Spectral evolution of the ultraluminous X-ray sources M82 X-1 and X-2

Abstract

M82 hosts two well-known ultraluminous X-ray sources (ULXs). X-1, an intermediate-mass black hole (IMBH) candidate, and X-2, an ultraluminous X-ray pulsar (ULXP). Here we present a broadband X-ray spectral analysis of both sources based on ten observations made simultaneously with Chandra and NuSTAR. Chandra provides the high spatial resolution to resolve the crowded field in the 0.5--8 keV band, and NuSTAR provides the sensitive hard X-ray spectral data, extending the bandpass of our study above 10 keV. The observations, taken in the period 2015--2016, cover a period of flaring from X-1, allowing us to study the spectral evolution of this source with luminosity. During four of these observations, X-2 was found to be at a low flux level, allowing an unambiguous view of the emission from X-1. We find that the broadband X-ray emission from X-1 is consistent with that seen in other ULXs observed in detail with NuSTAR, with a spectrum that includes a broadened disk-like component and a high-energy tail. We find that the luminosity of the disk scales with inner disk temperature as L~T^-3/2 contrary to expectations of a standard accretion disk and previous results. These findings rule out a thermal state for sub-Eddington accretion and therefore do not support M82 X-1 as an IMBH candidate. We also find evidence that the neutral column density of the material in the line of sight increases with L${}_X$, perhaps due to an increased mass outflow with accretion rate. For X-2, we do not find any significant spectral evolution, but we find the spectral parameters of the phase-averaged broadband emission are consistent with the pulsed emission at the highest X-ray luminosities.

In press at ApJ

References in corpus (14)

• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• A 400 solar mass black hole in the Ultraluminous X-ray source M82 X-1 accreting close to its Eddington limit
• Extensive HST Ultraviolet Spectra and Multi-wavelength Observations of SN 2014J in M82 Indicate Reddening and Circumstellar Scattering by Typical Dust
• A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
• An Iron K Component to the Ultrafast Outflow in NGC 1313 X-1
• Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
• A new, clean catalogue of extragalactic non-nuclear X-ray sources in nearby galaxies
• Identification of the X-Ray Thermal Dominant State in an Ultraluminous X-Ray Source in M82
• NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1
• A ~60-day super-orbital period originating from the ultraluminous X-ray pulsar in M82
• A broadband X-ray spectral study of the intermediate-mass black hole candidate M82 X-1 with NuSTAR, Chandra and Swift
• The hyperluminous X-ray source candidate in IC 4320: another HLX bites the dust

Cited by in corpus (10)

• Ultraluminous X-ray sources
• Average bolometric corrections and optical to X-ray flux measurements as a function of accretion rate for X-ray binaries
• Evolution of the spin, spectrum and super-orbital period of the ultraluminous X-ray pulsar M51 ULX7
• Study of accretion flows around an ultraluminous X-ray source M82 X-1 using NuSTAR data
• Digging a little deeper: characterising three new extreme ULX candidates
• The hunt for new pulsating ultraluminous X-ray sources: a clustering approach
• A Fast, Hot Wind from a Nuclear Starburst
• A NuSTAR study of quasi-periodic oscillations from the ultraluminous X-ray sources in M82
• Probing extreme black-hole outflows on short timescales via high spectral-resolution X-ray imagers
• A Multi-Mission Catalogue of Ultraluminous X-ray Source Candidates