Examining very-long-term variability in X-ray binaries: are ultraluminous X-ray sources intermediate mass black holes?

Abstract

The very-long-term (weeks to years) timing properties of high mass X-ray bina- ries (HMXBs) featuring black holes, particularly the so-called ultraluminous X-ray sources (ULXs), could be linked to the coupled precession of their accretion disks and relativistic jets. These modulations arise in environments that feature super- Eddington mass transfer onto their central compact objects, inviting comparisons with the archetype microquasar SS433. Measuring superorbital periods is difficult, especially for extragalactic sources, and this work highlights the need for long-term monitoring campaigns with good sampling plans. ULXs could host the long-sought intermediate-mass black holes. Alternately, they might be stellar-mass black holes that are either undergoing hyperaccretion, exhibiting beamed emission along our line of sight, or possibly a combination of these two scenarios, in which case ULXs would be a natural extension of the HMXB population. Utilizing a multiwaveband approach, we probe the long-term timing properties of HMXBs to learn if these modulations are a universal feature of ULXs.

We examine the very-long-term variability of X-ray binaries (XRBs) in the nearby galaxy M87. Constraining the variability of XRBs depends on obtaining meaningful lightcurves, which can only be done for the most luminous sources and with limited time resolution. Understanding the longer-term variability of extragalactic XRBs requires multi-epoch Chandra observations, which remain rare. We used archival Chandra data to search the long-term (∼10 year) light curves of 8 of the brightest XRBs in M87 for periodic modulations ranging from 30 days to 5 years. Our analysis shows that some of these sources exceed the ULX threshold luminosity 10^39 erg/s, but the relatively poor sampling makes it difficult to detect any very-long-term periods that might be present.

Finally, a spectroscopic analysis of a recently discovered HMXB found in the Large Magellanic Cloud and within a supernova remnant is presented. Its nebular environment invites comparisons to both SS433 and the ULX NGC 5408 X-1. The best explanation for the observed X-ray and optical properties is a HMXB containing a black hole undergoing wind accretion from its massive early-type donor.