S. S. Tsygankov

V0332+53 in the outburst of 2004-2005: luminosity dependence of the cyclotron line and pulse profile

ABSTRACT We present results of observations of the transient X-ray pulsar V0332+53 performedduring a very powerful outburst in Dec, 2004 – Feb, 2005 with the INTEGRAL andRXTE observatories in a wide (3 − 100 keV) energy band. A cyclotron resonancescattering line at an energy of ∼ 26 keV has been detected in the source spectrumtogether with its two higher harmonics at ∼ 50 and ∼ 73 keV, respectively. We showthat the energy of the line is not constant but linearly changes with the source lumi-nosity. Strong pulse profile variations, especially near the cyclotron line, are revealedfor different levels of the source intensity. We discuss the obtained results in terms ofthe theoretical models of X-ray pulsars.Key words: X-ray:binaries – (stars:)pulsars:individual – V0332+53 1 INTRODUCTIONThe transient X-ray pulsar V0332+53 was discovered bythe Vela 5B observatory in 1973 (Terrel & Predhorsky 1973)during an outburst when its intensity reached ∼ 1.4 Crabin the 3 − 12 keV energy band. The outburst lasted aboutthree months before the source became undetectable again.During observations in Nov 1983 – Jan 1984 with theEXOSAT observatory the pulsar’s parameters and orbitalones were determined: the pulse period ∼ 4.375 s, the or-bital period – 34.25 days, the eccentricity – 0.31, and theprojected semimajor axis of the neutron star – a

INTEGRAL/IBIS nine-year Galactic hard X-ray survey

Context. The INTEGRAL observatory operating in a hard X-ray/gamma domain gathered a large observational data set over nine years since 2003. Dominant fraction of the observing time was dedicated to the Galactic source population study making the possibility of the deepest Galactic survey in hard X-rays ever compiled. Aims. The aim of the current Galactic survey is to make a basis for Galactic source population studies, and perform mapping of the Milky Way in hard X-rays over the maximum exposure available at|b| † 17:5 . Methods. We used sky reconstruction algorithms specially developed for the high quality imaging of INTEGRAL/IBIS data. Results. We present sky images, sensitivity maps, and catalogs of detected sources in three energy bands: 17 60, 17 35, and 35 80 keV in the Galactic plane at |b| † 17:5 . The total number of sources in the referenced 17 60 keV band includes 402 (392) objects exceeding a 4:7 (5 ) detection threshold on the 9-years time-averaged map. Among the identified sources with known and tentatively identified nature, 251 are Galactic objects (107 low-mass X-ray binaries, 81 high-mass X-ray binaries, 35 cataclysmic variables, and 28 of other types) and 111 are extragalactic objects, including 108 active galactic nuclei (AGNs) and 3 galaxy clusters. The sample of Galactic sources with S{N i 5 has an identification completeness of 91%, which is valuable for population studies.

Completing the puzzle of the 2004–2005 outburst in V0332+53: the brightening phase included

Analysis of the data obtained with the RXTE observatory during a powerful outburst of the X-ray pulsar V0332+53 in 2004–2005 is presented. Observational data covering the outburst brightening phase are analysed in detail for the first time. A comparison of source parameters and their evolution during the brightening and fading phases shows no evidence for any hysteretic behaviour. It is found that the dependences of the energy of the cyclotron absorption line on the luminosity during the brightening and fading phases are almost identical. The complete data sequence including the outburst brightening and fading phases makes it possible to impose the more stringent constraints on the magnetic field in the source. The pulse profile and pulsed fraction are studied as functions of the luminosity and photon energy.

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

We present 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 high mass X-ray binaries 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 ∼ 2×10 erg s. 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 the its companion (wind-fed NS-HMXBs population). Using the elaborated model we argue that a flaring activity of so-called supergiant fast X-ray transients, the recently recognized sub-sample of HMXBs, is likely related with the magnetic arrest of their accretion.

4U 0115+63 from RXTE and INTEGRAL data: pulse profile and cyclotron line energy

We analyze the observations of the transient X-ray pulsar 4U 0115+63 with the RXTE and INTEGRAL observatories in a wide X-ray (3–100 keV) energy band during its intense outbursts in 1999 and 2004. The energy of the fundamental harmonic of the cyclotron resonance absorption line near the maximum of the X-ray flux from the source (luminosity range 5 × 1037–2 × 1038 erg s−1) is ∼11 keV. When the pulsar luminosity falls below ∼5 × 1037 erg s−1, the energy of the fundamental harmonic is displaced sharply toward the high energies, up to ∼16 keV. Under the assumption of a dipole magnetic field configuration, this change in cyclotron harmonic energy corresponds to a decrease in the height of the emitting region by ∼2 km, while other spectral parameters, in particular, the cutoff energy, remain essentially constant. At a luminosity ∼7 × 1037 erg s−1, four almost equidistant cyclotron line harmonics are recorded in the spectrum. This suggests that either the region where the emission originates is compact or the emergent spectrum from different (in height) segments of the accretion column is uniform. We have found significant pulse profile variations with energy, luminosity, and time. In particular, we show that the profile variations from pulse to pulse are not reduced to a simple modulation of the accretion rate specified by external conditions.

A reflection model for the cyclotron lines in the spectra of X-ray pulsars

Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line energy with the pulsar luminosity. At high luminosities, these variations are often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and of the cyclotron lines. However, this scenario inevitably implies a large gradient of the magnetic field strength within the line-forming region, which 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 here 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, where the gradient of the magnetic field strength is significantly smaller. Here we develop a reflection model and apply it 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.

Hard spectra of X-ray pulsars from INTEGRAL data

We present spectra for 34 accretion-powers X-ray pulsars and one millisecond pulsar that were within the field of view of the INTEGRAL observatory over two years (December 2002–January 2005) of its in-orbit operation and that were detected by its instruments at a statistically significant level (> 8σ in the energy range 18–60 keV). There are seven recently discovered objects of this class among the pulsars studied: 2RXP J130159.6-635806, IGR/AX J16320-4751, IGR J16358-4726, AX J163904-4642, IGR J16465-4507, SAX/IGR J18027-2017, and AX J1841.0-0535. We have also obtained hard X-ray (>20 keV) spectra for the accretion-powered pulsars RX J0146.9+6121, AX J1820.5-1434, and AX J1841.0-0535 for the first time. We analyze the evolution of spectral parameters as a function of the intensity of the sources and compare these with the results of previous studies.

Timing characteristics of the hard X-ray emission from bright X-ray pulsars based on INTEGRAL data

We review the results of a timing analysis of the observations for ten bright X-ray pulsars (with fluxes >100 mCrab in the 20–100 keV energy band) that fell within the INTEGRAL field of view from 2003 to 2007. The dependence of the pulse profile on the energy and intrinsic source luminosity has been investigated; particular attention has been paid to searching for changes in the pulse profile near the cyclotron frequency. The dependence of the pulsed fraction for X-ray pulsars on their luminosity and energy band has been studied in detail for the first time.

Observations of the transient X-ray pulsar KS 1947+300 by the INTEGRAL and RXTE observatories

We analyze the observations of the X-ray pulsar KS 1947+300 performed by the INTEGRAL and RXTE observatories over a wide (3-100 keV) X-ray energy range. The shape of the pulse profile was found to depend on the luminosity of the source. Based on the model of a magnetized neutron star, we study the characteristics of the pulsar using the change in its spin-up rate. We estimated the magnetic field strength of the pulsar and the distance to the binary.We analyze the observations of the X-ray pulsar KS 1947+300 performed by the INTEGRAL and RXTE observatories over a wide (3–100 keV) X-ray energy range. The shape of the pulse profile was found to depend on the luminosity of the source. Based on the model of a magnetized neutron star, we study the characteristics of the pulsar using the change in its spin-up rate. We estimated the magnetic field strength of the pulsar and the distance to the binary.

Long-term INTEGRAL and RXTE observations of the X-ray pulsar LMC X-4

We analyze the observations of the X-ray pulsar LMC X-4 performed by the INTEGRAL observatory and the All-Sky Monitor (ASM) of the RXTE observatory over a wide energy range. The observed hard X-ray flux from the source is shown to change by more than a factor of 50 (from ~70 mCrab in the high state to ~1.3 mCrab in the low state) on the time scale of the accretion-disk precession period, whose mean value for 1996-2004 was determined with a high accuracy, Pprec=30.275+/-0.004 days. In the low state, a flare about 10 h in duration was detected from the source; the flux from the source increased by more than a factor of 4 during this flare. The shape of the pulsars broadband spectrum is essentially invariable with its intensity; no statistically significant features associated with the possible resonance cyclotron absorption line were found in the spectrum of the source.We analyze the observations of the X-ray pulsar LMCX-4 performed by the INTEGRAL observatory and the All-Sky Monitor (ASM) of the RXTE observatory over a wide energy range. The observed hard X-ray flux from the source is shown to change by more than a factor of 50 (from ∼70 mCrab in the high state to ∼1.3 mCrab in the low state) on the time scale of the accretion-disk precession period, whose mean value for 1996–2004 was determined with a high accuracy, Pprec = 30.275 ± 0.004 days. In the low state, a flare about 10 h in duration was detected from the source; the flux from the source increased by more than a factor of 4 during this flare. The shape of the pulsar’s broadband spectrum is essentially invariable with its intensity; no statistically significant features associated with the possible resonance cyclotron absorption line were found in the spectrum of the source.