Abstract
We discuss the time-scales for alignment of black hole and accretion disc spins in the context of binary systems. We show that for black holes that are formed with substantial angular ...momentum, the alignment time-scales are likely to be at least a substantial fraction of the lifetimes of the systems. This result explains the observed misalignment of the disc and the jet in the microquasar GRO J 1655-40 and in SAX J 1819-2525 as probably being caused by the Bardeen-Petterson effect. We discuss the implications of these results on the mass estimate for GRS 1915+105, which has assumed the jet to be perpendicular to the orbital plane of the system and may hence be an underestimate. We show that the time-scales for the spin alignment in Cygnus X-3 are consistent with the likely misalignment of disc and jet in that system, and that this is suggested by the observational data.
The closest black holes Fender, R. P; Maccarone, T. J; Heywood, I
Monthly notices of the Royal Astronomical Society,
04/2013, Volume:
430, Issue:
3
Journal Article
Peer reviewed
Open access
Starting from the assumption that there is a large population (≥108) of stellar-mass isolated black holes (IBH) distributed throughout our Galaxy, we consider the detectable signatures of accretion ...from the interstellar medium (ISM) that may be associated with such a population. We simulate the nearby (radius 250 pc) part of this population, corresponding to the closest ∼35 000 black holes, using current best estimates of the mass distribution of stellar-mass black holes combined with two models for the velocity distribution of stellar-mass IBH which bracket likely possibilities. We distribute this population of objects appropriately within the different phases of the ISM and calculate the Bondi-Hoyle accretion rate, modified by a further dimensionless efficiency parameter λ. Assuming a simple prescription for radiatively inefficient accretion at low Eddington ratios, we calculate the X-ray luminosity of these objects, and similarly estimate the radio luminosity from relations found empirically for black holes accreting at low rates. The latter assumption depends crucially on whether or not the IBH accrete from the ISM in a manner which is axisymmetric enough to produce jets. Comparing the predicted X-ray fluxes with limits from hard X-ray surveys, we conclude that either the Bondi-Hoyle efficiency parameter λ is rather small (≤0.01), the velocities of the IBH are rather high, or some combination of both. The predicted radio flux densities correspond to a population of objects which, while below current survey limits, should be detectable with the Square Kilometre Array (SKA). Converting the simulated space velocities into proper motions, we further demonstrate that such IBH could be identified as faint high proper motion radio sources in SKA surveys.
Abstract
The microquasar Cygnus X-3 was observed several times with the Gemini North Infrared Spectrograph while the source was in the hard X-ray state. We describe the observed 1.0–2.4 μm spectra as ...arising from the stellar wind of the companion star and suggest its classification as a WN 4–6 Wolf–Rayet star. We attribute the orbital variations of the emission line profiles to the variations in the ionization structure of the stellar wind caused by the intense X-ray emission from the compact object. The strong variability observed in the line profiles will affect the mass function determination. We are unable to reproduce earlier results, from which the mass function for the Wolf–Rayet star was derived. Instead, we suggest that the system parameters are difficult to obtain from the infrared spectra. We find that the near-infrared continuum and the line spectra can be represented with non-LTE Wolf–Rayet atmosphere models if taking into account the effects arising from the peculiar ionization structure of the stellar wind in an approximative manner. From the representative models we infer the properties of the Wolf–Rayet star and discuss possible mass ranges for the binary components.
Abstract
We present a detailed analysis of the X-ray/IR fast variability of the Black-Hole Transient GX 339-4 during its low/hard state in 2008 August. Thanks to simultaneous high time resolution ...observations made with the VLT and RXTE, we performed the first characterization of the subsecond variability in the near-infrared band – and of its correlation with the X-rays – for a low-mass X-ray binary, using both time- and frequency-domain techniques. We found a power-law correlation between the X-ray and infrared fluxes when measured on time-scales of 16 s, with a marginally variable slope, steeper than the one found on time-scales of days at similar flux levels. We suggest the variable slope – if confirmed – could be due to the infrared flux being a non-constant combination of both optically thin and optically thick synchrotron emission from the jet, as a result of a variable self-absorption break. From cross spectral analysis, we found an approximately constant infrared time lag of ≈0.1 s, and a very high coherence of ∼90 per cent on time-scales of tens of seconds, slowly decreasing towards higher frequencies. Finally, we report on the first detection of a linear rms–flux relation in the emission from a low-mass X-ray binary jet, on time-scales where little correlation is found between the X-rays and the jet emission itself. This suggests that either the inflow variations and jet IR emission are coupled by a non-linear or time-variable transform, or that the IR rms–flux relation is not transferred from the inflow to the jet, but is an intrinsic property of emission processes in the jet.
ABSTRACT
We present the second data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 177 nearby galaxies from the Palomar sample, observed with the e-MERLIN array, as part of the ...Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies Sample (LeMMINGs) survey. Together with the 103 targets of the first LeMMINGs data release, this represents a complete sample of 280 local active (LINER and Seyfert) and inactive galaxies (H ii galaxies and absorption line galaxies, ALG). This large program is the deepest radio survey of the local Universe, ≳1017.6 W Hz−1, regardless of the host and nuclear type: we detect radio emission ≳0.25 mJy beam−1 for 125/280 galaxies (44.6 per cent) with sizes of typically ≲100 pc. Of those 125, 106 targets show a core which coincides within 1.2 arcsec with the optical nucleus. Although we observed mostly cores, around one third of the detected galaxies features jetted morphologies. The detected radio core luminosities of the sample range between ∼1034 and 1040 erg s−1. LINERs and Seyferts are the most luminous sources, whereas H ii galaxies are the least. LINERs show FR I-like core-brightened radio structures while Seyferts reveal the highest fraction of symmetric morphologies. The majority of H ii galaxies have single radio core or complex extended structures, which probably conceal a nuclear starburst and/or a weak active nucleus (seven of them show clear jets). ALGs, which are typically found in evolved ellipticals, although the least numerous, exhibit on average the most luminous radio structures, similar to LINERs.
In this paper, we investigate the relationship between the maximal luminosity of X-ray outburst and the orbital period in transient low mass X-ray binaries (or soft X-ray transients) observed by the ...Rossi X-ray Timing Explorer (RXTE) in the past decade. We find that the maximal luminosity (3-200 keV) in Eddington units generally increases with increasing orbital period, which does not show a luminosity saturation but in general agrees with theoretical prediction. The peak luminosities in ultra-compact binaries might be higher than those with an orbital period of 2-4 hr, but more data are needed to make this claim. We also find that there is no significant difference in the 3-200 keV peak outburst luminosity between neutron star (NS) systems and black hole (BH) systems with orbital periods above 4 hr; however, there might be a significant difference at smaller orbital periods where only NS systems are observed and radiatively inefficient accretion flow is expected to work at low luminosities for BH accreters.
Abstract
We present simultaneous multiband radio and X-ray observations of the black hole X-ray binary Cygnus X-1, taken with the Karl G. Jansky Very Large Array and the Nuclear Spectroscopic ...Telescope Array. With these data, we detect clear flux variability consistent with emission from a variable compact jet. To probe how the variability signal propagates down the jet flow, we perform detailed timing analyses of our data. We find that the radio jet emission shows no significant power at Fourier frequencies f ≳ 0.03 Hz (below ∼30 s time-scales), and that the higher frequency radio bands (9/11 GHz) are strongly correlated over a range of time-scales, displaying a roughly constant time lag with Fourier frequency of a few tens of seconds. However, in the lower frequency radio bands (2.5/3.5 GHz), we find a significant loss of coherence over the same range of time-scales. Further, we detect a correlation between the X-ray/radio emission, measuring time lags between the X-ray/radio bands on the order of tens of minutes. We use these lags to solve for the compact jet speed, finding that the Cyg X-1 jet is more relativistic than usually assumed for compact jets, where $\beta =0.92^{+0.03}_{-0.06}$ and ($\Gamma =2.59^{+0.79}_{-0.61}$). Lastly, we constrain how the jet size scale changes with frequency, finding a shallower relation (∝ν−0.4) than predicted by simple jet models (∝ν−1), and estimate a jet opening angle of ϕ ∼ 0.4–1.8 deg. With this study we have developed observational techniques designed to overcome the challenges of radio timing analyses and created the tools needed to connect rapid radio jet variability properties to internal jet physics.
We hypothesize that at least some of the recently discovered class of calcium-rich gap transients are tidal detonation events of white dwarfs (WDs) by black holes (BHs) or possibly neutron stars. We ...show that the properties of the calcium-rich gap transients agree well with the predictions of the tidal detonation model. Under the predictions of this model, we use a follow-up X-ray observation of one of these transients, SN 2012hn, to place weak upper limits on the detonator mass of this system that include all intermediate-mass BHs (IMBHs). As these transients are preferentially in the stellar haloes of galaxies, we discuss the possibility that these transients are tidal detonations of WDs caused by random flyby encounters with IMBHs in dwarf galaxies or globular clusters. This possibility has been already suggested in the literature but without connection to the calcium-rich gap transients. In order for the random flyby cross-section to be high enough, these events would have to be occurring inside these dense stellar associations. However, there is a lack of evidence for IMBHs in these systems, and recent observations have ruled out all but the very faintest dwarf galaxies and globular clusters for a few of these transients. Another possibility is that these are tidal detonations caused by three-body interactions, where a WD is perturbed towards the detonator in isolated multiple star systems. We highlight a number of ways this could occur, even in lower mass systems with stellar-mass BHs or neutron stars. Finally, we outline several new observational tests of this scenario, which are feasible with current instrumentation.
Abstract We present a Chandra observation of SN 2016hnk, a candidate Ca-rich gap transient. This observation was specifically designed to test whether or not this transient was the result of the ...tidal detonation of a white dwarf by an intermediate-mass black hole. Since we detect no X-ray emission 28 d after the discovery of the transient, as predicted from fall-back accretion, we rule out this model. Our upper limit of ∼10 M⊙ does not allow us to rule out a neutron star or stellar-mass black hole detonator due limits on the sensitivity of Chandra to soft X-rays and unconstrained variables tied to the structure of super-Eddington accretion discs. Together with other Chandra and multiwavelength observations, our analysis strongly argues against the intermediate-mass black hole tidal detonation scenario for Ca-rich gap transients more generally.
SDSS J080710+485259 is the longest-period outbursting ultracompact white dwarf binary. Its first-ever detected superoutburst started in 2018 November and lasted for a year, the longest detected so ...far for any short orbital period accreting white dwarf. Here we show the superoutburst duration of SDSS J080710+485259 exceeds the ∼2 month viscous time of its accretion disk by a factor of about 5. Consequently it follows that neither the empirical relation nor the theoretical relation between the orbital period and the superoutburst duration for AM CVn systems. Six months after the end of the superoutburst the binary remained 0.4 mag brighter than its quiescent level before the superoutburst. We detect a variable X-ray behavior during the post-outburst cooling phase, demonstrating changes in the mass accretion rate. We discuss how irradiation of the donor star, a scenario poorly explored so far and that ultimately can have important consequences for AM CVns as gravitational-wave sources, might explain the unusual observed features of the superoutburst.
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