Elena Gallo

A dark jet dominates the power output of the stellar black hole Cygnus X-1

Black holes undergoing accretion are thought to emit the bulk of their power in the X-ray band by releasing the gravitational potential energy of the infalling matter. At the same time, they are capable of producing highly collimated jets of energy and particles flowing out of the system with relativistic velocities. Here we show that the 10-solar-mass (10M[circdot]) black hole in the X-ray binary Cygnus X-1 (refs 3–5) is surrounded by a large-scale (∼5 pc in diameter) ring-like structure that appears to be inflated by the inner radio jet. We estimate that in order to sustain the observed emission of the ring, the jet of Cygnus X-1 has to carry a kinetic power that can be as high as the bolometric X-ray luminosity of the binary system. This result may imply that low-luminosity stellar-mass black holes as a whole dissipate the bulk of the liberated accretion power in the form of ‘dark’, radiatively inefficient relativistic outflows, rather than locally in the X-ray-emitting inflow.

Integrin-modulating therapy prevents fibrosis and autoimmunity in mouse models of scleroderma

In systemic sclerosis (SSc), a common and aetiologically mysterious form of scleroderma (defined as pathological fibrosis of the skin), previously healthy adults acquire fibrosis of the skin and viscera in association with autoantibodies. Familial recurrence is extremely rare and causal genes have not been identified. Although the onset of fibrosis in SSc typically correlates with the production of autoantibodies, whether they contribute to disease pathogenesis or simply serve as a marker of disease remains controversial and the mechanism for their induction is largely unknown. The study of SSc is hindered by a lack of animal models that recapitulate the aetiology of this complex disease. To gain a foothold in the pathogenesis of pathological skin fibrosis, we studied stiff skin syndrome (SSS), a rare but tractable Mendelian disorder leading to childhood onset of diffuse skin fibrosis with autosomal dominant inheritance and complete penetrance. We showed previously that SSS is caused by heterozygous missense mutations in the gene (FBN1) encoding fibrillin-1, the main constituent of extracellular microfibrils. SSS mutations all localize to the only domain in fibrillin-1 that harbours an Arg-Gly-Asp (RGD) motif needed to mediate cell–matrix interactions by binding to cell-surface integrins. Here we show that mouse lines harbouring analogous amino acid substitutions in fibrillin-1 recapitulate aggressive skin fibrosis that is prevented by integrin-modulating therapies and reversed by antagonism of the pro-fibrotic cytokine transforming growth factor β (TGF-β). Mutant mice show skin infiltration of pro-inflammatory immune cells including plasmacytoid dendritic cells, T helper cells and plasma cells, and also autoantibody production; these findings are normalized by integrin-modulating therapies or TGF-β antagonism. These results show that alterations in cell–matrix interactions are sufficient to initiate and sustain inflammatory and pro-fibrotic programmes and highlight new therapeutic strategies.

Assessing luminosity correlations via cluster analysis: evidence for dual tracks in the radio/X-ray domain of black hole X-ray binaries

The radio/X-ray correlation for hard and quiescent state black hole X-ray binaries is critically investigated in this paper. New observations of known sources, along with newly discovered ones (since 2003), have resulted in an increasingly large number of outliers lying well outside the scatter about the quoted best-fitting relation. Most of these outliers tend to cluster below the best-fitting line, possibly indicative of two distinct tracks which might reflect different accretion regimes within the hard state. Here, we employ and compare state of the art data clustering techniques in order to identify and characterize different data groupings within the radio/X-ray luminosity plane for 18 hard and quiescent state black hole X-ray binaries with nearly simultaneous multiwavelength coverage. Linear regression is then carried out on the

Observational constraints on the powering mechanism of transient relativistic jets

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 (analogous 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.

The X-ray spectral evolution of galactic black hole x-ray binaries toward quiescence

Most transient black hole X-ray binaries (BHXBs) spend the bulk of their time in a quiescent state, where they accrete matter from their companion star at highly sub-Eddington luminosities (we define quiescence here as a normalized Eddington ratio lx = L 0.5-10 keV/L Edd < 10–5). Here, we present Chandra X-ray imaging spectroscopy for three BHXB systems (H 1743–322, MAXI J1659–152, and XTE J1752–223) as they fade into quiescence following an outburst. Multiple X-ray observations were taken within one month of each other, allowing us to track each individual systems X-ray spectral evolution during its decay. We compare these three systems to other BHXB systems. We confirm that quiescent BHXBs have softer X-ray spectra than low-hard-state BHXBs, and that quiescent BHXB spectral properties show no dependence on the binary systems orbital parameters. However, the observed anti-correlation between X-ray photon index (Γ) and lx in the low-hard state does not continue once a BHXB enters quiescence. Instead, Γ plateaus to an average Γ = 2.08 ± 0.07 by the time lx reaches ~10–5. lx ~ 10–5 is thus an observationally motivated upper limit for the beginning of the quiescent spectral state. Our results are discussed in the context of different accretion flow models and across the black hole mass scale.

Jet spectral breaks in black hole X-ray binaries

In X-ray binaries, compact jets are known to commonly radiate at radio to infrared frequencies, whereas at optical to γ-ray energies, the contribution of the jet is debated. The total luminosity, and hence power of the jet, is critically dependent on the position of the break in its spectrum, between optically thick (self-absorbed) and optically thin synchrotron emission. This break, or turnover, has been reported in just one black hole X-ray binary (BHXB) thus far, GX 339−4, and inferred via spectral fitting in two others, A0620−00 and Cyg X−1. Here, we collect a wealth of multi-wavelength data from the outbursts of BHXBs during hard X-ray states, in order to search for jet breaks as yet unidentified in their spectral energy distributions. In particular, we report the direct detection of the jet break in the spectrum of V404 Cyg during its 1989 outburst, at νb = (1.8 ± 0.3) × 1014 Hz (1.7 ± 0.2 μm). We increase the number of BHXBs with measured jet breaks from three to eight. Jet breaks are found at frequencies spanning more than two orders of magnitude, from νb = (4.5 ± 0.8) × 1012 Hz for XTE J1118+480 during its 2005 outburst, to νb > 4.7 × 1014 Hz for V4641 Sgr in outburst. A positive correlation between jet break frequency and luminosity is expected theoretically; νb∝L∼ 0.5ν, jet if other parameters are constant. With constraints on the jet break in a total of 12 BHXBs including two quiescent systems, we find a large range of jet break frequencies at similar luminosities and no obvious global relation (but such a relation cannot be ruled out for individual sources). We speculate that different magnetic field strengths and/or different radii of the acceleration zone in the inner regions of the jet are likely to be responsible for the observed scatter between sources. There is evidence that the high-energy cooling break in the jet spectrum shifts from UV energies at LX ∼ 10−8LEdd (implying the jet may dominate the X-ray emission in quiescence) to X-ray energies at ∼10−3LEdd. Finally, we find that the jet break luminosity scales as Lν, jet∝L0.56 ± 0.05X (very similar to the radio-X-ray correlation), and radio-faint BHXBs have fainter jet breaks. In quiescence the jet break luminosity exceeds the X-ray luminosity.

The radio/X-ray domain of black hole X-ray binaries at the lowest radio luminosities

We report on deep, coordinated radio and X-ray observations of the black hole X-ray binary XTE J1118+480 in quiescence. The source was observed with the Karl G. Jansky Very Large Array for a total of 17.5 h at 5.3 GHz, yielding a 4.8 ± 1.4 µJy radio source at a position consistent with the binary system. At a distance of 1.7 kpc, this corresponds to an integrated radio luminosity between 4 and 8 × 10 25 ergs −1 , depending on the spectral index. This is the lowest radio luminosity measured for any accreting black hole to date. Simultaneous observations with the Chandra X-ray Telescope detected XTE J1118+480 at 1.2 × 10 −14 ergs −1 cm −2 (1–10 keV), corresponding to an Eddington ratio of ∼4 × 10 −9

A ?50,000 M? Solar Mass Black Hole in the Nucleus of RGG 118

Scaling relations between black hole (BH) masses and their host galaxy properties have been studied extensively over the last two decades, and point toward co-evolution of central massive BHs and their hosts. However, these relations remain poorly constrained for BH masses below . Here we present optical and X-ray observations of the dwarf galaxy RGG 118 taken with the Magellan Echellette Spectrograph on the 6.5 m Clay Telescope and Chandra X-ray Observatory. Based on Sloan Digital Sky Survey spectroscopy, RGG 118 was identified as possessing narrow emission line ratios indicative of photoionization partly due to an active galactic nucleus. Our higher resolution spectroscopy clearly reveals broad H? emission in the spectrum of RGG 118. Using virial BH mass estimate techniques, we calculate a BH mass of ?50,000 . We detect a nuclear X-ray point source in RGG 118, suggesting a total accretion powered luminosity of , and an Eddington fraction of ?1%. The BH in RGG 118 is the smallest ever reported in a galaxy nucleus and we find that it lies on the extrapolation of the relation to the lowest masses yet.

An Evolving Compact Jet in the Black Hole X-Ray Binary MAXI J1836–194

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 × 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.

X-RAY CONSTRAINTS ON THE LOCAL SUPERMASSIVE BLACK HOLE OCCUPATION FRACTION

Distinct seed formation mechanisms are imprinted upon the fraction of dwarf galaxies currently containing a central supermassive black hole. Seeding by Population III remnants is expected to produce a higher occupation fraction than is generated with direct gas collapse precursors. Chandra observations of nearby early-type galaxies can directly detect even low-level supermassive black hole activity, and the active fraction immediately provides a firm lower limit to the occupation fraction. Here, we use the volume-limited AMUSE surveys of ~200 optically selected early-type galaxies to characterize simultaneously, for the first time, the occupation fraction and the scaling of L X with M star, accounting for intrinsic scatter, measurement uncertainties, and X-ray limits. For early-type galaxies with M star 20% (at 95% confidence), but full occupation cannot be excluded. The preferred dependence of log L X upon log M star has a slope of ~0.7-0.8, consistent with the downsizing trend previously identified from the AMUSE data set, and a uniform Eddington efficiency is disfavored at ~2σ. We provide guidelines for the future precision with which these parameters may be refined with larger or more sensitive samples.