November 2013

Postdoctoral Position in High Energy Astrophysics is available in our group

Please, see http://jobregister.aas.org/job_view?JobID=46447 for details.
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September 2013

On the origin of cyclotron lines in the spectra of X-ray pulsars

Cyclotron resonance scattering features are observed in the spectra of some X-ray pulsars and show significant changes in the line energy with the pulsar luminosity. In a case of bright sources, the line centroid energy is anti-correlated with the luminosity. Such a behaviour is often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, and it makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface. The idea is based on the facts that a substantial part of column luminosity is intercepted by the neutron star surface and the reflected radiation should contain absorption features. The reflection model was developed and applied to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities. The model has profound implications for the interpretation of the data on the cyclotron line observed in X-ray pulsars.
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August 2013

Open Call for 7 four-year doctoral student positions in exact sciences at the University of Oulu

The Doctoral Programme in Exact Sciences (Exactus–DP) at University of Oulu, Finland invites applications for 7 four-year doctoral student positions in the fields of astronomy, physics, chemistry, mathematics and statistics.
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July 2013

The European Week of Astronomy and Space Science (EWASS) 2013

The European Week of Astronomy and Space Science (EWASS-2013) held on 8 - 13 July 2013 in Logomo Centre in Turku, Finland. EWASS is the annual meeting of the EAS. A few special sessions and symposia were organized by our group.
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June 2013

High energy astrophysics group has joined MAGIC collaboration

The MAGIC telescopes at La Palma island is one of the largest Cherenkov telescopes in the world. It operates at energies above 25 GeV and detects very high energy photons that produce cascades in the atmosphere and as a consequence Cherenkov light. Majority of the sources that MAGIC detects are active galaxies, pulsars and pulsar wind nebulae. The Oulu high-energy astrophysics group has applied in 2013 to join MAGIC collaboration.
The collaboration board has accepted our application during its meeting in Turku, Finland on 12 June 2013.
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May 2013

The peculiar cataclysmic variable FS Aurigae with a magnetic and freely precessing white dwarf

In this work based on two extensive sets of optical photometric observations and three X-ray data sets we show that the observed properties of the peculiar dwarf nova FS Aur closely resemble those of other intermediate polars, confirming this cataclysmic variable as a member of the class. Thus, FS Aur is the first cataclysmic variable which received an observational confirmation to contain a magnetic and freely precessing white dwarf.
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April 2013

Visit of Prof. Dmitrij Nagirner

Prof. Dmitrij I. Nagirner from St. Petersburg University, Russia, is visiting the group for three months. He is an expert on radiative transfer and formation of spectral lines as well on theories of kinetic equations. His recent work is devoted to Compton scattering in super-strong magnetic field of magnetars. Prof. Nagirner is also a former supervisor of three member of the HEA group: its leader, Prof. Juri Poutanen, as well as two PhD students Alexandra Veledina and Alexander Mushtukov. On April 19, 2013, Prof. Nagirner turns 75 years and on that occasion the HEA group organizes a 1-day workshop on high-energy astrophysics. The workshop takes place actually two days before, on April 17.
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April 2013

Ultraluminous X-ray sources in the Antennae galaxies are associated with young stellar clusters

Using accurate astrometry from the Hubble Space Telescope and Chandra satellite we showed that the ultraluminous X-ray sources (ULX) in the star-forming colliding Antennae galaxies are closely associated with young stellar clusters. With the help of the data obtained by us using VIMOS Integral Field Unit on Melipal Very Large Telescopes at ESO, we were able to measure the ages of the clusters that turned out to be below 6 Myr. This implies that the ULX progenitor masses certainly exceed 30 and for some objects are closer to 100 solar masses. These findings are consistent with the idea that majority of ULXs are massive X-ray binaries that have been ejected in the process of formation of stellar clusters by a few-body encounters.
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March 2013

Population of persistent high-mass X-ray binaries in the Milky Way

This work presents results of the study of persistent high-mass X-ray binaries (HMXBs) in the Milky Way, obtained from the deep INTEGRAL Galactic plane survey. This survey provides us a new insight into the population of HMXBs because almost half of the whole sample consists of sources discovered with INTEGRAL. It is demonstrated for the first time that the majority of persistent HMXBs have supergiant companions and their luminosity function steepens somewhere around ~2 × 1036 erg s-1. We show that the spatial density distribution of HMXBs correlates well with the star formation rate distribution in the Galaxy. The vertical distribution of HMXBs has a scale-height h ≃ 85 pc, that is somewhat larger than the distribution of young stars in the Galaxy. We propose a simple toy model, which adequately describes general properties of HMXBs in which neutron stars accrete a matter from the wind of its companion (wind-fed NS-HMXBs population). Using the elaborated model we argue that a flaring activity of the so-called supergiant fast X-ray transients, the recently recognized sub-sample of HMXBs, is likely related with the magnetic arrest of their accretion.
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Febrauary 2013

Young rotation-powered pulsars as ultra-luminous X-ray sources

The nature of Ultraluminous X-ray sources (ULX) in nearby galaxies is a matter of heated debates. There are two main competing models: intermediate mass black holes (IMBH) and stellar-mass black holes accreting at super-Eddington rates. Here we show that young pulsars can contribute significantly to the ULX population.
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January 2013

Hot accretion flow in black hole binaries: a link connecting X-rays to the infrared

Multiwavelength observations of galactic black hole transients have opened a new path to understanding the physics of the innermost parts of the accretion flows. While the processes giving rise to their X-ray continuum have been studied extensively, the emission in the optical and infrared (OIR) energy bands was less investigated and remains poorly understood. The standard accretion disc, which may contribute to the flux at these wavelengths, is not capable of explaining a number of observables: the infrared excesses, fast OIR variability and a complicated correlation with the X-rays. It was suggested that these energy bands are dominated by the jet emission, however, this scenario does not work in a number of cases. We suggest here an alternative, namely that most of the OIR emission is produced by the extended hot accretion flow.
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December 2012

Jari Kajava defended his PhD thesis "X-raying compact stars in the Galaxy and beyond"

On December 14, 2012 Jari Kajava defended his PhD thesis "X-raying compact stars in the Galaxy and beyond". The thesis is devoted to the studies of the ultra-luminous X-ray sources in external galaxies and to the accreting millisecond pulsars. These two types of objects are among the most intriguing in the modern high-energy astrophysics.
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November 2012

Visit to the Centre for Underground Physics in Pyhäsalmi

Astronomy division of the University of Oulu visited the Centre for Underground Physics in Pyhäsalmi (CUPP) on November 7, 2012. CUPP is an underground physics laboratory located in the deepest metal mine of Europe, the Pyhäsalmi Mine (1,444 m deep). The centre is currently (2012) hosting the EMMA experiment, and it is a candidate site for the proposed European LAGUNA-observatory.
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October 2012

Radioactive decay of titanium powers supernova remnant

The first direct detection of radioactive titanium associated with supernova remnant 1987A has been made by ESA’s Integral space observatory. This discovery has been made by the international team of astronomers (including Sergey Tsygankov - staff member of the University of Oulu and FINCA) and published recently in Nature. The radioactive decay has likely been powering the glowing remnant around the exploded star for the last 20 years. The total estimated mass of titanium-44 that must have been produced just after the core collapse of SN1987A’s progenitor star amounted to 0.03% of the mass of our own Sun.
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September 2012

Deep X-ray Survey of the Large Magellanic Cloud

In 2003-2012, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observatory has performed long-term observations of the Large Magellanic Cloud (LMC). At present, this is one of the deepest hard X-ray (20-60 keV) surveys of extragalactic fields. The large exposure ∼7 Ms spent observing this region allowed to detect more than twenty sources: ten belonging to the LMC itself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and others owning to other galaxies from the Local Group - the Milky Way and Small Magellanic Cloud.
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August 2012

The deepest X-ray picture of the Milky Way

The deepest ever image of the central area of the Milky Way in the hard X-rays (17-60 keV) has been produced. The majority of the X-ray light emitting sources are black holes and neutron stars. A large fraction of them are situated close to the Galactic Centre and the Galactic plane, with the distribution following well the structure of the Milky Way marked on the picture by green contours.
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July 2012

Strong outburst activity of the X-ray pulsar X Persei during 2001-2011

Results of a comprehensive analysis of the X-ray pulsar X Persei over the period 1996–2011 are now published in the Monthly Notices of the Royal Astronomical Society. Work done by a collaboration of russian, finnish and irish scientists, including staff member of FINCA and University of Oulu Sergey Tsygankov. Using data from the Rossi X-ray Timing Explorer (RXTE) and Swift, several consecutive outbursts were detected, during which the source luminosity increased by a factor of ∼5 up to LX≃ 1.2 × 1035 erg s−1. With the help of data from the International Gamma-Ray Astrophysics Laboratory (Integral), the highly significant cyclotron absorption line in the source spectrum was registered and, for the first time, detected a significantly hard X-ray emission from the pulsar up to ∼160 keV.
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May 2012

Relativistic kinetic equation for Compton scattering of polarized radiation in a strong magnetic field

Staff members of the University of Oulu with collaborators from Russia derived the relativistic kinetic equation for Compton scattering of polarized radiation in strong magnetic field using the Bogolyubov method. The induced scattering and the Pauli exclusion principle are taken into account. The electron polarization is also considered in the general form of the kinetic equation. The special forms of the equation for the cases of the non-polarized electrons, the rarefied electron gas and the two polarization mode description of radiation are found. The derived equations are valid for any photon and electron energies and the magnetic field strength below about 1016 G. These equations provide the basis for formulation of the equation for polarized radiation transport in atmospheres and magnetospheres of strongly magnetized neutron stars.
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April 2012

Discovery of a cyclotron absorption line in the persistent Be/X-ray pulsar RX J0440.9+4431

In a paper published in the Monthly Notices of the Royal Astronomical Society, staff member of FINCA and University of Oulu Sergey Tsygankov and collaborators from Germany and Russia have investigated X-ray pulsar RX J0440.9+4431. Spectral analysis of data obtained with INTEGRAL during the 2010 September outburst revealed a ~32 keV cyclotron resonant scattering feature in the source spectrum, that allowed to estimate the magnetic field strength of the neutron star as B = 3.2 x 1012 G . It makes RX J0440.9+4431 only the second persistent Be/XRP system (after X Persei) with known magnetic field strength and orbital period.
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December 2011

Photospheric radius expansion X-ray burst constrains the neutron star equation of state

Photospheric radius expansion (PRE) X-ray bursts are powerful probes of the neutron star (NS) parameters. During their cooling stage, when the photosphere is at the NS surface, the evolution of the apparent NS radius can be explained only by evolution of the color correction factor describing how the NS spectrum deviates from the black body. Using recently developed NS atmosphere models and theoretically determined color correction factors, we have fitted the spectral evolution of the PRE burst from NS 4U 1724-307 and constrained the NS radius to be above 14 km. This result is a strong argument in favor of the stiff equation of state of cold dense matter.
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November 2011

Discovery of simultaneous accretion disc and pulse profile variations in accreting millisecond pulsar SAX J1808.4-3658

Accreting millisecond pulsars show significant variability of their pulse profiles, especially at low accretion rates. On the other hand, their X-ray spectra are remarkably similar with not much variability over the course of the outbursts. For the first time, we have discovered that during the 2008 outburst of SAX J1808.4−3658 a major pulse profile change was accompanied by a dramatic variation of the disc luminosity at almost constant total luminosity. These findings put strong constraints on the theories of accretion on to magnetized neutron stars, and allow to better understand the accretion geometry as well as the physics of disc–magnetosphere coupling.
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October 2011

Fermi spectra of blazar 3C 454.3 reveal the location of the gamma-ray emitting region within the broad-line region

We study spectral properties of the brightest gamma-ray blazar 3C 454.3 using 138 weeks of observations by the Fermi Gamma-ray Space Telescope (Fermi). We probe the behaviour of the source as a function of time at different brightness levels. We show that the Fermi spectra in the GeV range can be well described by a wide underlying lognormal distribution with the photon–photon absorption breaks produced by the He II and H I Lyman recombination continua (LyC). We discover anticorrelation between the column density of the He II LyC and flux. This implies that the gamma-ray emission zone lies close to the boundary of the high-ionization part of the broad-line region and moves away from the black hole when the flux increases.
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September 2011

Gamma-ray bursts from magnetized collisionally heated jets

Jets producing gamma-ray bursts (GRBs) are likely to carry a neutron component that drifts with respect to the proton component. The neutron–proton collisions strongly heat the jet and generate electron–positron pairs. We have investigated radiation produced by this heating using a new numerical code. Our results show that collisional heating models is fully consistent with the observed GRB spectra.
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August 2011

A Synchrotron Self-Compton-Disk Reprocessing Model for Optical/X-Ray Correlation in Black Hole X-Ray Binaries

Physical picture of the emission mechanisms operating in the X-ray binaries was put under question by the simultaneous optical/X-ray observations with high time resolution. The light curves of the two energy bands appeared to be connected and the cross-correlation functions observed in three blackhole binaries exhibited a complicated shape. They show a dip of the optical emission a few seconds before the X-ray peak and the optical flare just after the X-ray peak. This behavior could not be explained in terms of standard optical emission candidates (e.g., emission from the cold accretion disk or a jet). We propose a novel model, which explains the broadband optical to the X-ray spectra and the variability properties. We suggest that the optical emission consists of two components: synchrotron radiation from the non-thermal electrons in the hot accretion flow and the emission produced by reprocessing of the X-rays in the outer part of the accretion disk. The first component is anti-correlated with the X-rays, while the second one is correlated, but delayed and smeared relative to the X-rays. The interplay of the components explains the complex shape of the cross-correlation function, the features in the optical power spectral density as well as the time lags.
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July 2011

A self-consistent hybrid Comptonization model for broad-band spectra of accreting supermassive black holes

The nature of the broad-band spectra of supermassive accreting black holes in active galactic nuclei (AGN) is an unknown issue. The X-ray spectra of Seyfert galaxies resemble those of accreting Galactic stellar-mass black holes (GBH) suggesting that similar physical processes operate in both sources. The spectral properties are well explained by thermal Comptonization, but the origin of the seed photons is less certain. The MeV tails observed in GBHs provide evidence in favour of non-thermal electron tails and it is possible that such electrons are also present in the X-ray emitting regions of AGNs. Using simulations with the kinetic code that self-consistently models electron and photon distributions, we find that the power-law-like X-ray spectra in AGNs can be explained in terms of the synchrotron self-Compton radiation of hybrid thermal/non-thermal electrons, similarly to the hard/low state of GBHs. This interpretation allows to understand a number of their observational properties, namely, the correlations of the spectral slope with the Eddington ratio.
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Events

Group meeting

Group meetings take place on Wednesdays, 16.15 in the 2nd floor coffee room.

The European Week of Astronomy and Space Science (EWASS) 2013

During EWASS-2013 a few special sessions and symposia were organized by our group. More details below:

1. Symposium "Extreme physics of neutron stars".
2. Symposium "The physics of accretion on compact objects: a multi-messenger approach".

Oulu high-energy astrophysics workshop

April 17, 11:00-17:30. Department of Physics, University of Oulu, room TA219.