Chandra Localization of MAXI J1848-015

ATel #14424; Deepto Chakrabarty (MIT), Peter G. Jonker (SRON and Radboud Univ.), Jeroen Homan (Eureka Scientific and SRON), Maureen van den Berg (Harvard-Smithsonian CfA)
on 26 Feb 2021; 12:58 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

Tweet

As part of an ongoing Chandra X-ray Observatory program for precise localization of X-ray transients in low-mass X-ray binaries, we obtained a short Chandra/ACIS-S observation of the new X-ray transient MAXI J1848-015, following its discovery by MAXI on 2020 Dec 20 (ATel #14282) and its subsequent arcminute-scale localization by NuSTAR (ATel #14290). X-ray follow-up observations were temporarily delayed by the sky region's proximity to the Sun during January. It is not yet clear whether the source contains a neutron star or a black hole. An improved 2.3 arcsec Swift position was recently reported by Kennea et al. (2021, ATel #14420), localizing the source in the core of the star cluster GLIMPSE-C01.

Our 1008.9 s observation was made on 2021 Feb 21, 17:05 TT (MJD 59266.71), which was 63 days after the source was first detected by MAXI. We used a (1/4)-subarray of the ACIS S3 chip with 0.8 s frame time in order to minimize photon pileup while allowing for the arcminute-scale position uncertainty from NuSTAR. The source lay near the edge of the subarray (but not near the edge of the S3 chip). It was detected with a measured count rate of 0.63(2) count/s or 0.50(2) count/frame, uncorrected for pileup. We estimate a pileup fraction of around 20 percent.

The best-fit X-ray source position is:

RA(J2000) = 282.207546 deg = 18h 48m 49.81s
Dec(J2000) = -1.497037 deg = -01d 29m 49.3s

with an uncertainty radius of 0.8 arcsec (90-percent confidence). This is close to, but just outside, the 2.3 arcsec Swift error circle, lying 2.5 arcsec from its center (ATel #14420). Our position is consistent with both of the larger MAXI and NuSTAR error regions.

We inspected deep Chandra archival observations of the field from 2006 (ACIS-S, 46 ks) and 2019 (ACIS-I, 30 ks), and we confirm the finding by Kennea et al. that the source is located in the core of the star cluster GLIMPSE-C01. It is not coincident with any of the faint X-ray sources detected in the archival observations. The closest of these faint X-ray sources is X5 from Pooley et al. (2007 arXiv:0708.3365). Using the position of X5 from Hare et al. (2018, ApJ, 865, 33) we find a separation of 2.45 arcsec. We consider this separation too large for MAXI J1848-015 to be possibly associated with X5. Our observation was too short to significantly detect any of the archival faint point sources, the brightest of which had an ACIS-S3 count rate of order 0.001 count/s.

The X-ray spectrum of the source was well fit with an absorbed power law model. Accounting for photon pileup, we found column density N_H = (8.4 +/- 1.4)e22 atoms/cm^2, power-law photon index Gamma = 1.8 +/- 0.4, and an unabsorbed 0.5-10 keV X-ray flux of (1.2 +0.9/-0.4)e-10 erg/s/cm^2. Assuming a cluster distance of 3.3 kpc (Hare et al.), this gives a luminosity of 1.e35 erg/s. For comparison, the faintest source detected by Hare et al. had a 0.5-8 keV luminosity of 8e30 erg/s, which thus provides an upper limit on the quiescent luminosity of MAXI J1848-015.