Radio monitoring of the hard state jets in the 2011 outburst of MAXI J1836−194

Abstract

MAXI J1836−194 is a Galactic black hole candidate X-ray binary that was discovered in 2011 when it went into outburst. In this paper, we present the full radio monitoring of this system during its 'failed' outburst, in which the source did not complete a full set of state changes, only transitioning as far as the hard intermediate state. Observations with the Karl G. Jansky Very Large Array (VLA) and Australia Telescope Compact Array (ATCA) show that the jet properties changed significantly during the outburst. The VLA observations detected linearly polarised emission at a level of ∼1% early in the outburst, increasing to ∼3% as the outburst peaked. High-resolution images with the Very Long Baseline Array (VLBA) show a ∼15 mas jet along the position angle −21 ± 2 • , in agreement with the electric vector position angle found from our polarisation results (−21 ± 4 • ), implying that the magnetic field is perpendicular to the jet. Astrometric observations suggest that the system required an asymmetric natal kick to explain its observed space velocity. Comparing quasi-simultaneous X-ray monitoring with the 5 GHz VLA observations from the 2011 outburst shows an unusually steep hard-state radio/X-ray correlation of L R ∝ L 1.8±0.2 X , where L R and L X denote the radio and X-ray luminosities, respectively. With ATCA and Swift monitoring of the source during a period of re-brightening in 2012, we show that the system lay on the same steep correlation. Due to the low inclination of this system, we then investigate the possibility that the observed correlation may have been steepened by variable Doppler boosting.

Polarized synchrotron emission in quiescent black hole X-ray transients

“…Recently, multiple frequency observations carried out with the Karl G. Jansky Very Large Array in 2013 have revealed a highly inverted radio spectrum, with α ∼ +0.7 (where Fν ∝ ν α ) and long-term variability of the radio emission (Dincer et al in preparation; see also MacDonald, Bailyn, & Buxton 2015). This is not unreasonable; a jet spectral index consistent with being this steep has been observed during outbursts in XTE J1118+480 and MAXI J1836-194, and in the latter source the radio jet was spatially resolved (Fender et al 2001;Russell et al 2015). If this steep spectrum is not due to low frequency selfabsorption, and instead extends to higher frequencies, it would have to either curve towards being flat (curvature is seen in some sources, such as V404 Cyg; Russell et al 2013), or the spectral break would have to reside at much lower frequencies than the NIR, in order for the NIR spectrum to be optically thin synchrotron.…”

A Connection Between Plasma Conditions Near Black Hole Event Horizons and Outflow Properties

“…MAXI 1836-194 is suspected to have a slightly higher bulk Lorentz factor (Γ b = 1-4) and a jet angle oriented close to our line-of-sight (4°-15°, Russell et al 2015). It is likely that the Doppler factor is variable depending on the X-ray flux of the source (see Figure 10 in Russell et al 2015). We use Russell et al's estimation of the bulk Lorentz factors depending on the X-ray flux and the jet angle of 10°and correct for the effect of Doppler boosting (δ = 2-5) in Figure 2 (right panel) and Table 4.…”

Radio polarimetry as a probe of unresolved jets: the 2013 outburst of XTE J1908+094

“…When detected, polarisation can be used to infer properties of the magnetic field and the surrounding medium (e.g., Stirling et al 2004;Miller-Jones et al 2008), as well as properties of the jet itself. An alignment between the compact jet axis and the intrinsic polarisation angle has now been observed in a number of sources (e.g., Corbel et al 2000;Russell & Shahbaz 2014;Russell et al 2015) and, if common to all black hole X-ray LMXBs, this allows us to infer the orientation of unresolved compact jets.…”