ART-P/Granat observations of the X-ray pulsar 4U0115+634 during the outburst in February 1990

Observations of the transient X-ray pulsar 4U0115+634 with the ART-P telescope aboard the Granat Observatory during the outburst in February 1990 are presented. The source exhibited a strong, regular and irregular variability, including X-ray bursts of duration 300–500 s. Two absorption features were detected in the source's photon spectrum at ～12 and ～22 keV, which were interpreted as the lines of resonance scattering of its emission at the first and second cyclotron harmonics. The magnetic-field strength B on the neutron-star surface that corresponds to these lines is ～1.3 × 10¹² G. The ratio of the line energies slightly differs from the harmonic ratio 1: 2. Moreover, this ratio and the line energies themselves vary appreciably with phase on a time scale of one pulsation period. The dependence of other spectral parameters for 4U0115+634 on pulse phase is investigated.     

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.     

ASCA observations of OAO 1657−415 and its dust-scattered X-ray halo

We report on two ASCA observations of the high-mass X-ray binary pulsar OAO 1657−415. A short observation near mid-eclipse caught the source in a low-intensity state, with a weak continuum and iron emission dominated by the 6.4-keV fluorescent line. A later, longer observation found the source in a high-intensity state and covered the uneclipsed through mid-eclipse phases. In the high-intensity state, the non-eclipse spectrum has an absorbed continuum component due to scattering by material near the pulsar and 80 per cent of the fluorescent iron emission comes from less than 19 light-second away from the pulsar. We find a dust-scattered X-ray halo whose intensity decays through the eclipse. We use this halo to estimate the distance to the source as 7.1 ± 1.3 kpc.     

On timing and spectral characteristics of the X-ray pulsar 4U 0115+63: Evolution of the pulsation period and the cyclotron line energy

An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, amember of a binary system with a Be star, since its discovery to the present day (～40 years) based on data from more than dozen observatories and instruments is presented. An overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source’s energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities 10³⁷?7 × 10³⁷ erg s^?1. We show that apart from the fundamental harmonic of the cyclotron absorption line at energy～11 keV, its four higher harmonics at energies ?24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the source’s spectra in the 4–28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995–2011. We have confirmed that modifying the source’s continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the source’s luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it.     

The central point source in G76.9+1.0

We describe the serendipitous discovery of a very steep-spectrum radio point source in low-frequency Giant Metrewave Radio Telescope (GMRT) images of the supernova remnant (SNR) G76.9+1.0. The steep spectrum, as well as the location of the point source near the centre of this SNR confirm that this indeed is the pulsar J2022+3842. Archival Chandra X-ray data shows a point source coincident with the radio point source. However, no pulsed radio emission was detected despite deep searches at 610 MHz and 1160 MHz – which can be understood to be due to temporal broadening of the pulses. Weak pulsed emission has indeed been seen at 2 GHz with the Green Bank Telescope (GBT), establishing the fact that scattering is responsible for its non-detection at low radio frequencies. We underline the usefulness of low-frequency radio imaging as a good technique to prospect for pulsar candidates.     

Modelling the thermal absorption and radio spectra of the binary pulsar B1259–63

We present the results of modelling of the radio spectrum evolution and dispersion measure variations of PSR B1259–63, a pulsar in a binary system with Be star LS 2883.We base our model on a hypothesis that the observed variations of the spectrum are caused by thermal free-free absorption occurring in the pulsar surroundings. We reproduce the observed pulsar spectral shapes in order to examine the influence of the stellar wind of LS 2883 and the equatorial disc on the pulsar’s radiation.The simulations of the pulsar’s radio emission and its consequent free-free absorption give us an insight into the impact of stellar wind and equatorial disc of LS 2883 has on the shapes of PSR B1259–63 radio spectra, providing an evidence for the connection between gigahertz-peaked spectra phenomenon and the close environment of the pulsar. Additionally, we supplement our model with an external absorbing medium, which results in a good agreement between simulated and observational data.     

Non-pulsed Emission from the Binary System B1259-63

PSR B1259-63 is the only known binary system with a radio pulsar from which the non-pulsed radio and X-ray emission was detected. The companion star in this system is a Be star SS 2883. A rapidly rotating radio pulsar is expected to produce a wind of relativistic particles. Be stars are known to produce highly asymmetric mass loss. Due to the interaction of the pulsar wind and the Be star wind the system of two shocks between the pulsar and the Be star forms. In this paper we show that the observed non-pulsed radio emission from the system is a result of the synchrotron emission of the relativistic particles in the outflow beyond the shock wave and that the non-pulsed X-ray emission is due to the inverse Compton scattering of the Be star photons on this particles. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-ray observations of PSR B0355+54 and its pulsar wind nebula

We present X-ray data of the middle-aged radio pulsar PSR B0355+54. The XMM-Newton and Chandra observations show not only emission from the pulsar itself, but also compact diffuse emission extending ∼50″ in the opposite direction to the pulsar’s proper motion. Our analysis also indicates the presence of fainter diffuse emission extending ∼5′ from the point source. The morphology of the diffuse component is similar to the ram-pressure confined pulsar wind nebulae detected for other sources. We find that the compact diffuse component is well-fitted with a power-law, with an index that is consistent with the values found for other pulsar wind nebulae. The core emission from the pulsar can be characterized with a thermal plus power-law fit, with the thermal emission most likely originating in a hot polar cap.     

X-ray observations of PSR B1259−63 near the 2007 periastron passage

PSR B1259−63 is a 48-ms radio pulsar in a highly eccentric 3.4-yr orbit with a Be star SS 2883. Unpulsed γ-ray, X-ray and radio emission components are observed from the binary system. It is likely that the collision of the pulsar wind with the anisotropic wind of the Be star plays a crucial role in the generation of the observed non-thermal emission. The 2007 periastron passage was observed in unprecedented details with Suzaku , Swift , XMM–Newton and Chandra missions. We present here the results of this campaign and compare them with previous observations. With these data we are able, for the first time, to study the details of the spectral evolution of the source over a 2-month period of the passage of the pulsar close to the Be star. New data confirm the pre-periastron spectral hardening, with the photon index reaching a value smaller than 1.5, observed during a local flux minimum. If the observed X-ray emission is due to the inverse Compton (IC) losses of the 10-MeV electrons, then such a hard spectrum can be a result of Coulomb losses, or can be related to the existence of the low-energy cut-off in the electron spectrum. Alternatively, if the X-ray emission is a synchrotron emission of very high-energy electrons, the observed hard spectrum can be explained if the high-energy electrons are cooled by IC emission in Klein–Nishina regime. Unfortunately, the lack of simultaneous data in the TeV energy band prevents us from making a definite conclusion on the nature of the observed spectral hardening and, therefore, on the origin of the X-ray emission.     

A candidate gamma-ray pulsar in the supernova remnant CTA 1

We present a detailed analysis of the high-energy gamma-ray source 2EG J0008+7307. The source has a steady flux and a hard spectrum, softening above 2 GeV. The properties of the gamma-ray source are suggestive of emission from a young pulsar in the spatially coincident CTA 1 supernova remnant, which has recently been found to have a non-thermal X-ray plerion. Our 95 per cent uncertainty contour around the > 1 GeV source position includes the point-like X-ray source at the centre of the plerion. We propose that this object is a young pulsar and is the most likely counterpart of 2EG J0008+7307.     

A transient I-band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4–3658

The optical counterpart of the transient, millisecond X-ray pulsar SAX J1808.4–3658 was observed in four colours ( BVRI ) for five weeks during the 2005 June–July outburst. The optical fluxes declined by ∼2 mag during the first 16d and then commenced quasi-periodic secondary outbursts, with time-scales of several days, similar to those seen in 2000 and 2002. The broad-band spectra derived from these measurements were generally consistent with emission from an X-ray heated accretion disc. During the first 16d decline in intensity the spectrum became redder. We suggest that the primary outburst was initiated by a viscosity change driven instability in the inner disc and note the contrast with another accreting millisecond pulsar, XTE J0929−314, for which the spectrum becomes bluer during the decline. On the night of 2005 June 5 (HJD 245 3527) the I -band flux was ∼0.45-mag brighter than on the preceding or following nights whereas the BV and R bands showed no obvious enhancement. A type I X-ray burst was detected by the Rossi X-ray Timing Explorer spacecraft during this I -band integration. It seems unlikely that reprocessed radiation from the burst was sufficient to explain the observed increase. We suggest that a major part of the I -band excess was due to synchrotron emission triggered by the X-ray burst. Several other significant short duration changes in V − I were detected. One occurred at about HJD 245 3546 in the early phase of the first secondary outburst and may be due to mass-transfer instability or to another synchrotron emission event.     

NuSTAR observations of the X-ray pulsar LMC X-4: A constraint on the magnetic field and tomography of the system in the fluorescent iron line

We present the results of the spectral and timing analysis of the X-ray pulsar LMC X-4 based on data from the NuSTAR observatory in the broad X-ray energy range 3–79 keV. Along with a detailed analysis of the source’s averaged spectrum, high-precision spectra corresponding to different phases of the neutron star spin cycle have been obtained for the first time. The Comptonization model is shown to describe best the source’s spectrum, and the evolution of its parameters as a function of the pulse phase has been traced. For all spectra (the averaged and phase-resolved ones) in the energy range 5–55 keV we have searched for the cyclotron absorption line. The derived upper limit on the optical depth of the cyclotron line τ ~ 0.15 (3σ) points to the absence of this feature in the given energy range, which provides a constraint on the magnetic field of the neutron star: B <3 × 10¹¹ or >6.5 × 10¹² G. The latter constraint is consistent with the magnetic field estimate obtained by analyzing the pulsar’s power spectrum, B ? 3 × 10¹³ G. Based on our analysis of the phase-resolved spectra, we have determined the delay between the emission peaks and the equivalent width of the fluorescent iron line. This delay depends on the orbital phase and is apparently associated with the travel time of photons between the emitting regions in the vicinity of the neutron star and the region where the flux is reflected (presumably in the inflowing stream or at the place of interaction between the stream and the outer edge of the accretion disk).     

G359.95-0.04: an energetic pulsar candidate near Sgr A*

We report the discovery of a prominent non-thermal X-ray feature located near the Galactic centre that we identify as an energetic pulsar wind nebula. This feature, G359.95-0.04, lies 1-lyr north of Sgr A* (in projection), is comet like in shape, and has a power-law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging to the rare class of ram-pressure confined pulsar wind nebulae. The luminosity of the nebula at the distance of Sgr A*, consistent with the inferred X-ray absorptions, is   L_x ∼ 1 × 10³⁴ erg s⁻¹  in the 2–10 keV energy band. The cometary tail extends back to a region centred at the massive stellar complex IRS 13 and surrounded by an enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic centre. We also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complexes IRS 13 and SNR Sgr A East.     

The inner scale of the plasma turbulence towards PSR J1644−4559

By measuring the decaying shape of the scatter-broadened pulse from the bright distant pulsar PSR J1644−4559, we probe waves scattered at relatively high angles by very small spatial scales in the interstellar plasma, which allows us to test for a wavenumber cutoff in the plasma density spectrum. Under the hypothesis that the density spectrum is due to plasma turbulence, we can thus investigate the (inner) scale at which the turbulence is dissipated. We report observations carried out with the Parkes radio telescope at 660 MHz from which we find strong evidence for an inner scale in the range 70–100 km, assuming an isotropic Kolmogorov spectrum. By identifying the inner scale with the ion inertial scale, we can also estimate the mean electron density of the scattering region to be 5–10 cm⁻³. This is comparable with the electron density of H  ii region G339.1−0.4, which lies in front of the pulsar, and so confirms that this region dominates the scattering. We conclude that the plasma inside the region is characterized by fully developed turbulence with an outer scale in the range 1–20 pc and an inner scale of 70–100 km. The shape of the rising edge of the pulse constrains the distribution of the strongly scattering plasma to be spread over about 20 per cent of the 4.6 kpc path from the pulsar, but with similarly high electron densities in two or more thin layers, their thicknesses can only be 10–20 pc.     

X-ray absorption in the strong Fe ii narrow-line Seyfert 1 galaxy Markarian 507

We present results from spectral analysis of ASCA data on the strong Fe  ii narrow-line Seyfert 1 galaxy Mrk 507. This galaxy was found to have an exceptionally flat ROSAT spectrum among the narrow-line Seyfert 1 galaxies (NLS1s) studied by Boller, Brandt & Fink. The ASCA spectrum, however, shows a clear absorption feature in the energy band below 2 keV, which partly accounts for the flat spectrum observed with the ROSAT Position Sensitive Proportional Counter (PSPC). Such absorption is rarely observed in other NLS1s. The absorption is mainly the result of cold (neutral or slightly ionized) gas with a column density of (2–3) × 10²¹ cm⁻². A reanalysis of the PSPC data shows that an extrapolation of the best-fitting model for the ASCA spectrum underpredicts the X-ray emission observed with the PSPC below 0.4 keV if the absorber is neutral (which indicates that the absorber is slightly ionized), covers only part of the central source, or there is extra soft thermal emission from an extended region. There is also evidence that the X-ray absorption is complex; an additional edge feature marginally detected at 0.84 keV suggests the presence of an additional high-ionization absorber, which imposes a strong O  viii edge on the spectrum. After correction for the absorption, the photon index of the intrinsic continuum, Γ ≃ 1.8, obtained from the ASCA data is quite similar to that of ordinary Seyfert 1 galaxies. Mrk 507 still has one of the flattest continuum slopes among the NLS1s, but is no longer exceptional. The strong optical Fe  ii emission remains unusual in the light of the correlation between Fe  ii strengths and steepness of soft X-ray slope.     

An unexpected outburst from A0535+262

A0535+262 is a transient Be/X-ray binary system which was in a quiescent phase from 1994 to 2005. In this paper we report on the timing and spectral properties of the INTEGRAL detection of the source in 2003 October. The source is detected for ∼6000 s in the 18–100 keV energy band at a luminosity of  ∼3.8 × 10³⁵ erg s⁻¹  ; this is compatible with the high end of the range of luminosities expected for quiescent emission. The system is observed to be outside of the centrifugal inhibition regime and pulsations are detected with periodicity,   P = 103.7 ± 0.1 s  . An examination of the pulse history of the source shows that it had been in a constant state of spin-down since it entered the quiescent phase in 1994. The rate of spin-down implies the consistent presence of an accretion disc supplying torques to the pulsar. The observations show that the system is still active and highly variable even in the absence of recent Type I or Type II X-ray outbursts.     

Discovery of a New Pulsating X-Ray Source with a 1549.1 Second Period, AX J183220-0840

A new pulsating X-ray source, AX J183220-0840, with a 1549.1 s period was discovered at R.A.=18h32m20s and decl.=-8&j0;40'30" (J2000, with an uncertainty of 0&farcm;6) during an ASCA observation on the Galactic plane. The source was observed two times, in 1997 and in 1999. A phase-averaged X-ray flux of 1.1x10-11 ergs cm-2 s-1 and a pulsation period of 1549.1+/-0.4 s were consistently obtained from these two observations. The X-ray spectrum was represented by a flat, absorbed power law with a photon index of Gamma approximately 0.8 and an absorption column density of NH approximately 1.3x1022 cm-2. Also, a signature of iron K-shell line emission with a centroid of 6.7 keV and an equivalent width of approximately 450 eV was detected. From the pulsation period and the iron-line feature, AX J183220-0840 is likely to be a magnetic white dwarf binary with a complexly absorbed thermal spectrum with a temperature of about 10 keV.     

INTEGRAL observations of TeV plerions

Amongst the sources seen in very high gamma-rays several are associated with Pulsar Wind Nebulae (“TeV plerions”). The study of hard X-ray/soft gamma-ray emission is providing an important insight into the energetic particle population present in these objects. The unpulsed emission from pulsar/pulsar wind nebula systems in the energy range accessible to the INTEGRAL satellite is mainly synchrotron emission from energetic and fast cooling electrons close to their acceleration site. Our analyses of public INTEGRAL data of known TeV plerions detected by ground based Cherenkov telescopes indicate a deeper link between these TeV plerions and INTEGRAL detected pulsar wind nebulae. The newly discovered TeV plerion in the northern wing of the Kookaburra region (G313.3+0.6 powered by the middle aged PSR J1420-6048) is found to have a previously unknown INTEGRAL counterpart which is besides the Vela pulsar the only middle aged pulsar detected with INTEGRAL. We do not find an INTEGRAL counterpart of the TeV plerion associated with the X-ray PWN “Rabbit” G313.3+0.1 which is possibly powered by a young pulsar.     

Hard X-ray observations of PSR J1833−1034 and its associated pulsar wind nebula

PSR J1833−1034 and its associated pulsar wind nebula (PWN) have been investigated in depth through X-ray observations ranging from 0.1 to 200 keV. The low-energy X-ray data from Chandra reveal a complex morphology that is characterized by a bright central plerion, no thermal shell and an extended diffuse halo. The spectral emission from the central plerion softens with radial distance from the pulsar, with the spectral index ranging from  Γ= 1.61  in the central region to  Γ= 2.36  at the edge of the PWN. At higher energy, INTEGRAL detected the source in the 17–200 keV range. The data analysis clearly shows that the main contribution to the spectral emission in the hard X-ray energy range is originated from the PWN, while the pulsar is dominant above 200 keV. Recent High Energy Stereoscopic System (HESS) observations in the high-energy gamma-ray domain show that PSR J1833−1034 is a bright TeV emitter, with a flux corresponding to ∼2 per cent of the Crab in 1–10 TeV range. In addition, the spectral shape in the TeV energy region matches well with that in the hard X-rays observed by INTEGRAL . Based on these findings, we conclude that the emission from the pulsar and its associated PWN can be described in a scenario where hard X-rays are produced through synchrotron light of electrons with Lorentz factor  γ∼ 10⁹  in a magnetic field of ∼10 μG. In this hypothesis, the TeV emission is due to inverse-Compton interaction of the cooled electrons off the cosmic microwave background photons. Search for PSR J1833−1034 X-ray pulsed emission, via RXTE and Swift X-ray observations, resulted in an upper limit that is about 50 per cent.     

Peculiarities of giant pulses from the crab pulsar at frequencies of 594 and 2228 MHz

We present the results of our simultaneous observations of giant pulses from the Crab pulsar B0531+21 at frequencies of 594 and 2228 MHz with a high (62.5 ns) time resolution. The pulse broadening by scattering was found to be 25 and 0.4 µs at 594 and 2228 MHz, respectively. We obtained the original giant-pulse profiles compensated for interstellar scattering. The measured profile widths at the two frequencies are approximately equal, ≈0.5 µs; i.e., the giant pulses are narrower than the integrated profile by a factor of about 1000. We detected an extremely high brightness temperature of radio emission, T_b≥10³⁶ K radio emission, which is higher than the previous estimates of this parameter by five orders of magnitude. The decorrelation bandwidth of the radio-spectrum diffraction distortions has been determined for this pulsar for the first time: 10 kHz at 594 MHz and 300 kHz at 2228 MHz.