The Spectral Energy Distribution of Quiescent Black Hole X‐Ray Binaries: New Constraints fromSpitzer

Abstract

Among the various issues that remain open in the field of accretion onto black hole X-ray binaries (BHBs) is the question of how gas accretes at very low Eddington ratios, in the so-called quiescent regime. While there is general agreement that X-rays are produced by a population of high-energy electrons near the BH, there is controversy concerning the modeling of the contributions of inflowing versus outflowing particles and their relative energy budget. Recent Spitzer observations of three quiescent BHBs have shown evidence for excess emission with respect to the Rayleigh-Jeans tail of the companion star between 8Y24 m. We suggest that synchrotron emission from a partially self-absorbed outflow might be responsible for the observed mid-IR excess, in place of, or in addition to, thermal emission from circumbinary material. If so, then the jet synchrotron luminosity, integrated from radio to near-IR frequencies, exceeds the measured 2Y10 keV luminosity by a factor of a few in these systems. In turn, the mechanical power stored in the jet exceeds the bolometric X-ray luminosity by at least 4 orders of magnitude. We compile the broadband spectral energy distribution (SED) of A0620À00, the lowest Eddington-ratio stellar mass BH with a known radio counterpart, by means of simultaneous radio, optical, and X-ray observations, and the archival Spitzer data. We are able to fit the SED of A0620À00 with a maximally jet-dominated model, in which the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base. The fitted parameters land in a range of values reminiscent of the Galactic center supermassive black hole Sgr A Ã . Most notably, the inferred ratio of the jet acceleration rate to local cooling rates is 2 orders of magnitude weaker than higher luminosity, hard-state sources.

Constraints on relativistic jets in quiescent black hole X-ray binaries from broad-band spectral modelling

“…Both fits converge toward relatively small nozzle radii r0 ∼ 2 − 3 rg. This nozzle radius is smaller than the best-fit values for J1118 in the hard state (10 − 20 rg; Maitra et al 2009), and comparable to the best-fit value for A0620-00 in quiescence (3.9 +2.2 −0.1 rg; Gallo et al 2007). When compared to the hard state, both fits also converge toward lower electron temperatures (Maitra et al 2009 found Te ∼ 4 × 10 10 K).…”

“…The same model has previously been applied to J1118 in the hard state during its 2000 and 2005 outbursts (Maitra et al 2009), and also to A0620-00 in quiescence (Gallo et al 2007). We obtain two model fits for J1118 in quiescence, of comparable statisti- Parameters calculated by model are reported at the top of the nozzle (z = h 0 ) and at the particle acceleration zone (zacc).…”

“…Almost all of our observational constraints on very low luminosity BHXBs (LX ∼10 −8.5 L Edd ) are derived from A0620-00, because it is the only one with a well sampled SED from the radio through the X-ray wavebands. 2 Interestingly, broadband modeling of A0620-00 in quiescence indeed supports the idea of quiescent emission properties similar to Sgr A* (Gallo et al 2007). However, we cannot determine from a single source if such a trend applies to all BHXBs.…”

“…We adopt flux densities of 69 (4.5µm) and 59 µJy (8.0 µm), and an upper limit of <50 µJy at 24 µm, as measured by Gallo et al (2007) who analyzed the same data originally presented in Muno & Mauerhan (2006). Gallo et al (2007) estimated that the statistical uncertainties in the flux densities are at the 10% level.…”

GRS1915+105: a comparison of the plateau state to the canonical hard state

“…MNW05 has already been applied successfully to various Galactic sources in the HS: Cyg X-1, GX 339-4, XTE J1118+480, GRO J1655-40 and A0620-00 (MNW05, Maitra et al (2009, Gallo et al 2007). In addition MNW05 has been successfully used to fit spectra from Supermassive Black Holes M81 * , SGR A * and NGC4258 (Markoff et al 2008, Yuan et al 2002.…”