Institute For Astronomie Und Astrophysik

Sand, Germany

Institute For Astronomie Und Astrophysik

Sand, Germany
SEARCH FILTERS
Time filter
Source Type

Nolan S.J.,Durham University | Puhlhofer G.,University of Heidelberg | Puhlhofer G.,Institute For Astronomie Und Astrophysik | Rulten C.B.,Durham University
Astroparticle Physics | Year: 2010

The current generation of imaging atmospheric Cherenkov telescopes are allowing the sky to be probed with greater sensitivity than ever before in the energy range around and above 100 GeV. To minimise the systematic errors on derived fluxes a full calibration of the atmospheric properties is important given the calorimetric nature of the technique. In this paper we discuss an approach to address this problem by using a ceilometer co-pointed with the H.E.S.S. telescopes and present the results of the application of this method to a set of observational data taken on the active galactic nucleus (AGN) PKS 2155-304 in 2004. © 2010 Elsevier B.V. All rights reserved.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Tsygankov S.,University of Turku | Santangelo A.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2016

We present the updated orbital solution for the transient Be X-ray binary V 0332+53, which we obtained by complementing historical measurements with the data from the gamma-ray burst monitor onboard Fermi. These were acquired during the outburst in June-October 2015. We modeled the observed changes in the spin-frequency of the pulsar and deduced the orbital parameters of the system. We significantly improved existing constrains and show that contrary to previous findings, no change in orbital parameters is required to explain the spin evolution of the source during the outbursts in 1983, 2005, and 2015. The reconstructed intrinsic spin-up of the neutron star during the latest outburst is found to be comparable with previously observed values and predictions of the accretion torque theory. © ESO, 2016.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Ducci L.,Institute For Astronomie Und Astrophysik | Ducci L.,University of Geneva
Astronomy and Astrophysics | Year: 2015

Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large when compared to the Eddington limit for isotropic accretion onto stellar-mass object. Most often either supercritical accretion onto stellar mass black holes or accretion onto intermediate mass black holes is invoked the high luminosities of ULXs. However, the recent discovery of coherent pulsations from ULX M82 X-1 with NuStar has shown that another scenario implying accretion onto a magnetized neutron star is possible for ULXs. Motivated by this discovery, we re-visited the available XMM-Newton archival observations of several bright ULXs with a targeted search for pulsations to check whether accreting neutron stars might power other ULXs as well. We have found no evidence for significant coherent pulsations in any of the sources including the M82 X-2. We provide upper limits for the amplitude of possibly undetected pulsed signals for the sources in the sample. © ESO, 2015.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Kreykenbohm I.,Dr. Karl Remeis Sternwarte | Kreykenbohm I.,Erlangen Center for Astroparticle Physics | Doroshenko R.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2012

We present an analysis of the spectral properties of the peculiar X-ray pulsar X Per based on INTEGRAL observations. We show that the source exhibits an unusually hard spectrum and is confidently detected by ISGRI up to more than 100 keV. We find that two distinct components may be identified in the broadband 4-200 keV spectrum of the source. We interpret these components as the result of thermal and bulk Comptonization in the vicinity of the neutron star and describe them with several semi-phenomenological models. The previously reported absorption feature at ≈30a keV is not required in the proposed scenario and therefore its physical interpretation must be taken with caution. We also investigated the timing properties of the source in the framework of existing torque theory, concluding that the observed phenomenology can be consistently explained if the magnetic field of the neutron star is ∼10 14G. © 2012 ESO.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Suleimanov V.,Institute For Astronomie Und Astrophysik | Suleimanov V.,Kazan Federal University
Astronomy and Astrophysics | Year: 2011

We present an analysis of the Vela X-1 "off-states" based on Suzaku observations taken in June 2008. Defined as states in which the flux suddenly decreases below the instrumental sensitivity, these "off-states" have been interpreted by several authors as the onset of the "propeller regime". For the first time ever, however, we find that the source does not turn off and, although the flux drops by a factor of 20 during the three recorded "off-states", pulsations are still observed. The spectrum and the pulse profiles of the "off-states" are also presented. We discuss our findings in the framework of the "gated accretion" scenario and conclude that most likely the residual flux is due to the accretion of matter leaking through the magnetosphere by means of Kelvin-Helmholz instabilities (KHI). © 2011 ESO.


Hertfelder M.,Institute For Astronomie Und Astrophysik | Kley W.,Institute For Astronomie Und Astrophysik | Suleimanov V.,Institute For Astronomie Und Astrophysik | Suleimanov V.,Kazan Federal University | Werner K.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2013

Context. Disk accretion onto stars leads to the formation of a boundary layer (BL) near the stellar surface where the disk makes contact with the star. Although a large fraction of the total luminosity of the system originates from this tiny layer connecting the accretion disk and the accreting object, its structure has not been fully understood yet. Aims. It is the aim of this work to obtain more insight into the BL around the white dwarf in compact binary systems. There are still many uncertainties concerning the extent and temperature of the BL and the rotation rate of the white dwarf. Methods. We perform numerical hydrodynamical simulations, where the problem is treated in a one-dimensional, radial approximation (slim disk). The turbulence is described by the α parameter viscosity. We include both cooling from the disk surfaces and radial radiation transport. The radiation energy is treated in a one-temperature approximation. Results. For a given M our results show a strong dependence on the stellar mass and rotation rate. The midplane and the effective temperature rise considerably with increasing stellar mass or decreasing stellar rotation rate. Our simulations also show that the radiation energy and pressure are indeed important in the BL. However, some models show a low optical depth in the BL, making it necessary to find a better representation for optically thin regions. Conclusions. The combination of a high mass and a small radius, characteristic of white dwarfs, can lead to an enormous energy release in the BL, provided the WD rotates slowly. Since the radial extent of BLs is typically very small (about 0.02 to 0.05 R*), this leads to surface temperatures of a few hundred thousand Kelvin. All of our models showed subsonic infall velocities with Mach numbers of <0.4 at most. © ESO, 2013.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Ducci L.,Institute For Astronomie Und Astrophysik | Klochkov D.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2012

We have investigated the dipping activity observed in the high-mass X-ray binary 4U 1907+09 and shown that the source continues to pulsate in the "off" state, noting that the transition between the "on" and "off" states may be either dip-like or flare-like. This behavior may be explained in the framework of the "gated accretion" scenario proposed to explain the flares in supergiant fast X-ray transients (SFXTs). We conclude that 4U 1907 + 09 might prove to be a missing link between the SFXTs and ordinary accreting pulsars. © 2012 ESO.


Doroshenko V.,Institute For Astronomie Und Astrophysik | Ducci L.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Sasaki M.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2014

The population of the Galactic X-ray binaries has been mostly probed with moderately sensitive hard X-ray surveys so far. The eRosita mission will provide for the first time a sensitive all-sky X-ray survey in the 2-10 keV energy range, where the X-ray binaries emit most of the flux and discover the still unobserved low-luminosity population of these objects. In this paper, we briefly review the current constraints for the X-ray luminosity functions of high- and low-mass X-ray binaries and present our own analysis based the INTEGRAL 9-year Galactic survey, which yields improved constraints. Based on these results, we estimate the number of new XRBs to be detected in the eRosita all-sky survey. © 2014 ESO.


Vasco D.,Institute For Astronomie Und Astrophysik | Klochkov D.,Institute For Astronomie Und Astrophysik | Staubert R.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2011

Studies of some bright, super-Eddington transient pulsars show a negative correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the bolometric luminosity. For Her X-1, using repeated RXTE observations during 1996-2005, the inverse dependence was found: the energy of the cyclotron line increases as the luminosity increases. The X-ray flux measured by the RXTE/ASM (2-10keV) has been assumed to represent the luminosity-more precisely: the maximum X-ray flux reached during the respective 35d Main-On. Here, we question whether the ASM flux is really an accurate measure of the bolometric luminosity of the source. We redetermined the energy of the cyclotron line and performed spectral fits using the combined data from the PCA (3.5-60keV) and HEXTE (20-75keV) instruments on RXTE of the same 35d cycles as used in the original work to determine the bolometric flux from those spectra. We confirm the result of the original analysis that the cyclotron line energy changes by ~7% for a change in flux by a factor of two. © 2011 ESO.


Hertfelder M.,Institute For Astronomie Und Astrophysik | Kley W.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2015

Context. Disk accretion onto weakly magnetized stars leads to the formation of a boundary layer (BL) where the gas loses its excess kinetic energy and settles onto the star. There are still many open questions concerning the BL, for instance the transport of angular momentum (AM) or the vertical structure. Aims. It is the aim of this work to investigate the AM transport in the BL where the magneto-rotational instability (MRI) is not operating owing to the increasing angular velocity Ω(r) with radius. We will therefore search for an appropriate mechanism and examine its efficiency and implications. Methods. We perform 2D numerical hydrodynamical simulations in a cylindrical coordinate system (r,φ) for a thin, vertically integrated accretion disk around a young star. We employ a realistic equation of state and include both cooling from the disk surfaces and radiation transport in radial and azimuthal direction. The viscosity in the disk is treated by the α-model; in the BL there is no viscosity term included. Results. We find that our setup is unstable to the sonic instability which sets in shortly after the simulations have been started. Acoustic waves are generated and traverse the domain, developing weak shocks in the vicinity of the BL. Furthermore, the system undergoes recurrent outbursts where the activity in the disk increases strongly. The instability and the waves do not die out for over 2000 orbits. Conclusions. There is indeed a purely hydrodynamical mechanism that enables AM transport in the BL. It is efficient and wave mediated; however, this renders it a non-local transport method, which means that models of a effective local viscosity like the α-viscosity are probably not applicable in the BL. A variety of further implications of the non-local AM transport are discussed. © ESO, 2015.

Loading Institute For Astronomie Und Astrophysik collaborators
Loading Institute For Astronomie Und Astrophysik collaborators