Young Black Hole and Neutron Star Systems in the Nearby Star-forming Galaxy M33: The NuSTAR View

Abstract

We can learn about the formation and evolution of compact objects, such as neutron stars and black holes (BHs), by studying the X-ray emission from accreting systems in nearby star-forming galaxies. The hard (E > 10 keV) X-ray emission in particular allows strong discrimination among the accretion states and compact object types. We conducted a NuSTAR survey (∼600 ks) of the Local Group spiral galaxy M33 to study the distribution of X-ray binary (XRB) accretors in an actively star-forming environment. We constructed color–intensity and color–color diagrams to infer XRB accretion states. Using these diagrams, we have classified 28 X-ray sources in M33 by comparing their hard X-ray colors to those of known systems. Four sources lie in the parameter space occupied by X-ray pulsars, while 8, 10, and 4 sources lie in the parameter space occupied by BHs in the hard, intermediate, and soft states, respectively. The known ultraluminous X-ray source M33 X-8 is also found to be consistent with that source type. Some sources overlap within the Z/Atoll sources due to the overlap of the two categories of BHs and Z/Atoll sources. In contrast to a similar NuSTAR survey of M31 (with a low-mass XRB-dominant population), the source population in M33 is dominated by high-mass XRBs (HMXBs), allowing the study of a very different population with similar sensitivity due to the galaxy's similar distance. This characterization of a population of HMXB accretion states will provide valuable constraints for theoretical XRB population synthesis studies to their formation and evolution.

Multiwavelength Characterization of the High-mass X-Ray Binary Population of M33

“…The HST survey we use to identify HMXB companion star candidates is outlined by the solid black line (PHATTER; Williams et al 2021). We also outline the overlapping NuSTAR survey of M33 (Yang et al 2022).…”

“…We list the source ID and classification from the (Yang et al 2022, Y22) NuSTAR survey of M33. Yang et al (2022) classified NuSTAR-detected sources based on their hard X-ray colors and luminosities. The abbreviations are as follows: ULX = ultraluminous X-ray source, S = soft accretion state black hole, I = intermediate accretion state black hole, H = hard accretion state black hole, P = pulsar, and Z = Z-type neutron star.…”

“…We cross-match our sample with previously published X-ray catalogs of M33 (Grimm et al 2005;Williams et al 2015;Yang et al 2022) and previous catalogs of HMXB candidates in M33 (Garofali et al 2018).…”

“…We identify counterparts to our HMXB candidates in the Yang et al (2022) NuSTAR survey of M33. NuSTAR-detected sources in that paper are listed with their counterpart IDs in the ChASeM33 survey (Tüllmann et al 2011), which we use to identify counterparts between the two catalogs.…”

“…M33 has been observed extensively at X-ray wavelengths with the Einstein Observatory (Long et al 1981;Markert & Rallis 1983;Trinchieri et al 1988), ROSAT (Schulman & Bregman 1995;Long et al 1996;Haberl & Pietsch 2001), XMM-Newton (Pietsch et al 2004;Misanovic et al 2006;Williams et al 2015), Chandra (Grimm et al 2005;Tüllmann et al 2011), and NuSTAR (Yang et al 2022). The Chandra ACIS Survey of M33 (ChASeM33) observed 70% of the area within the D 25 isophote of M33, providing a deep catalog of X-ray point sources with high angular resolution, which is critical for identifying the optical counterparts to accreting compact objects in HMXBs.…”