Recent advancements in the understanding of jet-disc coupling in black hole candidate X-ray binaries (BHXBs) have provided close links between radio jet emission and X-ray spectral and variability behaviour. In soft X-ray states the jets are suppress
ed, but the current picture lacks an understanding of the X-ray features associated with the quenching or recovering of these jets. Here we show that a brief, ~4 day infrared (IR) brightening during a predominantly soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray Type B quasi-periodic oscillation (QPO), a slight spectral hardening and an increase in the rms variability, indicating an excursion to the soft-intermediate state (SIMS). This IR flare has a spectral index consistent with optically thin synchrotron emission and most likely originates from the steady, compact jet. This core jet emitting in the IR is usually only associated with the hard state, and its appearance during the SIMS places the jet line between the SIMS and the soft state in the hardness-intensity diagram for this source. IR emission is produced in a small region of the jets close to where they are launched (~ 0.1 light-seconds), and the timescale of the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete ejection. We also present a summary of the evolution of the jet and X-ray spectral/variability properties throughout the whole outburst, constraining the jet contribution to the X-ray flux during the decay.
We report striking changes in the broadband spectrum of the compact jet of the black hole transient MAXI J1836-194 over state transitions during its discovery outburst in 2011. A fading of the optical-infrared (IR) flux occurred as the source entered
the hard-intermediate state, followed by a brightening as it returned to the hard state. The optical-IR spectrum was consistent with a power law from optically thin synchrotron emission, except when the X-ray spectrum was softest. By fitting the radio to optical spectra with a broken power law, we constrain the frequency and flux of the optically thick/thin break in the jet synchrotron spectrum. The break gradually shifted to higher frequencies as the source hardened at X-ray energies, from ~ 10^11 to ~ 4 x 10^13 Hz. The radiative jet luminosity integrated over the spectrum appeared to be greatest when the source entered the hard state during the outburst decay (although this is dependent on the high energy cooling break, which is not seen directly), even though the radio flux was fading at the time. The physical process responsible for suppressing and reactivating the jet (neither of which are instantaneous but occur on timescales of weeks) is uncertain, but could arise from the varying inner accretion disk radius regulating the fraction of accreting matter that is channeled into the jet. This provides an unprecedented insight into the connection between inflow and outflow, and has implications for the conditions required for jets to be produced, and hence their launching process.
We revisit the paradigm of the dependency of jet power on black hole spin in accreting black hole systems. In a previous paper we showed that the luminosity of compact jets continuously launched due to accretion onto black holes in X-ray binaries (an
alogous to those that dominate the kinetic feedback from AGN) do not appear to correlate with reported black hole spin measurements. It is therefore unclear whether extraction of the black hole spin energy is the main driver powering compact jets from accreting black holes. Occasionally, black hole X-ray binaries produce discrete, transient (ballistic) jets for a brief time over accretion state changes. Here, we quantify the dependence of the power of these transient jets (adopting two methods to infer the jet power) on black hole spin, making use of all the available data in the current literature, which includes 12 BHs with both measured spin parameters and radio flares over the state transition. In several sources, regular, well-sampled radio monitoring has shown that the peak radio flux differs dramatically depending on the outburst (up to a factor of 1000) whereas the total power required to energise the flare may only differ by a factor ~< 4 between outbursts. The peak flux is determined by the total energy in the flare and the time over which it is radiated (which can vary considerably between outbursts). Using a Bayesian fitting routine we rule out a statistically significant positive correlation between transient jet power measured using these methods, and current estimates of black hole spin. Even when selecting subsamples of the data that disregard some methods of black hole spin measurement or jet power measurement, no correlation is found in all cases.
[abridged] The black hole X-ray binary XTE J1550-564 was monitored extensively at X-ray, optical and infrared wavelengths throughout its outburst in 2000. We show that it is possible to separate the optical/near-infrared (OIR) jet emission from the O
IR disc emission. Focussing on the jet component, we find that as the source fades in the X-ray hard state, the OIR jet emission has a spectral index consistent with optically thin synchrotron emission (alpha ~ -0.6 to -0.7, where F_nu propto nu^alpha). This jet emission is tightly and linearly correlated with the X-ray flux; L_OIR,jet propto L_X^(0.98 +- 0.08) suggesting a common origin. This is supported by the OIR, X-ray and OIR to X-ray spectral indices being consistent with a single power law (alpha = -0.73). Ostensibly the compact, synchrotron jet could therefore account for ~ 100 % of the X-ray flux at low luminosities in the hard state. At the same time, (i) an excess is seen over the power law decay of the X-ray flux at the point in which the jet would start to dominate, (ii) the X-ray spectrum slightly softens, which seems to be due to a high energy cut-off or break shifting to a lower energy, and (iii) the X-ray rms variability increases. This may be the strongest evidence to date of synchrotron emission from the compact, steady jet dominating the X-ray flux of an X-ray binary. For XTE J1550-564, this is likely to occur within the luminosity range ~ (2 e-4 - 2 e-3) L_Edd on the hard state decline of this outburst. However, on the hard state rise of the outburst and initially on the hard state decline, the synchrotron jet can only provide a small fraction (~ a few per cent) of the X-ray flux. Both thermal Comptonization and the synchrotron jet can therefore produce the hard X-ray power law in accreting black holes.
A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black
holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.
[abridged] Three years of optical monitoring of the low-mass X-ray binary (LMXB) 4U 1957+11 is presented. The source was observed in V, R and i-bands using the Faulkes Telescopes North and South. The light curve is dominated by long-term variations w
hich are correlated (at the > 3 sigma level) with the RXTE ASM soft X-ray flux. The variations span one magnitude in all three filters. We find no evidence for periodicities in our light curves, contrary to a previous short-timescale optical study in which the flux varied on a 9.3-hour sinusoidal period by a smaller amplitude. The optical spectral energy distribution is blue and typical of LMXBs in outburst, as is the power law index of the correlation beta = 0.5, where F_{nu,OPT} propto F_X^beta. The discrete cross-correlation function reveals a peak at an X-ray lag of 2 - 14 days, which could be the viscous timescale. However, adopting the least squares method the strongest correlation is at a lag of 0 +/- 4 days, consistent with X-ray reprocessing on the surface of the disc. We therefore constrain the optical lag behind X-ray to be between -14 and +4 days. In addition, we use the optical - X-ray luminosity diagram for LMXBs as a diagnostic tool to constrain the nature of the compact object in 4U 1957+11, since black hole (BH) and neutron star (NS) sources reside in different regions of this diagram. If the system contains a BH (as is the currently favoured hypothesis), its distance must exceed ~ 20 kpc for the optical and X-ray luminosities to be consistent with other soft state BH systems. For distances < 20 kpc, the data lie in a region of the diagram populated only by NS sources. 4U 1957+11 is unique: it is either the only BH LMXB to exist in an apparent persistent soft state, or it is a NS LMXB which behaves like a black hole.
Near-infrared (NIR) and optical polarimetric observations of a selection of X-ray binaries are presented. The targets were observed using the Very Large Telescope and the United Kingdom Infrared Telescope. We detect a significant level (3 sigma) of l
inear polarisation in four sources. The polarisation is found to be intrinsic (at the > 3 sigma level) in two sources; GRO J1655-40 (~ 4-7% in H and Ks-bands during an outburst) and Sco X-1 (~ 0.1-0.9% in H and K), which is stronger at lower frequencies. This is likely to be the signature of optically thin synchrotron emission from the collimated jets in these systems, whose presence indicates a partially-ordered magnetic field is present at the inner regions of the jets. In Sco X-1 the intrinsic polarisation is variable (and sometimes absent) in the H and K-bands. In the J-band (i.e. at higher frequencies) the polarisation is not significantly variable and is consistent with an interstellar origin. The optical light from GX 339-4 is also polarised, but at a level and position angle consistent with scattering by interstellar dust. The other polarised source is SS 433, which has a low level (0.5-0.8%) of J-band polarisation, likely due to local scattering. The NIR counterparts of GRO J0422+32, XTE J1118+480, 4U 0614+09 and Aql X-1 (which were all in or near quiescence) have a linear polarisation level of < 16% (3 sigma upper limit, some are < 6%). We discuss how such observations may be used to constrain the ordering of the magnetic field close to the base of the jet in such systems.
Optical/near-infrared (optical/NIR; OIR) light from low-mass neutron star X-ray binaries (NSXBs) in outburst is traditionally thought to be thermal emission from the accretion disc. Here we present a comprehensive collection of quasi-simultaneous OIR
and X-ray data from 19 low-magnetic field NSXBs, including new observations of three sources: 4U 0614+09, LMC X-2 and GX 349+2. The average radio-OIR spectrum for NSXBs is alpha ~ +0.2 (where L_nu propto nu^alpha) at least at high luminosities when the radio jet is detected. This is comparable to, but slightly more inverted than the alpha ~ 0.0 found for black hole X-ray binaries. The OIR spectra and relations between OIR and X-ray fluxes are compared to those expected if the OIR emission is dominated by thermal emission from an X-ray or viscously heated disc, or synchrotron emission from the inner regions of the jets. We find that thermal emission due to X-ray reprocessing can explain all the data except at high luminosities for some NSXBs, namely the atolls and millisecond X-ray pulsars (MSXPs). Optically thin synchrotron emission from the jets (with an observed OIR spectral index of alpha_thin < 0) dominate the NIR light above L_x ~ 10^36 erg/s and the optical above L_x ~ 10^37 erg/s in these systems. For NSXB Z-sources, the OIR observations can be explained by X-ray reprocessing alone, although synchrotron emission may make a low level contribution to the NIR, and could dominate the OIR in one or two cases.
We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the infra
red emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalisation) between infrared and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled infrared and X-ray coverage: L_NIR propto L_x^(0.5-0.7). In the high/soft state this slope is much shallower; L_NIR propto Lx^(0.1-0.2), and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.
We present H-alpha and [O III] (5007 Angstroms) images of the nebula powered by the jet of the black hole candidate and microquasar Cygnus X-1, observed with the 2.5m Isaac Newton Telescope (INT). The ring-like structure is luminous in [O III] and th
ere exists a thin outer shell with a high [O III] / H-alpha flux ratio. This outer shell probably originates in the collisionally excited atoms close to the front of the bow shock. Its presence indicates that the gas is shock excited as opposed to photoionised, supporting the jet-powered scenario. The shock velocity was previously constrained at 20 < v < 360 km/s; here we show that v >= 100 km/s (1 sigma confidence) based on a comparison of the observed [O III] / H-alpha ratio in the bow shock with a number of radiative shock models. From this we further constrain the time-averaged power of the jet: P_Jet = (4 - 14)*10^36 erg/s. The H-alpha flux behind the shock front is typically 4*10^-15 erg/s/cm^2/arcsec^2, and we estimate an upper limit of ~8*10^-15 erg/s/cm^2/arcsec^2 (3 sigma) to the optical (R-band) continuum flux of the nebula. The inferred age of the structure is similar to the time Cyg X-1 has been close to a bright H II region (due to the proper motion of the binary), indicating a dense local medium is required to form the shock wave. In addition, we search a > 1 square degree field of view to the south of Cyg X-1 in H-alpha (provided by the INT Photometric H-alpha Survey of the Northern Galactic Plane; IPHAS) for evidence of the counter jet interacting with the surrounding medium. Two candidate regions are identified, whose possible association with the jet could be confirmed with follow-up observations in [S II] and deeper observations in [O III] and radio.
