arXiv subjects

J. -P. Lasota

Publications and source records attributed to J. -P. Lasota.

At least 37 records · Page 2Linked to original sources

arXiv2016

Outbursts in ultracompact X-ray binaries

Very faint X-ray binaries appear to be transient in many cases with peak luminosities much fainter than that of usual soft X-ray transients, but their nature still remains elusive. We investigate the possibility that this transient behaviour is due to the same thermal/viscous instability which is responsible for outbursts of bright soft X-ray transients, occurring in ultracompact binaries for adequately low mass-transfer rates. More generally, we investigate the observational consequences of this instability when it occurs in ultracompact binaries. We use our code for modelling the thermal-viscous instability of the accretion disc, assumed here to be hydrogen poor. We also take into account the effects of disc X-ray irradiation, and consider the impact of the mass-transfer rate on the outburst brightness. We find that one can reproduce the observed properties of both the very faint and the brighter short transients (peak luminosity, duration, recurrence times), provided that the viscosity parameter in quiescence is slightly smaller (typically a factor of between two and four) than in bright soft X-ray transients and normal dwarf nova outbursts, the viscosity in outburst being unchanged. This possibly reflects the impact of chemical composition on non-ideal MHD effects affecting magnetically driven turbulence in poorly ionized discs.

astro-ph.HE↗

arXiv2016

The slimming effect of advection on black-hole accretion flows

At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio $H/R \lesssim 1$) or thick (H/R >1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. We wish to establish under what conditions slim accretion flows become thick. We use analytical equations, numerical 1+1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates.We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.

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arXiv2014

Mass of a Black Hole Firewall

Quantum entanglement of Hawking radiation has been supposed to give rise to a Planck density "firewall" near the event horizon of old black holes. We show that Planck density firewalls are excluded by Einstein's equations for black holes of mass exceeding the Planck mass. We find an upper limit of $1/(8πM)$ to the surface density of a firewall in a Schwarzschild black hole of mass $M$, translating for astrophysical black holes into a firewall density smaller than Planck density by more than 30 orders of magnitude. A strict upper limit on the firewall density is given by the Planck density times the ratio $M_{\rm Pl}/(8πM)$.

gr-qc↗

arXiv2014

Extracting black-hole rotational energy: The generalized Penrose process

In the case involving particles the necessary and sufficient condition for the Penrose process to extract energy from a rotating black hole is absorption of particles with negative energies and angular momenta. No torque at the black-hole horizon occurs. In this article we consider the case of arbitrary fields or matter described by an unspecified, general energy-momentum tensor $T_{μν}$ and show that the necessary and sufficient condition for extraction of a black hole's rotational energy is analogous to that in the mechanical Penrose process: absorption of negative energy and negative angular momentum. We also show that a necessary condition for the Penrose process to occur is for the Noether current (the conserved energy-momentum density vector) to be spacelike or past directed (timelike or null) on some part of the horizon. In the particle case, our general criterion for the occurrence of a Penrose process reproduces the standard result. In the case of relativistic jet-producing "magnetically arrested disks" we show that the negative energy and angular-momentum absorption condition is obeyed when the Blandford-Znajek mechanism is at work, and hence the high energy extraction efficiency up to $\sim 300\%$ found in recent numerical simulations of such accretion flows results from tapping the black hole's rotational energy through the Penrose process. We show how black-hole rotational energy extraction works in this case by describing the Penrose process in terms of the Noether current.

gr-qc↗

arXiv2013

Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst

We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large Telescope (VLT), V- and R-band) observations of the intermediate mass black hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show that the V-band magnitudes vary with time, thus proving that a portion of the observed emission originates in the accretion disk. Using the first quiescent optical observations of HLX-1, we show that the stellar population surrounding HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may increase before the X-ray emission consistent with the scenario proposed by Lasota et al. (2011) in which the regular outbursts could be related to the passage at periastron of a star circling the intermediate mass black hole in an eccentric orbit, which triggers mass transfer into a quasi-permanent accretion disk around the black hole. Further, if there is indeed a delay in the X-ray emission we estimate the mass-transfer delivery radius to be ~1e11 cm.

astro-ph.HE↗

arXiv2012

The accretion disc, jets and environment of the intermediate mass black hole candidate ESO 243-49 HLX-1

The Ultra Luminous X-ray (ULX) source HLX-1 in the galaxy ESO 243-49 has an observed maximum unabsorbed X-ray luminosity of 1.3e42 erg/s (0.2-10.0 keV). From the conservative assumption that this value exceeds the Eddington limit by at most a factor of 10, the minimum mass is then 500 solar masses. The X-ray luminosity varies by a factor of 40 with an apparent recurrence timescale of approximately one year. This X-ray variability is associated with spectral state transitions similar to those seen in black hole X-ray binaries. Here we discuss our recent modelling of all the X-ray data for HLX-1 and show that it supports the idea that this ULX is powered by sub- and near Eddington accretion onto an intermediate mass black hole. We also present evidence for transient radio emission which is consistent with a discrete jet ejection event as well as comment on the nature of the environment around HLX-1 in light of recent Hubble Space Telescope photometry.

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arXiv2012

Investigating slim disk solutions for HLX-1 in ESO 243-49

The hyper luminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the best intermediate mass black hole candidate, displays spectral transitions similar to those observed in Galactic black hole binaries, but with a luminosity 100-1000 times higher. We investigated the X-ray properties of this unique source fitting multi-epoch data collected by Swift, XMM-Newton & Chandra with a disk model computing spectra for a wide range of sub- and super-Eddington accretion rates assuming a non-spinning black hole and a face-on disk (i = 0 deg). Under these assumptions we find that the black hole in HLX-1 is in the intermediate mass range (~2 x 10^4 M_odot) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is eta = 0.11 +/-0.03. We also show that the source does follow the L_X ~ T^4 relation for our mass estimate. At the outburst peaks, the source radiates near the Eddington limit. The accretion rate then stays constant around 4 x 10^(-4) M_odot yr^(-1) for several days and then decreases exponentially. Such "plateaus" in the accretion rate could be evidence that enhanced mass transfer rate is the driving outburst mechanism in HLX-1. We also report on the new outburst observed in August 2011 by the Swift-X-ray Telescope. The time of this new outburst further strengthens the ~1 year recurrence timescale.

astro-ph.HE↗

arXiv2011

The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49

The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO 243-49 galaxy exhibits variability with a possible recurrence time of a few hundred days. Finding the origin of this variability would constrain the still largely unknown properties of this extraordinary object. Since it exhibits an intensity-hardness behavior characteristic of black-hole X-ray transients, we have analyzed the variability of HLX-1 in the framework of the disk instability model that explains outbursts of such systems. We find that the long-term variability of HLX-1 is unlikely to be explained by a model in which outbursts are triggered by thermal-viscous instabilities in an accretion disc. Possible alternatives include the instability in a radiation-pressure dominated disk but we argue that a more likely explanation is a modulated mass-transfer due to tidal stripping of a star on an eccentric orbit around the intermediate-mass black hole. We consider an evolutionary scenario leading to the creation of such a system and estimate the probability of its observation. We conclude, using a simplified dynamical model of the post-collapse cluster, that no more than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar mergers in the dense collapsed cores of young clusters - could have a few times 1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for mass transfer at periapse, while avoiding collisional destruction or being scattered into the IMBH by 2-body encounters. The finite but low probability of this configuration is consistent with the uniqueness of HLX-1. We note, however, that the actual response of a standard accretion disk to bursts of mass transfer may be too slow to explain the observations unless the orbit is close to parabolic (and hence even rarer) and/or additional heating, presumably linked to the highly time-dependent gravitational potential, are invoked.

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arXiv2009

Restless quiescence: thermonuclear flashes between transient X-ray outbursts

For thermonuclear flashes to occur on neutron-star surfaces, fuel must have been accreted from a donor star. However, sometimes flashes are seen from transient binary systems when they are thought to be in their quiescent phase, during which no accretion, or relatively little, is expected to occur. We investigate the accretion luminosity during several such flashes, including the first-ever and brightest detected flash from Cen X-4 in 1969. We infer from observations and theory that immediately prior to these flashes the accretion rate must have been between about 0.001 and 0.01 times the equivalent of the Eddington limit, which is roughly 2 orders of magnitude less than the peak accretion rates seen in these transients during an X-ray outburst and 3-4 orders of magnitude more than the lowest measured values in quiescence. Furthermore, three such flashes, including the one from Cen X-4, occurred within 2 to 7 days followed by an X-ray outburst. A long-term episode of enhanced, but low-level, accretion is predicted near the end of the quiescent phase by the disk-instability model, and may thus have provided the right conditions for these flashes to occur. We discuss the possibility of whether these flashes acted as triggers of the outbursts, signifying a dramatic increase in the accretion rate. Although it is difficult to rule out, we find it unlikely that the irradiance by these flashes is sufficient to change the state of the accretion disk in such a dramatic way.

astro-ph↗

arXiv2008

Evolutionary models of short-period soft X-ray transients: comparison with observations

We consider evolutionary models for the population of short-period (<10 hr) low-mass black-hole binaries (LMBHBs) and compare them with observations of soft X-ray transients (SXTs). We show that assuming strongly reduced magnetic braking (as suggested by us before for low-mass semidetached binaries) the calculated masses and effective temperatures of secondaries are encouragingly close to the observed masses and effective temperatures (as inferred from their spectra) of donor stars in short-period LMBHBs. Theoretical mass-transfer rates in SXTs are consistent with the observed ones if one assumes that accretion discs in these systems are truncated (``leaky''). We find that the population of short-period SXTs is formed mainly by systems which had unevolved or slightly evolved main-sequence donors ($M_2 \lesssim 1.2M_\odot$) with a hydrogen abundance in the center $X_c > 0.35$ at the Roche-lobe overflow (RLOF). Longer period (0.5 - 1) day) SXTs might descend from systems with initial donor masses of about 1 $M_\odot$ and $X_c < 0.35$. Thus, one can explain the origin of short period LMBHB without invoking donors with cores almost totally depleted of hydrogen. Our models suggest that, unless the currently accepted empirical estimates of mass-loss rates by winds for massive O-stars and Wolf-Rayet stars are significantly over-evaluated, a very high efficiency of common-envelope ejection is necessary to form short-period LMBHBs.

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arXiv2008

The Formation, Evolution and Parameters of Short-Period Low-Mass X-Ray Binaries with Black-Hole Components

We discuss the formation, evolution and observational parameters of the population of short-period ($<10$ hr) low-mass black-hole binaries (LMBHB). Their evolution is determined by the orbital angular momentum loss and/or nuclear evolution of the donors. All observed semidetached LMBHB are observed as soft X-ray transients (SXTs). The absence of observed short-period stable luminous X-ray sources with black holes and low-mass optical components suggests that upon RLOF by the donor, the angular-momentum losses are substantially reduced. The model with reduced angular-momentum loss reasonably well reproduces the masses and effective temperatures of the observed secondaries of SXTs. Theoretical mass-transfer rates in SXTs are consistent with those deduced from observations only if the accretion discs in LMBHB are truncated. The population of short-period LMBHB is formed mainly by systems which at RLOF had unevolved or slightly evolved donors (abundance of hydrogen in the center $> 0.35$). Our models suggest that a very high efficiency of common envelopes ejection is necessary to form LMBHB.

astro-ph↗

arXiv2008

Evolution of low-mass binaries with black-hole components

We consider evolutionary models for the population of short-period (<10 hr) low-mass black-hole binaries (LMBHB) and compare them with observations of soft X-ray transients (SXT). Evolution of LMBHB is determined by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking by the stellar wind of the donors and gravitational wave radiation. We show that the absence of observed stable luminous LMBHB implies that upon RLOF by the low-mass donor angular momentum losses are substantially reduced with respect to the Verbunt and Zwaan "standard" prescription for magnetic braking. Under this assumption masses and effective temperatures of the model secondaries of LMBHB are in a satisfactory agreement with the masses and effective temperatures (as inferred from their spectra) of the observed donors in LMBHB. Theoretical mass-transfer rates in SXTs are consistent with the observed ones if one assumes that accretion discs in these systems are truncated ("leaky"). We find that the population of short-period SXT is formed mainly by systems which had unevolved or slightly evolved (X_c > 0.35) donors at the RLOF. Longer period (0.5 - 1 day) SXT might descend from systems with initial donor mass about 1 solar and X_c < 0.35. It is unnecessary to invoke donors with almost hydrogen-depleted cores to explain the origin of LMBHB. Our models suggest that a very high efficiency of common-envelopes ejection is necessary to form LMBHB, unless currently commonly accepted empirical estimates of mass-loss rates by winds for pre-WR and WR-stars are significantly over-evaluated.

astro-ph↗

arXiv2008

Radio-loudness of Active Galaxies and the Black Hole Evolution

Active galactic nuclei (AGNs) form two distinct sequences on the radio-loudness -- Eddington-ratio plane. The `upper' sequence contains radio selected AGNs, the `lower' sequence is composed mainly of optically selected AGNs. The sequences mark the upper bounds for the radio-loudness of two distinct populations of AGNs, hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that another parameter in addition to the accretion rate must play a role in determining the efficiency of jet production in AGNs. We speculate that this additional parameter is the spin of the black hole, assuming that black holes in giant elliptical galaxies have (on average) much larger spins than black holes in disc galaxies. Possible evolutionary scenarios leading to such a spin dichotomy are discussed. The galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars being hosted by giant ellipticals is radio quiet. This indicates that the production of powerful jets at high accretion rates is in most cases suppressed and, in analogy to X-ray binary systems (XRB) during high and very high states, may be intermittent. Such intermittency can be caused by switches between two different accretion modes, assuming that only during one of them an outflow from the central engine is sufficiently collimated to form a relativistic jet.

astro-ph↗

arXiv2007

Radio Loudness of AGNs: Host Galaxy Morphology and the Spin Paradigm

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. We find that AGNs form two distinct and well separated sequences on the radio-loudness - Eddington-ratio plane. We argue that these sequences mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that another parameter in addition to the accretion rate must play a role in determining the jet production efficiency in active galactic nuclei, and that this parameter is related to properties of the host galaxy. The revealed host-related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. We argue that the huge difference between the radio-loudness reachable by AGNs in disc and elliptical galaxies can be explained by the scenario according to which the spin of a black hole determines the outflow's power, and central black holes can reach large spins only in early type galaxies (following major mergers), and not (in a statistical sense) in spiral galaxies.

astro-ph↗

arXiv2006

Radio-Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about seven orders of magnitude in accretion luminosity and the full range of AGN black hole masses. We find that AGNs form two distinct and well separated sequences on the radio-loudness -- Eddington-ratio plane. The `upper' sequence is formed by radio selected AGNs, the `lower' sequence contains mainly optically selected objects. Whereas an apparent `gap' between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disc galaxies. This galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some BH X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks.

astro-ph↗

arXiv2006

The origin and fate of short-period low-mass black-hole binaries

We present results of a population synthesis study for semidetached short orbital period binaries which contain low-mass(<1.5 Msun) donors and black hole (>4 Msun) accretors. Evolution of these binaries is determined by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking by the stellar wind of the donors and gravitational wave radiation. According to our model, the estimated total number of this type of black-hole binaries in the Galaxy is about 10000. If the magnetic braking is described by the Verbunt & Zwaan formula, the model predicts around 3000 transient systems with periods >2 hours and around 300 luminous stable systems with periods between 3 and 8 hours. Several dozens of these bright systems should be above the RXTE ASM sensitivity limit. The absence of such systems implies that angular momentum losses are reduced by a factor more than 2 with respect to the Verbunt & Zwaan prescription. We show that it is unlikely that the transient behaviour of black-hole short-period X-ray binaries is explained by the evolved nature of the stellar companion. A substantial fraction of black-hole binaries with periods >3 hours could be faint with truncated, stable cold accretion discs as proposed by Menou et al. Most of the semidetached black-hole binaries are expected to have periods shorter than ~2 hours. Properties of such, still to be observed, very small mass-ratio (q<0.02) binaries are different from those of their longer period cousins.

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arXiv2004

Predictions of the disc instability model

Confronting the predictions of numerical calculations with observations is an important tool to progress with our understanding of the mechanism triggering the outbursts of dwarf novae and soft X-ray transients. Simultaneous multi-wavelength observations are of particular importance in this process as they contain information about the chronology of the outbursts as e.g. the famous UV-delay observed in dwarf novae. We review key-predictions of the disc instability model (DIM) and confront them with the corresponding observations.

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arXiv2004

X-ray sources and their optical counterparts in the globular cluster M 22

Using XMM-Newton EPIC imaging data, we have detected 50 low-luminosity X-ray sources in the field of view of M 22, where 5 +/- 3 of these sources are likely to be related to the cluster. Using differential optical photometry, we have identified probable counterparts to those sources belonging to the cluster. Using X-ray spectroscopic and timing studies, supported by the optical colours, we propose that the most central X-ray sources in the cluster are cataclysmic variables, millisecond pulsars, active binaries and a blue straggler. We also identify a cluster of galaxies behind this globular cluster.

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