Pike, Sean N.Sean N.PikeNegoro, HitoshiHitoshiNegoroTomsick, John A.John A.TomsickBACHETTI, MatteoMatteoBACHETTIBrumback, McKinleyMcKinleyBrumbackConnors, Riley M.T.Riley M.T.ConnorsGarcía, Javier A.Javier A.GarcíaGrefenstette, BrianBrianGrefenstetteHare, JeremyJeremyHareHarrison, Fiona A.Fiona A.HarrisonJaodand, AmrutaAmrutaJaodandLudlam, R. M.R. M.LudlamMASTROSERIO, GuglielmoGuglielmoMASTROSERIOMihara, TatehiroTatehiroMiharaShidatsu, MegumiMegumiShidatsuSugizaki, MutsumiMutsumiSugizakiTakagi, RyoheiRyoheiTakagi2022-05-302022-05-3020220004-637Xhttp://hdl.handle.net/20.500.12386/32115We present the results of Monitor of All-sky X-ray Image (MAXI) monitoring and two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the recently discovered faint X-ray transient MAXI J1848015. Analysis of the MAXI light curve shows that the source underwent a rapid flux increase beginning on 2020 December 20, followed by a rapid decrease in flux after only ∼5 days. NuSTAR observations reveal that the source transitioned from a bright soft state with unabsorbed, bolometric (0.1-100 keV) flux F = 6.9 ± 0.1 × 10-10 erg cm-2 s-1, to a low hard state with flux F = 2.85 ± 0.04 × 10-10 erg cm-2 s-1. Given a distance of 3.3 kpc, inferred via association of the source with the GLIMPSE-C01 cluster, these fluxes correspond to an Eddington fraction of the order of 10-3 for an accreting neutron star (NS) of mass M = 1.4M ⊙, or even lower for a more massive accretor. However, the source spectra exhibit strong relativistic reflection features, indicating the presence of an accretion disk that extends close to the accretor, for which we measure a high spin, a = 0.967 ± 0.013. In addition to a change in flux and spectral shape, we find evidence for other changes between the soft and hard states, including moderate disk truncation with the inner disk radius increasing from R in ≈ 3 R g to R in ≈ 8 R g, narrow Fe emission whose centroid decreases from 6.8 ± 0.1 keV to 6.3 ± 0.1 keV, and an increase in low-frequency (10-3-10-1 Hz) variability. Due to the high spin, we conclude that the source is likely to be a black hole rather than an NS, and we discuss physical interpretations of the low apparent luminosity as well as the narrow Fe emission.STAMPAenArticle10.3847/1538-4357/ac52582-s2.0-85126672845https://api.elsevier.com/content/abstract/scopus_id/85126672845https://iopscience.iop.org/article/10.3847/1538-4357/ac5258FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos