WATCHDOG: A Comprehensive All-Sky Database of Galactic Black Hole X-ray Binaries

Abstract

With the advent of more sensitive all-sky instruments, the transient Universe is being probed in greater depth than ever before. Taking advantage of available resources, we have established a comprehensive database of black hole (and black hole candidate) X-ray binary (BHXB) activity between 1996 and 2015 as revealed by all-sky instruments, scanning surveys, and select narrow-field X-ray instruments aboard the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), Monitor of All-Sky X-ray Image (MAXI), Rossi X-ray Timing Explorer (RXTE), and Swift telescopes; the Whole-sky Alberta Time-resolved Comprehensive black-Hole Database Of the Galaxy or WATCHDOG. Over the past two decades, we have detected 132 transient outbursts, tracked and classified behavior occurring in 47 transient and 10 persistently accreting BHs, and performed a statistical study on a number of outburst properties across the Galactic population. We find that outbursts undergone by BHXBs that do not reach the thermally dominant accretion state make up a substantial fraction (∼ 40%) of the Galactic transient BHXB outburst sample over the past ∼ 20 years. Our findings suggest that this "hard-only" behavior, observed in transient and persistently accreting BHXBs, is neither a rare nor recent phenomenon and may be indicative of an underlying physical process, relatively common among binary BHs, involving the mass-transfer rate onto the BH remaining at a low level rather than increasing as the outburst evolves. We discuss how the larger number of these "hard-only" outbursts and detected outbursts in general have significant implications for both the luminosity function and mass-transfer history of the Galactic BHXB population.

A ‘high-hard’ outburst of the black hole X-ray binary GS 1354−64

“…Many LMXBs show hard, or "failed", outbursts, where the source spectral state does not change to the soft state (Tetarenko et al 2015). These sources differ in orbital periods to each other, and thus sizes of the accretion discs, which leads to different amounts of accumulated matter in the disc.…”

“…Meyer-Hofmeister ( 2004) consider LX /L Edd < 0.05 as a requirement for the evaporation/condensation scheme to hold. Observationally, this limit can be as high as LX /L Edd < 0.11 (Tetarenko et al 2015). For larger luminosities the disc should approach the innermost stable orbit.…”

“…Belloni et al 2006). Tetarenko et al (2015) found that the limiting Eddington ratio for hard-only outburst is L Edd < 0.11. However, the peak luminosity of the 2015 outburst is several times brighter without the source transiting to the soft state.…”

On the primordial binary black hole mergings in LIGO-Virgo-Kagra data

“…The very first detection of the coalescing binary BH GW1250914 with unusually high masses of the components, ∼ 30M , much exceeding the dynamically estimated masses of BHs in the Galactic BH X-ray binaries [16], revived the interest to the primordial BHs (PBHs) that can be formed in the early Universe at the radiation-dominated stage [17][18][19]. Indeed, in the standard PBH formation mechanism from primordial cosmological perturbations [20,21], the mass of a PBH should be of the order of mass inside the cosmological horizon, M hor ≈ (2.2×10 5 M )(t/[s]).…”