2022ApJ...927..190P

We 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 x 10^–10 erg cm^–2 s^–1, to a low hard state with flux F = 2.85 ± 0.04 x 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.