Narrow Results

The detection of TeV gamma-rays from LS 5039 and the binary pulsar PSR B1259–63 by HESS, and from LS I +61 303 and the stellar-mass black hole Cygnus X-1 by MAGIC, provides clear evidence of very efficient acceleration of particles to multi-TeV energies in X-ray binaries. These observations demonstrate the richness of nonthermal phenomena in compact galactic objects containing relativistic outflows or winds produced near black holes and neutron stars. I review here some of the main observational results on very high energy (VHE) γ-ray emission from X-ray binaries, as well as some of the proposed scenarios to explain the production of VHE γ-rays. I put special emphasis on the flare TeV emission, suggesting that the flaring activity might be a common phenomena in X-ray binaries.

We present an analysis of two Chandra observations of LS 5039 performed in 2004 in two different orbital phases during the same orbital cycle. Our results show a clear flux variability, confirming a trend of increasing flux with orbital phase in the range 0.05 ≲ ϕ ≲ 0.7 as has been found in XMM observations carried out in 2005 during the same orbital cycle. We suggest that the X-ray variations are linked to orbital changes of the intrinsic properties of the emitter, which should have implications for possible emission models to explain the present multiwavelength knowledge of the source.

LS I +61 303 is a puzzling object detected from radio up to very high-energy gamma-rays. Variability has recently been observed in its high-energy emission. The object is a binary system, with a compact object and a Be star as primary. The nature of the secondary and the origin of the gamma-ray emission are not clearly established at present. Recent VLBA radio data have been used to claim that the system is a Be/neutron star colliding wind binary, instead of a microquasar. We review the main views on the nature of LS I +61 303 and present results of 3D SPH simulations that can shed some light on the nature of the system. Our results support an accretion powered source, compatible with a microquasar interpretation.

There are three Galactic jet sources from which TeV emission has been detected: LS 5039, LS I +61 303 and Cygnus X-1. These three sources show power-law tails in X-rays and soft gamma-rays that could indicate a nonthermal origin of this radiation. In addition, all three sources apparently show correlated and complex behavior at X-ray and TeV energies. In some cases, this complex behavior is related to the orbital motion (e.g. LS 5039, LS I +61 303), and in some others it is related to some transient events occurring in the system (e.g. Cygnus X-1, and likely also LS I +61 303 and LS 5039). Based on modeling results or on energetic grounds, it seems difficult to explain the emission in the X-/soft gamma-ray and the TeV bands as coming from the same (i.e. one-zone) region. We also stress the importance of the pair creation phenomena in these systems, which harbor a massive and hot star, for the radio and the X-ray emission, as a secondary pair radiation component may be significant in these energy ranges. Finally, we point out that the presence of the star can indeed have a strong impact on both the nonthermal radiation production and the jet dynamics.

Microquasars (MQs) are X-ray binary systems that display relativistic radio jets. These objects constitute a suitable laboratory for testing high energy astrophysical processes still not well understood, such as those present when jets interact with the interstellar medium (ISM). Focusing on the study of the nonthermal contribution from cocoon and bow-shock regions, we explore, under different ISM densities and ages of the jet source, the possibility to detect MQ jet termination regions. We conclude that emission from these regions may be faint, but still detectable in the radio, X-ray, and gamma-ray bands.

Unlike high-mass gamma-ray binaries, low-mass microquasars lack external sources of radiation and matter that could produce high-energy emission through interactions with relativistic particles. In this work, we consider the synchrotron emission of protons and leptons that populate the jet of a low-mass microquasar. In our model photohadronic and inverse Compton (IC) interactions with synchrotron photons produced by both protons and leptons result in a high-energy tail of the spectrum. We also estimate the contribution from secondary pairs injected through photopair production. The high-energy emission is dominated by radiation of hadronic origin, so we can call these objects "proton microquasars".

We introduce the use of a well-known parameter, the Alfvén Radius, R_A, as a new tool to discern whether an X-ray binary system may undergo a microquasar phase, i.e. ejecting relativistic particles orthogonal to the accretion disk. We study what we call the basic condition, R_A/R_* = 1 in its dependency on the magnetic field strength and the mass accretion rate. With this basic condition we establish under which combination of parameters any class of accreting neutron stars could become a microquasar instead of confining disk-material down to the magnetic poles and creating the two emitting caps typical for an X-ray pulsar. In the case of black-hole accreting binaries we equate the magnetic field pressure to the plasma pressure in the last stable orbit (i.e. R_A/R_LSO = 1) and we get upper limits for the magnetic field strength as a function of the mass accretion rate and the black hole mass.

In high-mass microquasars (HMMQ), strong interactions between jets and stellar winds at binary system scales could occur. In order to explore this possibility, we have performed numerical two-dimensional hydrodynamical simulations of jets crossing the dense stellar material to study how the jet will be affected by these interactions. We find that the jet head generates strong shocks in the wind. These shocks reduce the jet advance speed, and compress and heat up the jet and wind material. In addition, strong recollimation shocks can occur where pressure balance between the jet side and the surrounding medium is reached. All this, together with jet bending, could lead to the destruction of jets with power < 10³⁶erg/s. The conditions around the outflow shocks would be convenient for accelerating particles up to ~ TeV energies. These accelerated particles could emit via synchrotron and inverse Compton (IC) scattering if they were leptons, and via hadronic processes if they were hadrons.

We discuss the evidence for proton loading in relativistic jets from microquasars in light of recent constraints on the jet power. We argue that, both in the case of the Cygnus X-1 jet and the entire ensemble of Galactic microquasars, the evidence points towards a significant contribution to the total kinetic energy flux from cold protons. However, as with all other methods of constraining jet composition (except for the singular case of SS 433), a number of alternative, though maybe less plausible, explanations exist. In light of this continued elusiveness of a single slam-dunk argument for proton loading, the best we can hope for is a continuing accumulation of bits of evidence such as these which will, on the whole, form a preponderance of evidence against pure pair jets.

The high-mass microquasar Cygnus X-1 has been detected during a flaring state at very high energies, E > 200 GeV. The observation was performed by the Atmospheric Cherenkov Telescope MAGIC. It constitutes the first experimental evidence of very-high energy (VHE) emission produced by a Galactic stellar-mass black hole. The observed signal was detected in coincidence with an X–ray flare.

The gamma-ray flare occurred when the compact object was located behind the companion star with respect to the line of sight (superior conjunction of the compact object). In this configuration the absorption of VHE photons by annihilation with the stellar photons is expected to be maximum. This suggests that the emission has been originated far above the compact object. The energy spectrum is well fitted by a relatively soft power-law.

We present a model for the absorption and the emission of VHE gamma-rays in Cyg X-1. Detailed calculations of the gamma-ray opacity due to pair creation are provided and used to establish constraints on the emitting region. We propose that the high energy flare was the result of the interaction between the jet and a very dense clump from the stellar wind.

Since its launch on October 2002, the INTEGRAL satellite has revolutionized our knowledge of the hard X–ray sky thanks to its unprecedented imaging capabilities and source detection positional accuracy above 20 keV. Nevertheless, many of the newly-detected sources in the INTEGRAL sky surveys are of unknown nature. However, the combined use of available information at longer wavelengths (mainly soft X–rays and radio) and of optical spectroscopy on the putative counterparts of these new hard X–ray objects allows pinpointing their exact nature. Continuing our long-standing program running since 2004 (and with which we identified more than 100 INTEGRAL objects) here we report the classification, through optical spectroscopy, of 25 unidentified high-energy sources, mostly belonging to the recently published 4-th IBIS survey.

GRS 1758-258 is one of the two brightest persistent hard X–ray sources in the Galactic Center region, of which an optical/near infrared counterpart has not been unambiguously identified in previous work, mainly due to high absorption and the lack of suitable astrometric stars in early times. In this work, we take advantage of modern star catalogs to reanalyze archival images of the field in the optical and near infrared wavelengths, and compute a new astrometric solution. As a result, a single source is consistent with radio and X–ray sub-arcsec positions, which we propose as the counterpart of GRS 1758-258.

Evidence is presented indicating that in the hard state of Cygnus X-1, the coronal magnetic field might be below equipartition with radiation (suggesting that the corona is not powered by magnetic field dissipation) and that the ion temperature in the corona is significantly lower than what predicted by ADAF like models. It is also shown that the current estimates of the jet power set interesting contraints on the jet velocity (which is at least mildly relativistic), the accretion efficiency (which is large in both spectral states), and the nature of the X–ray emitting region (which is unlikely to be the jet).

Microquasar outbursts are characterized by spectral state transitions. The transitions between states characterized by a hard spectrum and those characterized by a soft spectrum are of particular interest. Besides drastic spectral and timing changes, these transitions often show discrete ejections detectable in the radio domain. The mechanisms giving birth to the ejections, the links with accretion and the exact nature of the ejected material are still largely unknown. We present systematic X–ray spectral analysis prior to the ejection in several microquasars, and show that, in each case, the properties of the corona drastically evolve, while that of the disc remain roughly constant. We intepret this behavior as possibly due to an ejection of the corona at the spectral transition.

We present the first three-dimensional simulations of the evolution of a microquasar jet inside the binary star system. The aim is to study the interaction of these jets with the stellar wind from a massive companion and the possible locations of high-energy emission sites. We have simulated two jets with different injection power in order to give a hint on the minimum power required for the jet to escape the system and become visible in larger scales. In the setup, we include a massive star wind filling the grid through which the jet evolves. We show that jets should have powers of the order of 10³⁷ erg s^-1 or more in order not to be destroyed by the stellar wind. The jet–wind interaction results in regions in which high-energy emission could be produced. These results imply the possible existence of a population of X–ray binaries undetected in the radio band due to jet disruption inside the region dominated by the stellar wind.

ASTROSAT is an astronomy satellite designed for simultaneous multi-wavelength studies in the Optical/UV and a broad X-ray energy range. With four X-ray instruments and a pair of UV-Optical telescopes, ASTROSAT will provide unprecedented opportunity for simultaneous multi-wavelength observations, which is of immense value in study of highly variable sources, especially X-ray binaries and active galactic nuclei. The Large Area X-ray Proportional Counters (LAXPC) of ASTROSAT, which has the largest effective area in the hard X-ray band compared to all previous X-ray missions, will enable high time resolution X-ray measurements in the 2–80 keV band with moderate energy resolution. Here we give a brief summary of the payload characteristics of ASTROSAT and discuss some of the main science topics that will be addressed with the LAXPC, and with simultaneous observations with the UVIT telescopes, with particular emphasis on X-ray binaries and compact objects. The possibility of aiding gravitational wave experiments is also briefly mentioned.

In accreting neutron star (NS) low-mass X-ray binary (LMXB) systems, NS accretes material from its low-mass companion via a Keplerian disk. In a viscous accretion disk, inflows orbit the NS and spiral in due to dissipative processes, such as the viscous process and collisions of elements. The dynamics of accretion flows in the inner region of an accretion disk is significantly affected by the rotation of NS. The rotation makes NS, thus the spacetime metric, deviate from the originally spherical symmetry, and leads to gravitational quadrupole, on one hand. On the other hand, a rotating NS drags the local inertial frame in its vicinity, which is known as the rotational frame-dragging effect. In this paper, we investigate the orbital motion of accretion flows of accreting NS/LMXBs and demonstrate that the rotational effects of NS result in a band of quasi-quantized structure in the inner region of the accretion disk, which is different, in nature, from the scenario in the strong gravity of black hole arising from the resonance for frequencies related to epicyclic and orbital motions. We also demonstrate that such a disk structure may account for frequencies seen in X-ray variability, such as quasi-periodic oscillations (QPOs), and can be a potential promising tool for the investigation of photon polarization.

Neutron-star and black hole X-ray binaries (XRBs) exhibit radio jets, whose properties depend on the X-ray spectral state and history of the source. There is general agreement about the type of the accretion disk around the compact object in the various spectral states. What is missing is a physical explanation for the appearance, disappearance, and re-appearance of jets. We will demonstrate that by invoking a simple physical mechanism proposed more than ten years ago, the so-called Poynting-Robertson Cosmic Battery (PRCB), we can explain in a natural way the disk–jet connection in XRBs.

We study the production of neutrons in the corona of an accreting black hole through the interaction of locally accelerated protons with matter and radiation. A fraction of these neutrons may escape and penetrate into the base of the jet, later decaying into protons. This is a possible mechanism for loading Poynting-dominated outflows with baryons.

We characterize the spatial and energy distribution of neutrons in the corona and that of the protons injected in the jet by neutron decay. We assess the contribution of these protons to the radiative spectrum of the jet. We also investigate the fate of the neutrons that escape the corona into the external medium.

We study a thin warped accretion disk around a spinning black hole in the viscous regime (i.e., α > H/R; α is the Shakura-Sunyaev viscosity parameter, H is the disk thickness and R is the radial distance), and calculate the steady state radial profile of the disk tilt angle for a wide range of relevant parameters of the system (like the Kerr parameter of the black hole). Although the inner part of such a disk was proposed to become aligned with the spin direction of the black hole by the Bardeen-Petterson effect, we show that for a reasonable range of the parameters of the system, the inner disk can stay significantly tilted with respect to the black hole spin. A tilt in the inner accretion disk can affect the observed X-ray spectral and timing features, and hence it makes the inner accretion disk particularly useful for probing the strong gravity region.