arXiv subjects

Marek Gierlinski

Publications and source records attributed to Marek Gierlinski.

At least 37 records · Page 2Linked to original sources

arXiv2005

Scaling variability from stellar to supermassive black holes

We investigate the correspondence between the variability seen in the stellar and supermassive black holes. Galactic Black Hole (GBH) power density spectra (PDS) are generally complex, and dependent on spectral state. In the low/hard state the high-frequency rollover in the PDS moves in a way which is not simply related to luminosity. Hence this feature can only be used as an approximate indicator rather than as an accurate tracer of black hole mass in AGN. The X-ray spectrum in the high/soft state is dominated by the disc in GBH, which is rather stable. We show that the PDS of the Comptonized tail in GBH can be much more variable, and that it is this which should be compared to AGN due to their much lower disc temperature. This bandpass effect removes a problem in interpreting the (often highly variable) Narrow Line Seyfert 1 (NLS1) galaxies as the counterparts of the high mass accretion rate GBH. Additionally, we speculate that some NLS1 (e.g. Akn 564) are counterparts of the very high state. The Comptonized tail in this state is also highly variable, but with PDS which can be roughly described as band-limited noise. This shape is similar to that seen in the low/hard state, so merely seeing such band-limited noise in the power spectrum of an AGN does not necessarily imply low luminosity. We also stress that Cyg X-1, often used for comparison with AGN, is not a typical black hole system due to its persistent nature. In particular, the shape of its power spectrum in the high/soft state is markedly different from that of other (transient) GBH systems in this state. The fact that the NLS1s NGC 4051 and MCG-6-30-15 do appear to show a power spectrum similar to that of the high/soft state of Cyg X-1 may lend observational support to theoretical speculation that the Hydrogen ionization disc instability does not operate in AGN.

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arXiv2005

Patterns of energy-dependent variability from Comptonization

We study fractional variability as a function of energy from black-hole X-ray binaries on timescales from milliseconds to hundreds of seconds. We build a theoretical model of energy-dependent variability in which the X-ray energy spectrum varies in response to a changing physical parameter. We compare these models to rms spectra obtained from RXTE PCA observations of black-hole binaries XTE J1550-564 and XTE J1650-500. We show that two main variability models are consistent with the data: variable seed photon input in the hard state and variable power in the Comptonized component in the soft and very high states. The lack of clear reflection features in the rms spectra implies that the reflection and the X-ray continuum, when integrated over Fourier frequencies, are correlated and vary with similar fractional amplitudes. Our models predict two important features of rms spectra, not possible to be clearly seen by the PCA due sensitivity limits. At soft X-rays, >~3 keV, we predict the presence of a break in the rms spectrum at energy directly related to the seed photon temperature. At higher energies, ~20--30 keV, we predict a peak in the rms spectrum originating from the variability of the spectrum produced by a hybrid thermal/non-thermal electron distribution. If these features are confirmed by broad-band observations, they will impose important constraints on the origin of the seed photons for Comptonization and the electron distribution in the hot plasma.

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arXiv2005

GRS 1915+105: the distance, radiative processes and energy-dependent variability

We present an exhaustive analysis of five broad-band observations of GRS 1915+105 in two variability states, chi and omega, observed simultaneously by the PCA, HEXTE and OSSE. We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a nonthermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the chi state are typical to the very high/intermediate state of black-hole binaries. The spectrum of the omega state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak nonthermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the QPOs present in the chi state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find 11 kpc as the most likely value, contrary to a recent claim of a much lower value.

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arXiv2005

Physics of accretion in the millisecond pulsar XTE J1751-305

We have undertaken an extensive study of X-ray data from the accreting millisecond pulsar XTE J1751-305 observed by RXTE and XMM-Newton during its 2002 outburst. In all aspects this source is similar to a prototypical millisecond pulsar SAX J1808.4-3658, except for the higher peak luminosity of 13 per cent of Eddington, and the optical depth of the hard X-ray source larger by factor ~2. Its broad-band X-ray spectrum can be modelled by three components. We interpret the two soft components as thermal emission from a colder (kT ~ 0.6 keV) accretion disc and a hotter (~1 keV) spot on the neutron star surface. We interpret the hard component as thermal Comptonization in plasma of temperature ~40 keV and optical depth of ~1.5 in a slab geometry. The plasma is heated by the accretion shock as the material collimated by the magnetic field impacts on to the surface. The seed photons for Comptonization are provided by the hotspot, not by the disc. The Compton reflection is weak and the disc is probably truncated into an optically thin flow above the magnetospheric radius. Rotation of the emission region with the star creates an almost sinusoidal pulse profile with rms amplitude of 3.3 per cent. The energy-dependent soft phase lags can be modelled by two pulsating components shifted in phase, which is naturally explained by a different character of emission of the optically thick spot and optically thin shock combined with the action of the Doppler boosting.

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arXiv2004

Radiative Processes, Spectral States and Variability of Black-Hole Binaries

We review radiative processes responsible for X-ray emission in hard (low) and soft (high) spectral states of black-hole binaries. The main process in the hard state appears to be scattering of blackbody photons from a cold disk by thermal electrons in a hot inner flow, and any contribution from nonthermal synchrotron emission is at most small. In the soft states, blackbody disk emission dominates energetically, and its high-energy tail is due to scattering by hybrid, thermal/nonthermal electrons, probably in active regions above the disk surface. State transitions appear to correspond to a variable inner radius of the cold disk driven by changes of the accretion rate. The existence of two accretion solutions, hot and cold, in a range of the accretion rate leads to hysteresis in low-mass X-ray binaries.

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arXiv2004

Spectral transitions in X-ray Binaries

We systematically analyzed about a terabyte of data from Galactic binary systems to characterize how these spectra change as a function of mass accretion rate. With this volume of data we are able to identify the differences in behaviour of black holes and neutron stars, implying that black holes really do have an event horizon. The spectral evolution seen as a function of L/L_Edd can be linked to current state-of-the-art numerical simulations of accretion flows, and we develop qualitative and quantitative models of the spectral changes in all types of binary systems. The key ingredient of this picture is that the major hard-soft spectral transition is driven by a changing inner radius of the accretion disc. This also qualitatively explains the changes seen in the variability power spectra of these objects. This lays the foundation for a self-consistent, theoretically motivated framework in which to explain both the spectral and timing properties of accreting compact objects.

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arXiv2004

GX 339-4: the distance, state transitions, hysteresis and spectral correlations

We study X-ray and variability and distance of GX 339-4. We derive d>7 kpc, based on recent determination of the binary parameters. We study data from the Ginga/ASM, the CGRO/BATSE, and the RXTE/ASM, PCA and HEXTE. From 1987 to 2004, GX 339-4 underwent 15 outbursts and went through all known states of black-hole binaries. We also present the PCA data from the initial stage of the 2004 outburst. We then study colour-colour and colour-flux correlations. In the hard state, there is a strong anticorrelation between the 1.5-5 and 3-12 keV spectral slopes, which we explain by thermal Comptonization of disc photons. There is also a softening of the spectrum above 3 keV with the increasing flux that becomes stronger with increasing energy up to 200 keV. This indicates an anticorrelation between the electron temperature and luminosity, explained by hot accretion models. In addition, we see a variable broad-band slope with a pivot at 200 keV. We confirm the presence of pronounced hysteresis, with the hard-to-soft state transitions occurring at much higher (and variable) luminosities than the soft-to-hard transitions. We fit the ASM data with a model consisting of an outer accretion disc and a hot inner flow. State transitions are associated then with variations in the disc truncation radius, which we fit as 6GM/c^2 in the soft state and several times that in the hard state. The disappearence of the inner disc takes place at a lower accretion rate than its initial appearance due to the dependence of the transitions on the source history. We provide further evidence against the X-ray emission in the hard state being nonthermal synchrotron, and explain the observed radio-X-ray correlation by the jet power being correlated with the accretion power.

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arXiv2004

Is the soft excess in AGN real?

We systematically analyse all publicly available XMM-Newton spectra of radio-quiet PG quasars. The soft X-ray excess in these objects is well modelled by an additional, cool, Compton scattering region. However, the remarkably constant temperature derived for this component over the whole sample requires a puzzling fine tuning of the parameters. Instead, we propose that the soft excess is an artifact of strong, relativistically smeared, partially ionized absorption. The strong jump in opacity at ~0.7 keV from OVII, OVIII and iron can lead to an apparent soft excess below this energy, which is trivially constant since it depends on atomic processes. This can have a dramatic effect on the derived spectrum, which has implications for fitting the relativistic smearing of the reflected iron line emission from the disc.

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arXiv2003

Black hole accretion discs: reality confronts theory

Disc spectra from highly luminous black hole binaries are observed to be rather simple, despite theoretical predictions to the contrary. We collate the disc-dominated spectra from multiple observations of 10 separate sources, and show that these overwhelmingly follow a multi-temperature disc blackbody spectrum with luminosity proportional to T^4, and that such discs are stable. These results are in conflict with standard Shakura-Sunyaev alpha-discs predictions, including proposed modifications such as additional energy loss in a jet, wind or corona. Accretion disc spectra can be useful, reliable guides to the disc structure, and should be used to test the next generation of accretion disc models, in which the disc viscosity is calculated self-consistently from the magnetically generated turbulent dynamo.

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arXiv2003

Discovery of powerful millisecond flares from Cygnus X-1

We have found a large number of very strong flares in the available XTE PCA data of Cyg X-1 (also seen in available HEXTE and BATSE data) with 13 flares satisfying our chosen threshold criterion, occuring both in the hard and the soft states. We analyze here in detail two of them. The strongest one took place in the soft state, with the 3-30 keV energy flux increasing 30 times with respect to the preceding 16-s average. The e-folding time is ~ 7 ms for the main flare and ~ 1 ms for its precursor. The spectrum strongly hardens during the flare. On the other hand, flares in the hard state have generally lower amplitudes and longer e-folding times, and their spectra soften during the flare, with the hardness of the spectrum at the flare peak similar for both types of the flares. The presence of the flares shows unusually dramatic events taking place in the accretion flow of Cyg X-1. On the other hand, the rate of occurence of hard-state flares shows they may represent a high-flux end of the distribution of shots present in usual lightcurves of Cyg X-1.

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arXiv2003

Observing the effects of the event horizon in black holes

The key difference between neutron stars and black holes is the presence/absence of a solid surface. Recent attempts to detect this difference have concentrated on the quiescent luminosity, but here these sources are faint and difficult to observe. Instead we look at these sources when they are bright, and show that there is a clear difference between black holes and neutron stars in the evolution of their X-ray spectra which is due to the presence of a surface in the neutron stars. We also show that there is a type of X-ray spectrum which is only seen from black holes, making it a good diagnostic for the nature of new transient sources.

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arXiv2003

The X-ray/gamma-ray spectrum of XTE J1550--564 in the very high state

We fit the broad-band X/gamma-ray spectrum (0.8-1000 keV) of the accreting black hole XTE J1550-564 in the very high state. The quasi-simultaneous data from ASCA, RXTE and OSSE show that the disc is strongly Comptonized, with a high energy tail extending out to several hundreds keV. However, inverse Compton scattering by a purely thermal or purely power-law electron distribution cannot explain the observed spectrum. Instead the data require a hybrid distribution, with both thermal and non-thermal electrons scattering the disc photons. This is very similar to the electron distribution inferred for other high and very high state black hole binaries, showing that it is a generic feature of high mass accretion rate black holes.

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arXiv2003

On the Nature of the X-ray Emission from the Accreting Millisecond Pulsar SAX J1808.4-3658

The pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies are studied. The two main emission component, the black body and the Comptonized tail that are clearly identified in the time-averaged spectrum, show strong variability with the first component lagging the second one. The observed variability can be explained if the emission is produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth ~0.3-1 at the neutron star surface. The emission patterns of the black body and the Comptonized radiation are different: a "knife"- and a "fan"-like, respectively. We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending in the Schwarzschild spacetime. We present also accurate analytical formulae for computations of the light curves from rapidly rotating neutron stars using formalism recently developed by Beloborodov (2002). Our model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time-averaged spectrum. We constrain the compact star mass to be bounded between 1.2 and 1.6 solar masses. By fitting the observed profiles, we determine the radius of the compact object to be R~11 km if M=1.6 M_sun, while for M=1.2 M_sun the best-fitting radius is ~6.5 km, indicating that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a lower limit on the inclination of the system of 65 degrees.

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arXiv2002

The X-ray spectrum of the atoll source 4U 1608-52

The transient atoll source 4U 1608-52 had been extensively observed by the Rossi X-ray Timing Explorer RXTE during its 1998 outburst. We analyse its X-ray spectra as a function of inferred accretion rate from both the 1998 outburst and from the non-outburst 1996 and 1998 data. We can fit all the spectra by a model in which seed photons from the neutron star surface are Comptonized in a boundary layer. The Comptonized emission illuminates the accretion disc surface, producing an ionized, relativistically broadened reflection signature, while the direct emission from the accretion disc can also be seen. The evolution of the source can be explained if the main parameter driving the spectral evolution is the average mass accretion rate, which determines the truncation radius of the inner accretion disc. At low mass accretion rates, in the island state, the disc truncates before reaching the neutron star surface and the inner accretion flow/boundary layer is mostly optically thin. The disc emission is at too low a temperature to be observed in the RXTE spectra, but some of the seed photons from the neutron star can be seen directly through the mostly optically thin boundary layer. At higher mass accretion rates, in the banana state, the disc moves in and the boundary layer becomes much more optically thick so its temperature drops. The disc can then be seen directly, but the seed photons from the neutron star surface cannot as they are buried beneath the increasingly optically thick boundary layer.

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arXiv2002

X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4

We analyse five observations of the X-ray binary GX 339-4 by the soft gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE observations. The source was bright during four of them, with the luminosity of L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s. Our broad-band spectral fits show the mean electron energy of electrons in the Comptonizing plasma decreasing with increasing luminosity within the hard (bright) state. For the observation with the best statistics at soft gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably carried by non-thermal electrons. Then, considering the efficiency of Comptonized hybrid synchrotron emission allows us to obtain an upper limit on the strength of the magnetic field in the X-ray source. Furthermore, this synchrotron emission is capable of producing the optical spectrum observed in an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum is consistent with being dominated by bremsstrahlung. The unusually strong Fe K alpha line observed by the PCA during that state is found not to be intrinsic to the source but to originate mostly in the Galactic diffuse emission.

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arXiv2002

Modeling the Energy Dependent Pulse Profiles of the Accreting Millisecond Pulsar SAX J1808.4-3658

The pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies are studied. The two main emission component, a black body and a power-law tail, clearly identified in the time-averaged spectrum, do not vary in phase. We show that the observed variability can be easily explained if the emission patterns of the black body and the Comptonized radiation are different: a "knife" and a "fan"-like, respectively. We suggest that Comptonization in a hot slab (radiative shock) of Thomson optical depth \~0.3 at the surface of the neutron star may be responsible for the emission. We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending. The model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time-averaged spectrum. By fitting the observed pulse profiles we obtain constraints on the neutron star radius (R=7.5+-1.0 km), the inclination of the system i>60 deg, and the angle between the magnetic dipole and the rotational axis delta=20deg+-5deg.

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arXiv2002

A comment on the colour-colour diagrams of Low Mass X-ray Binaries

Disc accreting neutron stars come in two distinct varieties, atolls and Z sources, named after their differently shaped tracks on a colour-colour diagram as the source luminosity changes. Here we present analysis of three transient atoll sources showing that there is an additional branch in the colour-colour diagram of atoll sources which appears at very low luminosities. This new branch connects to the top of previously known C-shaped (atoll) path, forming a horizontal track where the average source flux decrease from right to left. This turns the C-shape into a Z. Thus both atolls and Z sources share the same topology on the colour-colour diagram, and evolve in similar way as a function of increasing averaged mass accretion rate. This strongly favours models in which the underlying geometry of these sources changes in similar ways. A possible scenario is one where the truncated disc approaches the neutron star when the accretion rate increases, but that in the atolls the disc is truncated by evaporation (similarly to black holes) whereas in the Z sources it is truncated by the magnetic field.

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arXiv2001

Phase-resolved X-ray spectroscopy of the millisecond pulsar SAX J1808.4-3658

We present new results based on RXTE observations of SAX J1808.4-3658. The X-ray spectrum can be fitted by a two-component model. We interpret the soft component as blackbody emission from a heated spot on the neutron star, and the hard component as coming from Comptonization in plasma heated by the accretion shock as the material collimated by the magnetic field impacts onto the neutron star surface. The hotspot is probably the source of seed photons for Comptonization. The hard component illuminates the disc, giving rise to a reflected spectrum. The inferred evolution of the inner radius is not consistent with the magnetic field truncating the disc. Instead it seems more likely that the inner disc radius is set by some much longer time-scale process, most probably connected to the overall evolution of the accretion disc. This disc truncation mechanism would then have to be generic in all low mass accretion rate flows both in disc accreting neutron stars and black hole systems. The phase resolved spectra show clearly that the blackbody and hard Comptonized spectra pulse independently. This obviously gives an energy dependent phase lag. Full general relativistic effects are not required to explain this. We do not detect Compton reflection from the neutron star surface, though we predict that it should be present in the X-ray spectrum. This would give an unambiguous observational measure of M/R if there is any iron on the neutron star surface which is not completely ionized.

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