The X-ray source SLX 1732-304 in the globular cluster Terzan 1: The spectral states and an X-ray burst

ART-P/Granat observations of the X-ray burster SLX 1732-304 in the globular cluster Terzan 1 are presented. The X-ray (3–20 keV) fluxes from the source differed by more than a factor of 4 during the observing sessions on September 8 (F _x ? 6.95 × 10^?10 erg cm^?2 s^?1) and October 6, 1990 (F _x ? 1.64 × 10^?10 erg cm^?2 s^?1). The intensity variations of SLX 1732-304 were apparently accompanied by variations in its hardness: whereas the source in its high state had the spectrum with a distinct exponential cutoff typical of bright low-mass X-ray binaries, its low-state spectrum could be satisfactorily described by a simple power law with a photon index α?1.7. During the ART-P observation on September 8, a type I X-ray burst was detected from SLX 1732-304.     

Low state of the X-ray burster SLX 1732-304 in the globular cluster Terzan 1 according to RXTE data

Observations of the X-ray burster SLX 1732-304 in the globular cluster Terzan 1 with the PCA/RXTE instrument in April 1997 are presented. The source was in a low state; its flux in the standard X-ray band was half the flux recorded by the ART-P/Granat telescope in 1990 also during its low state. At the same time, its spectrum was softer than the ART-P spectrum; it was well described by a power law with a photon index of 2.3 without any evidence of a high-energy cutoff.     

The X-ray burst detected in 1991 from the transient source SAX J1747.0-2853

We present the observations of an intense X-ray burst from the recurrent transient source SAXJ1747.0-2853 located near the Galactic center. The burst was detected by the ART-P telescope onboard the Granat observatory on October 20, 1991. The burst time profile exhibits features that clearly point to an increase in the photospheric radius of the neutron star at the burst onset. This increase is attributable to an expansion of its outer hydrogen-rich atmospheric envelope under radiation pressure. After hydrogen-envelope outflow and photospheric contraction, the radiation flux emerging from deep within the star continued to rise, which several seconds later led to a recurrent, weaker photospheric expansion attributable to the outflow of the outer helium-envelope layers. Based on the described picture, we determined the distance to the source, d=7.9±0.4 kpc. No radiation was detected by the ART-P telescope from the source in quiescence. Actually, the source itself was discovered only seven years later by the BeppoSAX satellite during its X-ray activity.     

Type-I X-ray bursts from voids: tracers of weakly accreting bursters?

We obtained long-term (10–20 years) light curves for seven X-ray bursters. These sources exhibited no prolonged episodes of luminosities exceeding several percent of the Eddington luminosity over the entire observing period. For four sources, we found upper limits for the luminosity of over 5 years. These limits proved to be below 10³⁶ erg s^?1. We estimated the total number of such sources in our Galaxy.     

A deficit of type I X-ray bursts from low-accretion-rate binaries: Data from the TTM/COMIS telescope onboard the Mir-Kvant observatory

We analyze 175 sessions of Galactic-center observations with the TTM/COMIS telescope onboard the Mir-Kvant observatory from 1987 until 1998. Because of its wide field of view (～ 15°×15°), much of the Galaxy and, hence, a large number of X-ray sources were simultaneously within the telescope aperture. During the observations, 47 X-ray bursts were detected, 33 of which are most likely type I bursts related to unstable helium burning on the surfaces of neutron stars. All the detected type I bursts were identified with known X-ray sources; the pre-and post-burst luminosities of these sources measured with the TTM telescope were high. No bursts were detected from voids, i.e., from sources whose luminosities in quiescence did not exceed the TTM detection threshold. This result allows us to constrain the combination of the number of binary sources with low accretion rates and the properties of X-ray bursts from such sources, in particular, the peak luminosity during bursts and the frequency of their occurrence.     

Quasi-periodic X-ray oscillations in the source Cygnus X-2

The RXTE observations of Cyg X-2 during 1996–1999 are presented. The properties of quasi-periodic oscillations (QPOs) are analyzed in detail. A new method of averaging the power-density spectra obtained during various observations is used to search for kHz QPOs. Its distinctive feature is the grouping of observations not only by spectral characteristics of the source’s X-ray radiation but also by its temporal characteristics. The results obtained are used for an analysis in terms of the transition-layer model (TLM) and the relativistic-precession model (RPM) for a slowly rotating neutron star. Theoretical predictions of the two models are compared, and their self-consistency is verified. The tilt of the magnetosphere to the accretion-disk plane and the neutron-star mass and angular momentum are determined using these models. The distance to the source is estimated from observational data.     

Observations of the X-ray pulsar GX 301-2 with the ART-P telescope of the GRANAT Observatory

The variability of the X-ray flux from the pulsar GX 301-2 is analyzed by using data from the ART-P telescope of the GRANAT Observatory. The intensity variations with time scales of several thousand seconds are studied at various orbital phases. The high-state flux from the source exceeds its low-state flux by as much as a factor of 10. The hardness and spectrum of the source are shown to change greatly with its intensity. These intensity variations are most likely caused by substantial inhomogeneities in the stellar wind from the companion star.     

The Spin-X wide-field X-ray monitor of the Spectrum-X-Gamma astrophysical observatory

The Spin-X wide-field X-ray monitor of the Spectrum-X-Gamma astrophysical observatory, which is based on the principle of a coded-aperture telescope, is designed to detect and localize cosmic gammaray-burst (GRB) sources; to survey large areas of the sky in search of new transients; and to carry out long-term observations of bright Galactic sources, including X-ray bursters. The monitor consists of two noncoaxial identical modules, Spin-X1 and Spin-X2, which together cover 6.8% of the sky. The high-apogee, four-day orbit of the Spectrum-XG satellite allows the instrument to be in observing mode more than 50% of the time. Having simulated the rate of GRB detection by Spin-X, we show that extrapolating BATSE 50–300-keV average data on the number of GRBs, their duration, and their mean energy spectrum to the X-ray energy band leads to disagreement with the observed detection rate of GRBs in the X-ray band. The number of GRBs that can be detected and localized with an accuracy r≤3′ (3σ) (the error-circle radius) by Spin-X is estimated to be ten bursts per year. We present data on the Spin-X sensitivity achievable during long-term observations of persistent and transient sources and on its sensitivity to X-ray bursts from Galactic sources in the 2–30-keV energy band.     

Quasi-periodic X-ray oscillations in the source Scorpius X-1

The RXTE observations of Scorpius X-1 in 1996–1999 are presented. The properties of its quasi-periodic X-ray oscillations are studied in detail. The results obtained are used for analysis in terms of the transition-layer model (TLM) and the relativistic-precession model (RPM) for a slowly rotating neutron star. Theoretical predictions of the two models are compared and their self-consistency is verified. The tilt of the magnetosphere to the accretion-disk plane, the neutron-star mass, and its angular momentum are determined in terms of the models.     

A hard X-ray survey of the Galactic-Center region with the IBIS telescope of the INTEGRAL observatory: A catalog of sources

From August 23 through September 24, 2003, the INTEGRAL Observatory conducted a deep survey of the Galactic-Center region with a record-breaking sensitivity at energies above 20 keV. We have analyzed the images of the Galactic-Center region obtained with the ISGRI detector of the IBIS telescope (15–200 keV) and give a catalog of detected sources. We detected a total of 60 sources with fluxes above 1.5 mCrab, 44 and 3 of which were previously identified as binary systems of different classes in our Galaxy and as extragalactic objects, respectively. We discovered one new source.     

On the mechanism of X-ray emission from radio pulsars

We discuss the correlations between the luminosities of radio pulsars in various frequency ranges and the magnetic fields on the light cylinder. These correlations suggest that the observed emission is generated in outer layers of the pulsar magnetospheres by the synchrotron mechanism. To calculate the distribution functions of the relativistic particles in the generation region, we use a model of quasilinear interactions between the waves excited by cyclotron instability and particles of the primary beam and the secondary electron—positron plasma. We derive a formula for calculating the X-ray luminosity L _x of radio pulsars. A strong correlation was found between L _x and the parameter \(\dot P_{ - 15} /P^{3.5}\), where P is the neutron-star rotation period, in close agreement with this formula. The latter makes it possible to predict the detection of X-ray emission from more than a hundred (114) known radio pulsars. We show that the Lorentz factors of the secondary particles are small (γ _p = 1.5–8.5), implying that the magnetic field near the neutron-star surface in these objects is multipolar. It follows from our model that almost all of the millisecond pulsars must emit X-ray synchrotron radiation. This conclusion differs from predictions of other models and can be used to test the theory under consideration. The list of potential X-ray radiators presented here can be used to search for X-ray sources with existing instruments.     

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

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, P_prec = 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.     

Optical and X-ray observations of thermonuclear bursts from GS 1826-24 during September–October 2003

The results of optical (the RTT-150 telescope) and X-ray (the RXTE observatory) observations of the burster GS 1826-24 are presented. Emphasis was placed on analyzing the emissions during thermonuclear bursts. The results obtained allowed the size of the accretion disk in GS 1826-24 and the inclination of this binary to be estimated.     

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.     

X-ray variability of the pulsar Vela X-1 as observed with ART-P/Granat

Observations of the X-ray pulsar Vela X-1 with the ART-P telescope onboard the Granat Observatory are presented. Variability on a time scale of several thousand seconds was detected; intensity variations are shown to be accompanied by changes in the source’s spectrum. The hardness was also found to be highly variable on a scale of one pulsation period. The source’s spectrum exhibits an absorption feature at energy ～7 keV, which is apparently attributable to cyclotron scattering/absorption in the neutron-star magnetic field. Weak persistent emission was detected during an X-ray eclipse, which probably resulted from the scattering of pulsar emission in the stellar wind from an optical star.     

Two years of observations of the X-ray pulsar SMC X-1 with the ART-P telescope onboard the Granat observatory

We present the observations of the pulsar SMC X-1 with the ART-P telescope onboard the Granat observatory. We investigate the variability of the flux from the source on time scales of several tens of days. The intensity variation of the pulsar are shown to be consistent with the presence of a periodicity in the system with a characteristic time scale of ～61 days. The precession of an inclined accretion disk, as indirectly confirmed by the absence of low-state pulsations, may be responsible for the observed variability. The spectrum of the source is well described by a power-law energy dependence of the photon flux density with a slope of ～1.5 and an exponential cutoff at energies above ～14–18 keV. We estimated the inclinations between the planes of the orbit and the accretion disk and the magnetic field of the neutron star.     

Energy release on the surface of a rapidly rotating neutron star during disk accretion: A thermodynamic approach

The total energy E of a star as a function of its angular momentum J and mass M in the Newtonian theory, E=E(J, M) [in general relativity, the gravitational mass of a star as a function of its angular momentum J and rest mass m, M=M(J, m)], is used to determine the remaining parameters (angular velocity, chemical potential, etc.) in the case of rigid rotation. Expressions are derived for the energy release during accretion onto a cool (with constant entropy), rapidly rotating neutron star (NS) in the Newtonian theory and in general relativity. A separate analysis is performed for the cases where the NS equatorial radius is larger and smaller than the radius of the marginally stable orbit in the disk plane. An approximate formula is proposed for the NS equatorial radius for an arbitrary equation of state, which matches the exact equation of state at J=0.     

Spectral variability of the X-ray pulsar Hercules X-1

We present the results of our comparative timing and spectral analysis of the high and low (off) states in the X-ray pulsar Her X-1 based on data from the ART-P telescope onboard the Granat observatory. A statistically significant (several mCrab) persistent flux with a simple power-law spectrum was detected during the low state. The spectral slope changed from observation to observation by almost a factor of 2. Pulsations were detected only during the high state of the source, when its flux was a factor of ～25 larger than the low-state flux. The spectral shape of Her X-1 in its high state was complex, with the parameters depending on pulse phase.     

A young cooling neutron star in the remnant of Supernova 1987A

We describe the cooling theory for isolated neutron stars that are several tens of years old. Their cooling differs greatly from the cooling of older stars that has been well studied in the literature. It is sensitive to the physics of the inner stellar crust and even to the thermal conductivity of the stellar core, which is never important at later cooling stages. The absence of observational evidence for the formation of a neutron star during the explosion of Supernova 1987A is consistent with the fact that the star was actually born there. It may still be hidden in the dense center of the supernova remnant. If, however, the star is not hidden, then it should have a low thermal luminosity (below ～10³⁴ erg s^?1) and a short internal thermal relaxation time (shorter than 13 yr). This requires that the star undergo intense neutrino cooling (e.g., via the direct Urca process) and have a thin crust with strong superfluidity of free neutrons and/or an anomalously high thermal conductivity.