Investigating MAXI J1752–457 with NuSTAR in the Aftermath of a Superburst

Abstract We present two NuSTAR observations of the X-ray transient, MAXI J1752−457, following a superburst that was observed by MAXI/Gas Slit Camera (GSC) in 2024 November. NuSTAR follow-up confirmed that MAXI J1752−457 is coincident with the previously observed Einstein Probe source, EP240809a. We performed a spectral analysis of the source during both NuSTAR observations, and we find that the hard X-ray spectra are consistent with the inclusion of a spherical blackbody component and a steep, nonthermal power-law component. At about 79 hr after the onset of the superburst, we find a blackbody temperature of kT bb = 0.60 ± 0.1 keV and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">bb</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>6</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km (not including corrections for scattering in the neutron star atmosphere), where D 8 is the source distance, which is not yet known, in units of 8 kpc. We found that the blackbody temperature did not change significantly in the 1 day interval between successive NuSTAR observations, and we performed an energy-resolved timing analysis that showed that the source variability was dominated by red noise in the power-law component, suggesting coupling with an accretion disk. We infer that the source had entered an accretion-powered flux state. Furthermore, we measure a photon index of Γ ≈ 4, much steeper than those typically observed during accretion onto neutron stars at similar luminosities. This is suggestive of ongoing evolution of the electron energy distribution responsible for the power-law component several days after the superburst, but the lack of hard X-ray observations prior to and throughout the superburst makes it difficult to present a conclusive physical interpretation of this result.