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High energy astrophysics
Accreting neutron stars
Marja Annala, Natalia Babkovskaia, Askar Ibragimov, Juri Poutanen, Kerttu Viironen and also our collaborators: Andrei Beloborodov (Columbia Univ. USA), Maurizio Falanga (Paris), Marek Gierlinski (Durham, UK), Anatoly Spitkovsky (Princeton Univ.), Valery Suleimanov (Tuebingen).
Our major activities are concentrated on the studies of neutron stars in X-ray binaries:
low-magnetic field old neutron stars in low-mass X-ray binaries as well as young, with strong magnetic field, in high-mass X-ray binaries. We analyze the X-ray data from the NASA's Rossi X-ray Timing Explorer (RXTE) satellite and from the ESA's International Gamma-Ray Astrophysical Laboratory (INTEGRAL) and XMM-Newton satellite on accretion-powered millisecond pulsars. An important discovery was made on the further spin-up of the fastest known accreting pulsar IGR J00291+5934 that rotates 599 times per second. This confirms the theory of the origin of radio millisecond pulsars from the neutron stars accreting gas from a companion in low-mass X-ray binaries. These results were reported in the refereed papers in Astronomy and Astrophysics and MNRAS, the press-releases of the ESA, University of Oulu and other involved institutes in Europe as well as in the public press, Kaleva newspaper and astronomical Tähdet ja Avaruus magazine.
Fig. 1. The fastest X-ray millisecond pulsar with the spin period of 1,67 ms (599 Hz) was discovered in December 2004 by INTEGRAL and RXTE. From Falanga, Kuiper, Poutanen et al. (2005).
Fig. 2. Left: Pulse profile of IGR J00291+5934. Right: an artist impression how accretion from a low-mass star onto a millisecond pulsar occurs. Credit: NASA/Dana Berry.
We also have developed computer codes to compute light curves and polarization from rapidly rotating neutron stars around compact objects which included special (Doppler boosting, aberration) and general (light bending, redshift) relativistic effects. This can be used to get constraints on the equation of state of the matter at super-nuclear densities inside neutron stars. We construct theoretical model for the pulse profile from the X-ray pulsars (in high mass X-ray binaries). Comparing the statistics of the observed single- and double-pulsed pulsars with the theoretical model, we can get useful constraints on the magnetic field geometry in highly magnetized neutron stars.
Another direction of our research is the physics of the spreading/boundary layers between the accretion discs and the neutron star surface. We have modeled the spectral properties of these layers using previously developed 1D hydrodynamical models. We further develop the hydrodynamic model to 2D and study the temporal variability from such boundary layers with the purpose of understanding the production of the quasi-period oscillations observed in the kHz range from a number of accreting neutron stars in low-mass X-ray binaries.
Accreting black holes in X-ray binaries
Askar Ibragimov, Jari Kajava, Juri Poutanen, and our collaborators: Alexey Butkevich (Drezden Techn. Univ, Germany), Paolo Coppi (Yale Univ.), Andrew Fabian (Cambridge Univ., UK), Sergei Fabrika (Special Astrophysics Observatory, Russia), Marek Gierlinski (Durham, UK), Marat Gilfanov (MPA Garching, Germany), Diana Hannikainen (Helsinki), Indrek Vurm (Tartu Univ., Estonia), Andrzej Zdziarski (N. Copernicus Astron. Centre, Warsaw).
Accreting black holes in the X-ray binaries have been studied extensively with various X-ray and gamma-ray observatories such as RXTE, CGRO, INTEGRAL. We also constructed theoretical models describing the spectral properties. Our major results in this field are the first determination of the broad-band spectra of the most famous black hole, Cygnus X-1, from about 1 keV to 10 MeV as well as our theoretical hybrid thermal/non-thermal model that describes well such spectra. We continue studies of the spectral properties of Cygnus X-1 as well as other black holes, such as the microquasar GRS 1915+105. An unprecedented extensive study of more than 40 simultaneous RXTE and Compton Gamma-Ray Observatory observations of Cyg X-1 revealed a strong correlation between various spectral parameters, which provides interesting constraints on the theoretical models of this source.
Our interests include also studies and theoretical modeling of the the temporal variability of accreting black holes from the time-scales of milliseconds to very long time-scale of hundreds of days at their orbital and super-orbital periods.
A new field of our research is studies of the ultra-luminous X-ray sources (ULX) in nearby galaxies. These are non-nuclear sources with luminosities exceeding 10**39 erg/s. There are two main competing models: intermediate-mass black holes (IMBH) or super-critically (at rates above the Eddington) accreting stellar mass black holes. Our theoretical model of the super-critical accretion explains naturally many of the observed properties. On the observational side, we have obtained time at the 8m VLT/ESO telescopes to observe the nebulae around ULX. These nebulae are much larger than ordinary supernovae remnants and show signatures of UV/X-ray photoionization from the central source. Previous studies of the nebulae with the Russian 6m telescope at SAO show that these nebulae also are dynamically perturbed probably by the winds/jet from the central source. This argues in favour of super-critically accreting stellar mass black holes as the sub-critically accreting IMBH should not produce powerful winds.
Fig. 3. The luminosity-temperature relation for accreting black holes.
The lines are the theoretical models while the symbols are the observations. See details in Poutanen, Fabrika, Butkevich, Abolmasov 2006, MNRAS.
On the theory side, we continue to develop models of relativistic plasmas taking into account the most important microphysical processes such as Compton scattering, synchrotron radiation, and pair production. We develop computer codes that can handle self-consistently temporal evolution of the electron and photon distributions simultaneously. We will apply our models to the studies of the accretion disc-corona around black holes.
Relativistic flows in gamma-ray bursts and active galaxies
Juri Poutanen, Dmitrii Nagirner (St. Petersburg Univ., Russia), Boris Stern (Astro Space Centre, Moscow), Yana Tikhomirova (Astro Space Centre, Moscow), Indrek Vurm (Tartu Univ., Estonia).
Our activities in the field of relativistic flows and plasmas are both observational and theoretical. At the theory side, we develop models of the spectral production in relativistic flows in gamma-ray bursts (GRBs) and active galactic nuclei (AGN). With the discoveries of the optical counterparts of long gamma-ray bursts, their origin is now firmly associated with the death of massive stars. The short bursts are probably originate in collisions of compact objects. However, the nature of the gamma-ray burst spectra is still unclear. We have developed a novel theoretical model, based on synchrotron self-Compton emission from continuously heated electrons (Fig.4), which describes well the observed spectral properties as well as the temporal evolution of GRBs. Our model predicts rather blue optical-UV spectra and strong emission in the 100 MeV-10 GeV range, which will be probed by the GLAST space observatory (launch in 2007).
Fig. 4. Spectrum from synchrotron self-Compton radiation produced by continuously heated electrons. The electron distribution is shown by the dashed lines. From Stern & Poutanen 2004, MNRAS, 332, L35.
The nature of the emission of relativistic jets in AGNs such as for example in blazars is still an unsolved problem. We have proposed a straightforward and efficient mechanism for the high-energy
emission of relativistic astrophysical jets associated with an exchange of interacting
high-energy photons between the jet and the external environment. Physical processes playing the main role in this mechanism are electron-positron pair production by photons and the inverse Compton scattering. It was shown with numerical simulations that a relativistic jet (with the Lorentz
factor larger than 3-4) moving through the sufficiently dense, soft radiation field inevitably undergoes transformation into a luminous state. The process has a supercritical character: the high-energy photons breed exponentially being fed directly by the bulk kinetic energy of the jet.
As a result, a significant fraction (at least 20 per cent)
of the jet kinetic energy is converted into radiation mainly in the MeV-GeV energy range.
On the observational side, we have analysed the data from the BATSE instrument aboard of Compton Gamma-Ray Observatory which contain the largest sample of GRBs. The largest GRB uniform sample of long GRBs containing more than 3900 events can be found here.
Further studies of GRB properties with the new Swift observatory are underway.
Fig. 5. Distribution of gamma-ray bursts in the galactic coordinates observed by BATSE and detected by us in the continuous data records. From Stern et al. 2001.
Stellar Astrophysics
Stellar activity
Svetlana Berdyugina, Silva Järvinen, Juri Poutanen and our collaborators: Heidi Korhonen (AIP, Potsdam, Germany), David Moss (Univ. of Manchester, UK), Dmitrii Sokoloff (Moscow Univ.), Klaus Strassmeier (AIP, Potsdam), Ilya Usoskin (SGO, Oulu)
We study stellar activity from surface maps of stars using mostly the high-quality spectroscopic data with the high-resolution spectrograph SOFIN at the Nordic Optical Telescope on La Palma. The technique of surface imaging (combining high-quality observed spectra spanning a full rotation with theoretical ones, based on good atmospheric models, and inverting the surface temperature distribution from the fits) has been developed (including the Occamian approach and the Tikhonov-regularization) and applied to our observations.
Data spanning several years has been used to derive a time series of maps of the RS CVn-star II Peg. They show two active longitudes (strong spots are always close to these longitudes, separted by about 180 degrees). The spots are always at high latitudes; low latitude spots, if present, are weaker and short-lived. The main activity (the largest, coolest) spot of the two stays for some time at one of the latitudes, then - quite quickly - switches to the other. This so-called flip-flop phenomenon was previously detected in photometric data in the star FK Com, which is a different type of object. Combining the maps with photometric data showed that the phenomenon is roughly periodic; this can be used to define stellar activity cycles. A search through the photometric records revealed that three other RS CVn-stars showed the same phenomenon. Maps of FK Com showed also two active longitudes and are consistent with the photometrically identified flip-flop. The comparison of maps obtained one year apart even showed some evidence for differential rotation.
Our stellar activities are supplemented by the studies of the Sun and the sunspots. We have recently found that the distribution of sunspots is non-axisymmetric and spot group formation implies the existence of two persistent (on century scale) active longitudes separated by 180 deg (see Fig.6). These regions migrate with differential rotation and periodically alternate their activity levels showing a flip-flop cycle. The pattern and behaviour of active longitudes on the Sun is similar to that on cool, rapidly rotating stars with outer convective envelopes. This suggests that the magnetic dynamo, including non-axisymmetric magnetic fields and flip-flop cycles, is also similar in these stars. This allows us to understand better the phenomenon of stellar magnetic activity and to study it in detail on the Sun.
Fig.6. The longitudinal distribution of the sunspot area during cycle No. 19.
The upper panel shows the observed Carrington longitudes
and the expected migration path of the two active longitudes according to differential rotation law which
define our dynamic reference frame.
The lower panel shows the same plot but after the longitude correction, i.e.
subtraction of the migration path. One sees preferred longitudes. Interestingly, even at
a century scale, these preferred longitudes are visible (see Fig. 7). From Usoskin, Berdyugina, Poutanen, 2005, A&A, 441, 347.
Fig.7. Longitudinal distributions of sunspot occurrence in the Northern
hemisphere for the period 1878--1996.
a) Actual sunspots (area weighted) in the Carrington frame.
The distribution is nearly isotropic.
b) The same as panel a) but in the dynamic reference frame.
c) Only the position of one dominant spot for each Carrington rotation
is considered.
d) The same as in panel c) but for 6-months averages. From Usoskin, Berdyugina, Poutanen, 2005, A&A, 441, 347.
Molecular spectro-polarimetry
Svetlana Berdyugina, Sami Solanki (Max-Planck-Institut fuer Sonnensystemforschung), J.-O.Stenflo (ETH Zurich, Switzerland)
We have been developing novel techniques for studying cosmic magnetic fields based on molecular spectropolarimetry. Many diatomic molecules present in the atmospheres of the Sun and cool stars exhibit the Paschen-Back effect at field strengths typical of sunspots and active cool stars. We have presented a complete theoretical description of the molecular Paschen-Back effect, which together with the observations of the spectral line polarization profiles, holds promise to form the basis of new diagnostics of solar and stellar magnetic fields. Among our first results are the explanation of puzzling molecular polarization features observed in sunspots, direct observation of magneto-convection in the coldest parts of sunspot umbra revealed with the help of the molecular bands (see Fig. 8), and detection of the polarized radiation from spatially unresolved starspots.
Fig.8. An image of a sunspot obtained with the filter centered at the TiO band at Swedish Solar Telescope, La Palma. The high temperature sensitivity of the TiO molecule helps to reveal the previously unknown complex magneto-convection structure in the sunspot umbra.
Water masers in late-type stars and active galactic nuclei
Natalia Babkovskaia, Juri Poutanen, Ryszard Szczerba (N. Copernicus Centre, Torun, Poland), Anita Richards (Jodrell Bank Obs., Manchester, UK)
Masers (microwave amplification by stimulated emission of radiation) are relatives of lasers in physical laboratories. The difference is that masers are produced naturally in the astrophysical environments and have huge powers, e.g. emission from AGNs in only one spectral line of water at 22 GHz can exceed solar luminosity by two orders of magnitude. Some late-type stars also show strong water maser emission lines. Our research was concentrated on modeling of the dusty molecular disks around spectacular class of asymptotic giant branch (AGB) stars, so called silicate carbon stars, and its best studied representative V778 Cyg. Our approach is based on the extreme sensitivity of the maser emission to the density of molecular hydrogen and geometry of the masing material. Thus, modeling the water maser activity allows us to determine the physical parameters in the emitting region. The data on V778 Cyg show the presence of the doubly warped disk (see Fig.9) around a companion of the AGB stars which is a challenge to the theoretical models. We also continue our studies of the physics of masers in active galaxies.
Fig.9. A map of water maser features and the spectral profile in AGB star V778 Cyg observed by MERLIN
as well as the 3D view of the disk around a companion of an AGB star in V778 Cyg.
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