MAXI and NuSTAR Observations of the Faint X-Ray Transient MAXI J1848-015 in the GLIMPSE-C01 Cluster

We present the results of MAXI monitoring and two NuSTAR observations recently discovered faint X-ray transient J1848-015. Analysis light-curve shows that source underwent a rapid flux increase beginning on 2020 December 20, followed by decrease in after only $\sim5$ days. reveal transitioned from bright soft state with unabsorbed, bolometric ($0.1$-$100$ keV) $F=6.9 \pm 0.1 \times 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, to low hard $F=2.85 0.04 10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. Given distance $3.3$ kpc, inferred via association GLIMPSE-C01 cluster, these fluxes correspond an Eddington fraction order $10^{-3}$ for accreting neutron star mass $M=1.4M_\odot$, or even lower more massive accretor. However, spectra exhibit strong relativistic reflection features, indicating presence accretion disk which extends close accretor, we measure high spin, $a=0.967\pm0.013$. In addition change spectral shape, find evidence other changes between states, including moderate truncation inner radius increasing $R_\mathrm{in}\approx3\,R_\mathrm{g}$ $R_\mathrm{in}\approx8\,R_\mathrm{g}$, narrow Fe emission whose centroid decreases $6.8\pm0.1$ keV $6.3 0.1$ keV, low-frequency ($10^{-3}$-$10^{-1}$ Hz) variability. Due spin conclude is likely be black hole rather than star, discuss physical interpretations apparent luminosity as well emission.