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

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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*10³⁸ 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.