Broadband Spectrum of the X-ray Binary M33 X-6 from NuSTAR and Swift-XRT Data: An Extragalactic Z-Source?

TL;DR

This study presents the first broad-spectrum X-ray analysis of M33 X-6, revealing its potential as a Z-source with a thick accretion disk and a significant hard X-ray component, expanding understanding of extragalactic X-ray binaries.

Contribution

First broadband (0.3-20 keV) X-ray spectrum of M33 X-6 obtained, suggesting it is a Z-source with a thick accretion disk and a hard X-ray component.

Findings

X-6's spectrum fits a thick accretion disk model.

Detected a hard X-ray component in the spectrum.

X-6's luminosity is approximately 2×10^{38} erg/s.

Abstract

We present the results of our study of the X-ray spectrum for the source X-6 in the nearby galaxy M33 obtained for the first time at energies above 10 keV from the data of the NuSTAR orbital telescope. The archival Swift-XRT data for energy coverage below 3 keV have been used, which has allowed the spectrum of M33 X-6 to be constructed in the wide energy range 0.3-20 keV. The spectrum of the source is well described by the model of an optically and geometrically thick accretion disk with a maximum temperature of ~2 keV and an inner radius of ~5 cos^{-1/2}(theta) km (where "theta" is the unknown disk inclination angle with respect to the observer). There is also evidence for the presence of an additional hard component in the spectrum. The X-ray luminosity of M33 X-6 measured for the first time in the wide energy range 0.3-20 keV is ~2*10^{38} erg/s , with the luminosity in the hard 10-20 keV X-ray band being ~10% of the source's total luminosity. The results obtained suggest that X-6 may be a Z-source, i.e., an X-ray binary with subcritical accretion onto a weakly magnetized neutron star.