K-alpha emission, and the fit is significantly improved by the addition of a relativistic reflection component (relxill). We measure a source flux (4-10 keV) of (3.00 +/- 0.02) x 10^-10 erg/cm2/s (1-sigma error; equivalent to about 27 mCrab). Assuming a distance to the source of 8 kpc, this corresponds to a luminosity of 2.3x10^36 erg/s.