Bimodal distribution of short gamma-ray bursts: Evidence for two distinct types of short gamma-ray bursts

GRB 170817A was confirmed to be associated with GW170817, which was produced by a neutron star - neutron star merger. It indicates that at least some short gamma-ray bursts come from binary neutron star mergers. Theoretically, it is widely accepted that short gamma-ray bursts can be produced by two distinctly different mechanisms, binary neutron star mergers and neutron star - black hole mergers. These two kinds of bursts should be different observationally due to their different trigger mechanisms. Motivated by this idea, we collect a universal data set constituted of 51 short gamma-ray bursts observed by Swift/BAT, among which 14 events have extended emission component. We study the observational features of these 51 events statistically. It is found that our samples consist of two distinct groups. They clearly show a bimodal distribution when their peak photon fluxes at 15–150 keV band are plotted against the corresponding fluences. Most interestingly, all the 14 short bursts with extended emission lie in a particular region of this plot. When the fluences are plotted against the burst durations, short bursts with extended emission again tend to concentrate in the long duration segment. These features strongly indicate that short gamma-ray bursts really may come from two distinct types of progenitors. We argue that those short gamma-ray bursts with extended emission come from the coalescence of neutron stars, while the short gamma-ray bursts without extended emission come from neutron star - black hole mergers.     

Studying the SN‐GRB connection with X‐shooter: The GRB 100316D / SN 2010bh case

During the last ten years, observations of long‐duration gamma‐ray bursts brought to the conclusion that at least a fraction of them is associated with bright supernovae of type Ib/c. In this talk, after a short review on the previously observed GRB‐SN connection cases, we present the recent case of GRB 100316/SN 2010bh. In particular, during the observational campaign of SN 2010bh, a pivotal role was played by VLT/X‐shooter, sampling with unique high quality data the spectral energy distribution of the early evolution phases from the UV to the K band (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relativistic blue‐ and red‐shifted absorption lines in AGNs

Current, accumulating evidence for (mildly) relativistic blue‐ and red‐shifted absorption lines in AGNs is reviewed. XMM‐Newton and Chandra sensitive X‐ray observations are starting to probe not only the kinematics (velocity) but also the dynamics (accelerations) of highly ionized gas flowing in‐and‐out from, likely, a few gravitational radii from the black hole. It is thus emphasized that X‐ray absorption‐line spectroscopy provides new potential to map the accretion flows near black holes, to probe the launching regions of relativistic jets/outflows, and to quantify the cosmological feedback of AGNs. Prospects to tackle these issues with future high energy missions are briefly addressed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Constraining the neutron star equation of state using XMM‐Newton

We have identified three possible ways in which future XMM‐Newton observations can provide significant constraints on the equation of state of neutron stars. First, using a long observation of the neutron star X‐ray transient Cen X‐4 in quiescence one can use the RGS spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X‐ray spectral fitting of the pn and MOS spectra and allows us to investigate whether the variability observed in the quiescent X‐ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N_H. This will test whether the soft thermal spectral component can indeed be due to the hot thermal glow of the neutron star. Potentially such an observation could also reveal redshifted spectral lines from the neutron star surface. Second, XMM‐Newton observations of radius expansion type I Xray bursts might reveal redshifted absorption lines from the surface of the neutron star. Third, XMM‐Newton observations of eclipsing quiescent low‐mass X‐ray binaries provide the eclipse duration. With this the system inclination can be determined accurately. The inclination determined from the X‐ray eclipse duration in quiescence, the rotational velocity of the companion star and the semi‐amplitude of the radial velocity curve determined through optical spectroscopy, yield the neutron star mass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron star high‐mass binaries as the origin of SGR/AXP

A close high‐mass binary system consisting of a neutron star (NS) and a massive OB supergiant companion is expected to lead to a Thorne‐Żytkow object (TZO) structure, which consists of a NS core and a stellar envelope. We use the scenario machine program to calculate the formation tracks of TZOs in close high‐mass NS binaries and their subsequent evolution. We propose and demonstrate that the explosion and instant contraction of a TZO structure leave its stellar remnant as a soft gamma‐ray repeater and an anomalous X‐ray pulsar respectively. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gravitational and electromagnetic emission by magnetized coalescing binary systems

We discuss the possibility to obtain an electromagnetic emission accompanying the gravitational waves emitted in the coalescence of a compact binary system. Motivated by the existence of black hole configurations with open magnetic field lines along the rotation axis, we consider a magnetic dipole in the system, the evolution of which leads to (i) electromagnetic radiation, and (ii) a contribution to the gravitational radiation, the luminosity of both being evaluated. Starting from the observations on magnetars, we impose upper limits for both the electromagnetic emission and the contribution of the magnetic dipole to the gravitational wave emission. Adopting this model for the evolution of neutron star binaries leading to short gamma ray bursts, we compare the correction originated by the electromagnetic field to the gravitational waves emission, finding that they are comparable for particular values of the magnetic field and of the orbital radius of the binary system. Finally we calculate the electromagnetic and gravitational wave energy outputs which result comparable for some values of magnetic field and radius.     

Oscillations in radiation-driven outflows

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.     

XMM‐Newton and broad iron lines

Iron line emission is common in the X‐ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X‐ray irradiation of the accretion flow by the power‐law X‐ray continuum produces the X‐ray ‘reflection’ spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM‐Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GRB‐selected galaxies

We describe the current status and recent results from our Swift/VLT legacy survey, a VLT Large Programme aimed at characterizing the host galaxies of a homogeneously selected sub‐sample of Swift gamma‐ray bursts (GRBs). The immediate goals are to determine the host luminosity function, study the effects of reddening, determine the fraction of Lyα emitters in the hosts, and obtain redshifts for targets without a reported one. We have defined a very carefully selected sample, obeying strict and well‐defined criteria: 68 targets in total. Among the preliminary results is a large optical detection rate, the lack of extremely red objects (only one possible case in the sample), and 10 new GRB redshifts with the mean redshift of the host sample assessed to be 〈z 〉 ≳ 2 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Forced oscillations in fluid tori and quasi‐periodic oscillations

The kilo‐Hertz Quasi‐Periodic Oscillations in X‐ray binaries could originate within the accretion flow, and be a signature of non–linear fluid oscillations and mode coupling in strong gravity. The possibility to decipher these systems will impact our knowledge of fundamental parameters such as the neutron star mass, radius, and spin. Thus they offer the possibility to constrain the nuclear equation of state and the rotation parameter of stellar–mass black holes. We review the general properties of these oscillations from a hydrodynamical point of view, when the accretion flow is subject to external perturbations and summarize recent results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The science case of the PEPSI high‐resolution echelle spectrograph and polarimeter for the LBT

We lay out the scientific rationale for and present the instrumental requirements of a high‐resolution adaptiveoptics Echelle spectrograph with two full‐Stokes polarimeters for the Large Binocular Telescope (LBT) in Arizona. Magnetic processes just like those seen on the Sun and in the space environment of the Earth are now well recognized in many astrophysical areas. The application to other stars opened up a new field of research that became widely known as the solarstellar connection. Late‐type stars with convective envelopes are all affected by magnetic processes which give rise to a rich variety of phenomena on their surface and are largely responsible for the heating of their outer atmospheres. Magnetic fields are likely to play a crucial role in the accretion process of T‐Tauri stars as well as in the acceleration and collimation of jet‐like flows in young stellar objects (YSOs). Another area is the physics of active galactic nucleii (AGNs) , where the magnetic activity of the accreting black hole is now believed to be responsible for most of the behavior of these objects, including their X‐ray spectrum, their notoriously dramatic variability, and the powerful relativistic jets they produce. Another is the physics of the central engines of cosmic gamma‐ray bursts, the most powerful explosions in the universe, for which the extreme apparent energy release are explained through the collimation of the released energy by magnetic fields. Virtually all the physics of magnetic fields exploited in astrophysics is somehow linked to our understanding of the Sun's and the star's magnetic fields. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The correlations and anticorrelations in QPO data

Double peak kHz QPO frequencies in neutron star sources varies in time by a factor of hundreds Hz while in microquasar sources the frequencies are fixed and located at the line ν ₂ = 1.5ν ₁ in the frequency‐frequency plot. The crucial question in the theory of twin HFQPOs is whether or not those observed in neutron‐star systems are essentially different from those observed in black holes. In black hole systems the twin HFQPOs are known to be in a 3:2 ratio for each source. At first sight, this seems not to be the case for neutron stars. For each individual neutron star, the upper and lower kHz QPO frequencies, ν ₂ and ν ₁, are linearly correlated, ν ₂ = Aν ₁ + B , with the slope A < 1.5, i.e., the frequencies definitely are not in a 1.5 ratio. In this contribution we show that when considered jointly on a frequency‐frequency plot, the data for the twin kHz QPO frequencies in several (as opposed to one) neutron stars uniquely pick out a certain preferred frequency ratio that is equal to 1.5 for the six sources examined so far. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

QPO as the Rosetta Stone for understanding black hole accretion

Quasi‐periodic oscillations (QPO) seen in the X‐ray fluxes of individual neutron stars and black hole sources are one of most intriguing phenomena in today's astrophysics. The QPO nature is visibly determined by super‐strong Einstein's gravity. I argue here that it also profoundly depends on the MRI turbulence in accretion flows. Understanding the QPO physics may therefore guide accretion theory out of its present state of confusion. Readers will find here an up‐to‐date, comprehensible account of what is known, and what is not, about the QPO physics. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical analysis for the Q‐factor of twin kHz QPOs

Using the recently published data of twin kHz quasi‐period oscillations (QPOs) in neutron stars of low‐mass X‐ray binaries (LMXBs), we study the different profiles between bright Z sources and less luminous Atoll sources. The quality factors of upper kHz QPOs show a narrow distribution both for Z sources and Atoll sources, which concentrate at 7.98 and 9.75, respectively. The quality factors of lower kHz QPOs show a narrow distribution for Z sources and a broader distribution for Atoll sources, which concentrate at 5.25 and 86.22, respectively. In order to investigate the relation between the quality factor and the peak frequency of kHz QPOs, we fit the data with power‐law, linear, and exponential functions, respectively. There is an obvious trend that the quality factors increase with the peak frequencies both for upper and lower QPOs. The implications of our results are discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relativistic Fe Kα features in the XMM‐Newton spectrum of the intermediate Seyfert galaxy 4U 1344‐60

We present the analysis of optical and X‐ray XMM‐Newton data of the source 4U 1344‐60. On the basis of the optical data we propose to classify 4U 1344‐60 as a Seyfert 1.5 galaxy and we measured a redshift value z = 0.012 ± 0.001. The observed X‐ray spectrum is complex. The continuum emission can be described as a power law obscured by two neutral absorption components. 4U 1344‐60 exhibits a broad and skewed iron line at 6.4 keV most likely originated in a few gravitational radius of an accretion disc. The analysis also reveals the presence of two narrow emission line‐like features at ∼4.9 keV and ∼5.3 keV. Assuming that hot spots on the surface of the accretion disc, orbiting very close to the black hole is responsible of these emission lines, the accretion disc would present an inclination of 20° and the active regions would be located in the 6–10 R _g radius range. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic fields of coalescing neutron stars and the luminosity function of short gamma-ray bursts

Coalescing binary neutron stars are the most promising candidates for detection by gravitational-wave detectors and are considered to be most promising for explaining the phenomenon of short gamma-ray bursts. The magnetic fields of neutron stars during their coalescence can produce a number of interesting observational manifestations and can affect significantly the shape of the gravitationalwave signal. In this paper, we model the distribution of magnetic fields in coalescing neutron stars by the population synthesis method using various assumptions about the initial parameters of the neutron stars and the evolution laws of their magnetic fields. We discuss possible electromagnetic phenomena preceding the coalescence of magnetized neutron stars and the effect of magnetic field energy on the shape of the gravitational-wave signal during the coalescence. For a log-normal (Gaussian in logarithm) distribution of the initialmagnetic fields consistent with the observations of radio pulsars, the distribution inmagnetic field energy during the coalescence is shown to describe adequately the observed luminosity function of short gamma-ray bursts under various assumptions about the pattern of field evolution and initial parameters of neutron stars.     

The X‐ray source population of the Andromeda galaxy M 31

First studies of the X‐ray source population of M 31 were performed with the Einstein Observatory and ROSAT. High resolution Chandra Observatory images not only spatially resolved the center area but also supernova remnants (SNRs) in the galaxy. Source catalogues of restricted areas were presented with high astrometric accuracy. Also luminosity function studies and studies of individual sources based on Chandra and XMM‐Newton observations led to a better knowledge of the X‐ray source population. An XMM‐Newton source catalog based on archival observations revealed more than 850 sources down to a 0.2–4.5 keV luminosity of 10³⁵ erg s^–1. EPIC hardness ratios as well as informations from earlier X‐ray, optical, and radio catalogues were used to distinguish between different source classes (SNRs, supersoft sources (SSSs), X‐ray binaries (XRBs), globular cluster sources within M 31, and foreground stars and objects in the background). However, many sources could only be classified as “hard”. These sources may either be XRBs or Crab‐like SNRs in M 31 or background sources. Two of the globular cluster sources could be identified as low mass XRBs with a neutron star as compact object as they showed type I X‐ray bursts. Many of the SSSs were identified as optical novae. Inspired by these results an XMM‐Newton survey of the entire D₂₅ disk of M 31 and a dedicated program to monitor X‐ray counterparts of optical novae in M 31 was started. We discuss implications for further nearby galaxy studies. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The synergy of the cosmic ray and high energy atmospheric physics: Particle bursts observed by arrays of particle detectors

Particle bursts detected on the earth's surface during thunderstorms by various particle detectors originated from the relativistic runaway electron avalanches (RREAs) initiated by free electrons accelerated in the strong atmospheric electric fields. Two oppositely directed dipoles in the thundercloud accelerate electrons in the direction of the earth's surface, and to the open space. The particle bursts observed by orbiting gamma ray observatories are called terrestrial gamma ray flashes (TGFs, with energies of several MeV, only sometimes reaching tens of MeV); ones registered by particle detectors located on the ground – are called thunderstorm ground enhancements (TGEs, with energies, usually reaching 40-50 MeV). Balloons and aircraft in the troposphere register gamma ray glows (with energies of several MeV). Recently, high-energy atmospheric physics includes also, so-called, downward TGFs (DTGFs), intense particle bursts with a duration of a few milliseconds.Well-known extensive air showers (EASs) originate from the interactions of galactic protons and fully-stripped nuclei with the atmosphere atoms. EAS particles have very dense cores around the shower axes. However, high-energy particles in the EAS cores comprise a very thin disc of (a few tens of ns), and a particle detector traversed by an EAS core will not register a particle burst, but only one very large pulse. Only neutron monitor, by collecting delayed thermal neutrons from EAS core particle interactions with soil, can register particle bursts. We discuss the relation between short particle bursts available from the largest particle arrays with EAS phenomena. We demonstrate that the neutron monitors can extend the EAS “lifetime” up to a few milliseconds, a time comparable with DTGFs duration. The possibility to use the network of neutron monitors for high-energy cosmic ray research is also deliberated.Plain Language Summary: Short and extended particle bursts are registered in space, the troposphere, and the earth's surface. Coordinated monitoring of the particle fluxes, near-surface electric fields, and lightning flashes makes it possible to formulate a hypothesis on the origin of intense bursts and their relation to extensive air showers and atmospheric discharges. Analysis of the observational data and possible origination scenarios of particle bursts allows us to conclude that the bursts can be explained by the electron acceleration in the thunderous atmosphere and by gigantic showers developed in the terrestrial atmosphere by high-energy protons and fully-stripped nuclei accelerated in Galaxy.     

The iron emission line complex of MCG‐5‐23‐16: the long XMMNewton look

We present the results of the simultaneous XMM‐Newton and Chandra observations of the bright Seyfert 1.9 galaxy MCG–5‐23‐16, which is one of the best known examples of a relativistically broadened iron Kα line. We find that: a) the soft X‐ray emission is likely to be dominated by photoionized gas, b) the complex iron emission line is best modelled with a narrow and a broad component with a FWHM ∼44000 km/s. This latter component has an EW ∼50 eV and its profile is well described with an emission line mainly originating from the accretion disk a few tens of gravitational radii from the central black hole and viewed with an inclination angle ∼40°. We found evidence of a possible sporadic absorption line at ∼7.7 keV which, if associated with Fe XXVI Kα resonance absorption, is indicative of a possible high velocity (v ∼ 0.1c) outflow. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)