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The catalog of variable sources detected by INTEGRAL. I. Catalog and techniques
Context. During its 6 years of operation, INTEGRAL/ISGRI has detectedmore than 500 sources. Many of these sources are variable. Taking intoaccount that nearly half of INTEGRAL/ISGRI sources are new and many ofthem remain unidentified, the variability properties of the sources canprovide additional constraints to help us to classify and identify theunknown sources. Aims: To study the variability properties of thesources detected by INTEGRAL/ISGRI, we develop a method to quantify thevariability of a source. We describe here our techniques and compile acatalog of the sources that fit our criteria of variability. Methods: We use the natural time binning of INTEGRAL observations calledScience Window (? 2000 s) and test the hypothesis that the detectedsources are constant using a ?^2 all-sky map in three energy bands(20-40, 40-100, 100-200 keV). We calculate an intrinsic variance of theflux in individual pixels and use it to define the fractionalvariability of a source. The method is sensitive to the sourcevariability on timescales of one Science Window and higher. Weconcentrate only on the sources that were already reported to bedetected by INTEGRAL. Results: We present a catalog of 202sources found to be significantly variable. For the catalog sources, wegive the measure of variability and fluxes with corresponding errors inthe 20-40, 40-100 and 100-200 keV energy bands, and we present somestatistics about the population of variable sources. The description ofthe physical properties of the variable sources will be given in aforthcoming paper.Table 3 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A68
| Swift-BAT Survey of Galactic Sources: Catalog and Properties of the Populations
We study the populations of X-ray sources in the Milky Way in the 15-55keV band using a deep survey with the BAT instrument aboard the Swiftobservatory. We present the log N-log S distributions of the varioussource types and we analyze their variability and spectra. For thelow-mass X-ray binaries (LMXBs) and the high-mass X-ray binaries(HMXBs), we derive the luminosity functions to a limiting luminosity ofLX ~ 7 × 1034 erg s-1. Ourresults confirm the previously found flattening of the LMXB luminosityfunction below a luminosity of LX ~ 1037 ergs-1. The luminosity function of the HMXBs is found tobe significantly flatter in the 15-55 keV band than in the 2-10 keVband. From the luminosity functions we estimate the ratios of the hardX-ray luminosity from HMXBs to the star formation rate, and the LMXBluminosity to the stellar mass. We use these to estimate the X-rayemissivity in the local universe from X-ray binaries and show that itconstitutes only a small fraction of the hard X-ray background.
| The Mouse That Roared: A Superflare from the dMe Flare Star EV Lac Detected by Swift and Konus-Wind
We report on a large stellar flare from the nearby dMe flare star EV Lacobserved by the Swift and Konus-Wind satellites and the LiverpoolTelescope. It is the first large stellar flare from a dMe flare star toresult in a Swift trigger based on its hard X-ray intensity. Its peakfX from 0.3 to 100 keV of 5.3 × 10-8erg cm-2 s-1 is nearly 7000 timeslarger than the star's quiescent coronal flux, and the change inmagnitude in the white filter is >=4.7. This flare also caused atransient increase in EV Lac's bolometric luminosity (L bol)during the early stages of the flare, with a peak estimated L X/L bol ~ 3.1. We apply flare loop hydrodynamic modelingto the plasma parameter temporal changes to derive a loop semi-length ofl/R sstarf = 0.37 ± 0.07. The soft X-ray spectrum ofthe flare reveals evidence of iron K? emission at 6.4 keV. Wemodel the K? emission as fluorescence from the hot flare sourceirradiating the photospheric iron, and derive loop heights of h/Rsstarf = 0.1, consistent within factors of a few with theheights inferred from hydrodynamic modeling. The K? emissionfeature shows variability on timescales of ~200 s which is difficult tointerpret using the pure fluorescence hypothesis. We examine K?emission produced by collisional ionization from accelerated particles,and find parameter values for the spectrum of accelerated particleswhich can accommodate the increased amount of K? flux and the lackof observed nonthermal emission in the 20-50 keV spectral region.
| Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles
Context. The light curves observed from X-ray pulsars and magnetarsreflect the radiation emission pattern, the geometry of the magneticfield, and the neutron star compactness. Aims: We study thestatistics of X-ray pulse profiles in order to constrain the neutronstar compactness and the magnetic field geometry. Methods: Wecollect the data for 124 X-ray pulsars, which are mainly in high-massX-ray binary systems, and classify their pulse profiles according to thenumber of observed peaks seen during one spin period, dividing them intotwo classes, single- and double-peaked. We find that the pulsars aredistributed about equally between both groups. We also compute theprobabilities predicted by the theoretical models of two antipodalpoint-like spots that emit radiation according to the pencil-likeemission patterns. These are then compared to the observed fraction ofpulsars in the two classes. Results: Assuming a blackbodyemission pattern, it is possible to constrain the neutron starcompactness if the magnetic dipole has arbitrary inclinations to thepulsar rotational axis. More realistic pencil-beam patterns predict that79% of the pulsars are double-peaked independently of their compactness.The theoretical predictions can be made consistent with the data if themagnetic dipole inclination to the rotational axis has an upper limit of40° ± 4°. We also discuss the effect of limitedsensitivity of the X-ray instruments to detect weak pulses, which lowersthe number of detected double-peaked profiles and makes the theoreticalpredictions to be consistent with the data even if the magnetic dipoledoes have random inclinations. This shows that the statistics of pulseprofiles does not allow us to constrain the neutron star compactness. Incontrast to the previous claims by Bulik et al. (2003, A&A, 404,1023), the data also do not require the magnetic inclination to beconfined in a narrow interval.
| Exploring the Small Magellanic Cloud to the Faintest X-ray Fluxes: Source Catalog, Timing, and Spectral Analysis
We present the results of a pair of 100 ks Chandra observations in theSmall Magellanic Cloud (SMC) to survey high-mass X-ray binaries (HMXBs),stars, and low-mass X-ray binaries (LMXBs)/cataclysmic variables down toLx = 4.3 × 1032 erg s-1.The two SMC Deep Fields (DFs) are located in the most activestar-forming region of the bar, with Deep Field-1 positioned at the mostpulsar-rich location identified from previous surveys. Two new pulsarswere discovered in the outburst: CXOU J004929.7-731058 (P = 892 s), CXOUJ005252.2-721715 (P = 326 s), and three new HMXB candidates wereidentified. Of the 15 Be-pulsars now known in the field, 13 weredetected, with pulsations seen in 9 of them. Ephemerides demonstratethat 6 of the 10 pulsars known to exhibit regular outbursts were seenoutside their periastron phase, and quiescent X-ray emission atLX = 1033-1034 erg s-1is shown to be common. Comparison with ROSAT, ASCA, and XMM-Newtoncatalogs resulted in positive identification of several previouslyambiguous sources. Bright optical counterparts exist for 40 of the X-raysources, of which 33 are consistent with early-type stars (MV< -2, B - V < 0.2), and are the subject of a companionpaper. The results point to an underlying HMXB population density up todouble that of active systems. The full catalog of 394 point sources ispresented along with detailed analyses of timing and spectralproperties.
| Discovery of a magnetic neutron star in X-ray transient IGR J01583+6713
Aims: IGR J01583+6713 is a new transient source discovered by theINTEGRAL/IBIS hard X-ray surveys. Optical observations have providedevidence that it is a high mass X-ray binary, but its nature remainsunclear. Methods: We used the INTEGRAL/IBIS data to study IGRJ01583+6713 during its outburst. Results: We present the temporalprofiles and spectral properties of IGR J01583+6713 around its outburston 2005 Dec. 6. During the outburst, the mean X-ray luminosity reachedaround 4 × 1035 erg s-1 in the energy rangeof 20-100 keV. The continuum spectrum can be fitted by abremsstrahlung model of kT ~ 35 keV or a power-law model of ? ~2.1. In addition, the electron resonant cyclotron absorption lines weredetected at ˜ 35 keV and possible at ˜ 67 keV, implying thata magnetic neutron star of B ~ 4 × 1012 G is located inIGR J01583+6713. We class IGR J01583+6713 as a transient X-ray sourceswith a magnetic neutron star. After the outburst, the flux of IGRJ01583+6713 decreased and could not be detected by IBIS after 2005 Dec.10. During the outburst, we cannot confirm the possible pulse period at469 s previously reported and did not detect the modulation signals inthe range of 200-2000 s with the IBIS observations. Conclusions: The transient hard X-ray source IGR J01583+6713 contains amagnetic neutron star, which would help to understand its transientnature.
| Is there a highly magnetized neutron star in GX 301-2?
We present the results of an in-depth study of the long-period X-raypulsar GX 301-2. Using archival data of INTEGRAL, RXTE ASM, and CGROBATSE, we study the spectral and timing properties of the source.Comparison of our timing results with previously published work revealsa secular decay of the orbital period at a rate of ? - 3.25 ×10-5 d yr-1, which is an order of magnitude fasterthan for other known systems. We argue that this is probably resulteither of the apsidal motion or of gravitational coupling of the matterlost by the optical companion with the neutron star, although currentobservations do not allow us to distinguish between those possibilities.We also propose a model to explain the observed long pulse period. Wefind that a very strong magnetic field B ~ 1014 G can explainthe observed pulse period in the framework of existing models fortorques affecting the neutron star. We show that the apparentcontradiction with the magnetic field strength BCRSF ~ 4× 1012 G derived from the observed cyclotron lineposition may be resolved if the line formation region resides in a tallaccretion column of height ~2.5-3 RNS. The color temperaturemeasured from the spectrum suggests that such a column may indeed bepresent, and our estimates show that its height is sufficient to explainthe observed cyclotron line position.
| A compact pulsar wind nebula model of the ?-ray-loud binary LS I +61?303
We study a model of LS I +61°303 in which its radio to TeV emissionis due to interaction of a relativistic wind from a young pulsar withthe wind from its companion Be star. The detailed structure of thestellar wind plays a critical role in explaining the properties of thesystem. We assume the fast polar wind is clumpy, which is typical forradiatively driven winds. The clumpiness and some plasma instabilitiescause the two winds to mix. The relativistic electrons from the pulsarwind are retained in the moving clumps by inhomogeneities of themagnetic field, which explains the X-ray variability observed ontime-scales much shorter than the orbital period. We calculate detailedinhomogeneous spectral models reproducing the average broad-bandspectrum from radio to TeV. Given the uncertainties on the magneticfield within the wind and the form of the distribution of relativisticelectrons, the X-ray spectrum could be dominated by either Compton orsynchrotron emission. The recent Fermi observations constrain thehigh-energy cut-off in the electron distribution to be at the Lorentzfactor of 2 × 104 or ~108 in the former andlatter model, respectively. We provide formulae comparing the losses ofthe relativistic electrons due to Compton, synchrotron and Coulombprocesses versus the distance from the Be star. We calculate the opticaldepth of the wind to free-free absorption, showing that it will suppressmost of the radio emission within the orbit, including the pulsed signalof the rotating neutron star. We point out the importance of Compton andCoulomb heating of the stellar wind within and around the ?-rayemitting region. Then, we find the most likely mechanism explaining theorbital modulation at TeV energies is anisotropy of emission, withrelativistic electrons accelerated along the surface of equal rampressure of the two winds. Pair absorption of the TeV emissionsuppresses one of the two maxima expected in an orbit.
| Spin-Down of the Long-Period Accreting Pulsar 4U 2206+54
4U 2206+54 is a high-mass X-ray binary which has been suspected tocontain a neutron star accreting from the wind of its companion, BD+53° 2790. Reig et al. have recently detected 5560 s periodpulsations in both Rossi X-ray Timing Explorer (RXTE) and InternationalGamma-ray Astrophysics Laboratory observations which they conclude aredue to the spin of the neutron star. We present observations made withSuzaku which are contemporaneous with their RXTE observation of thissource. We find strong pulsations at a period of 5554 ± 9 s inagreement with their results. We also present a reanalysis of BeppoSAXobservations of 4U 2206+54 made in 1998, in which we find strongpulsations at a period of 5420 ± 28 s, revealing a spin-downtrend in this long-period accreting pulsar. Analysis of these datasuggests that the neutron star in this system is an accretion-poweredmagnetar.
| The Palermo Swift-BAT hard X-ray catalogue. II. Results after 39 months of sky survey
Aims: We present the Palermo Swift-BAT hard X-ray catalogueobtained from the analysis of data acquired during the first 39 monthsof the Swift mission. Methods: We developed a dedicated softwareto perform the data reduction, mosaicking, and source detection of theBAT survey data. We analyzed the BAT dataset in three energy bands(14-150 keV, 14-30 keV, 14-70 keV), obtaining a list of 962 detectionsabove a significance threshold of 4.8 standard deviations. Theidentification of the source counterparts was pursued using threestrategies: cross-correlation with published hard X-ray catalogues,analysis of field observations of soft X-ray instruments, andcross-correlation with SIMBAD databases. Results: The surveycovers 90% of the sky down to a flux limit of 2.5×10-11 erg cm-2 s-1 and 50% ofthe sky down to a flux limit of 1.8 ×10-11 ergcm-2 s-1 in the 14-150 keV band. We derived acatalogue of 754 identified sources, of which ˜69% areextragalactic, ˜27% are Galactic objects, and ˜4% arealready known X-ray or gamma ray emitters, whose nature has yet to bedetermined. The integrated flux of the extragalactic sample is ˜1%of the cosmic X-ray background in the 14-150 keV range.Table 2 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/510/A48
| Monitoring the hard X-ray sky with SuperAGILE
Context. SuperAGILE is the hard X-ray monitor of the AGILE gamma raymission, in orbit since 23 April 2007. It is an imaging experiment basedon a set of four independent silicon strip detectors, equipped withone-dimensional coded masks, operating in the nominal energy range 18-60keV. Aims: The main goal of SuperAGILE is the observation ofcosmic sources simultaneously with the main gamma-ray AGILE experiment,the Gamma Ray Imaging Detector (GRID). Given its ~steradian-wide fieldof view and its ~15 mCrab day-sensitivity, SuperAGILE is also wellsuited to the long-term monitoring of Galactic compact objects and thedetection of bright transients. Methods: The SuperAGILE detectorproperties and design allow for a 6 arcmin angular resolution in each ofthe two independent orthogonal projections of the celestial coordinates.Photon by photon data are continuously available by means of experimenttelemetry, and are used to derive images and fluxes of individualsources, with integration times depending on the source intensity andposition in the field of view. Results: We report on the mainscientific results achieved by SuperAGILE over its first two years inorbit, until April 2009. The scientific observations started in mid-July2007, with the science verification phase, continuing during thecomplete AGILE Cycle 1 and the first ~half of Cycle 2. Despite thelargely non-uniform sky coverage, due to the pointing strategy of theAGILE mission, a few tens of Galactic sources were monitored, sometimesfor unprecedently long continuous periods, leading to the detection alsoof several bursts and outbursts. Approximately one gamma ray burst permonth was detected and localized, allowing for prompt multiwavelengthobservations. A few extragalactic sources in bright states wereoccasionally detected as well. The light curves of sources measured bySuperAGILE are made publicly available on the web in almost real-time.To enable a proper scientific use of these, we provide the reader withthe relevant scientific and technical background.
| Correlated optical/X-ray variability in the high-mass X-ray binary SAX J2103.5+4545
SAX J2103.5+4545 is the Be/X-ray binary (BeX) with the shortest orbitalperiod. It shows extended bright and faint X-ray states that last for afew hundred days. The main objective of this work is to investigate therelationship between the X-ray and optical variability and tocharacterize the spectral and timing properties of the bright and faintstates. We have found a correlation between the spectral and temporalparameters that fit the energy and power spectra. Softer energy spectracorrespond to softer power spectra. That is to say, when the energyspectrum is soft, the power at high frequencies is suppressed. We alsopresent the results of our monitoring of the H? line of theoptical counterpart since its discovery in 2003. There is a correlationbetween the strength and shape of the H? line, originated in thecircumstellar envelope of the massive companion and the X-ray emissionfrom the vicinity of the neutron star. H? emission, indicative ofan equatorial disc around the B-type star, is detected whenever thesource is bright in X-rays. When the disc is absent, the X-ray emissiondecreases significantly. The long-term variability of SAX J2103.5+4545is characterized by fast episodes of disc loss and subsequentreformation. The time-scales for the loss and reformation of the disc(about 2 yr) are the fastest among BeXs.
| The Fourth IBIS/ISGRI Soft Gamma-ray Survey Catalog
In this paper, we report on the fourth soft gamma-ray source catalogobtained with the IBIS gamma-ray imager on board the INTEGRAL satellite.The scientific data set is based on more than 70 Ms of high-qualityobservations performed during the first five and a half years of theCore Program and public observations. Compared to previous IBIS surveys,this catalog includes a substantially increased coverage ofextragalactic fields, and comprises more than 700 high-energy sourcesdetected in the energy range 17-100 keV, including both transients andfaint persistent objects that can only be revealed with longer exposuretimes. A comparison is provided with the latest Swift/BAT surveyresults.Based on observations with INTEGRAL, an ESA project with instruments andScience Data Centre funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.
| Evidence for a magnetic neutron star in high-mass X-ray binary 4U 2206+54 with INTEGRAL/IBIS observations
The hard X-ray source 4U 2206+54 is a peculiar high-mass X-ray binarywith a main-sequence donor star. Recent X-ray observations suggestedthat the compact object in 4U 2206+54 may be a neutron star. The X-rayemission comes from the accretion of stellar winds from the massivedonor star, and variability of its luminosity may be due to changes inits orbital phase. To reveal further the nature of the compact object,we studied 4U 2206+54 with INTErnational Gamma-Ray AstrophysicsLaboratory (INTEGRAL)/IBIS observations over two years, and found thatfor most of this time 4U 2206+54 was in a quiescent state, sometimesundergoing an active phase. In the quiescent state the spectrum can befitted by a power-law model of ? ~ 2.1 with a hard X-rayluminosity of ~5 × 1034ergs-1 (20-100keV).While the source is in the active state, the 20-100keV hard X-rayluminosity reaches ~ 2 × 1035ergs-1 and thespectrum is fitted by a thermal bremsstrahlung model of kT ~ 43keV plustwo cyclotron absorption lines at ~30 and 60keV. We then derived amagnetic field of 3.3 × 1012G for the compact object in4U 2206+54. During the active state, we found a pulsation period of~5400s in the light curve of 4U 2206+54. The compact object in 4U2206+54 should therefore be a magnetic neutron star with slow pulsation.The detection of cyclotron absorption lines in the active state andtheir non-detection in the quiescent state suggested that the twodifferent accretion states have possible different hard X-ray emissionregions: the surface of the neutron star in the active state and themagnetic-accretion pressure equivalent point in the quiescent state.Re-analysis of the RXTE/ASM light curve found modulation periods at~9.56 and 19.11d, and the orbital period of 4U 2206+54 should be 19.11d.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| An XMM-Newton Spectral and Timing Study of IGR J16207 - 5129: An Obscured and Non-Pulsating HMXB
We report on a ~12 hr XMM-Newton observation of the supergiant high-massX-ray binary (HMXB) IGR J16207 - 5129. This is only the secondsoft X-ray (0.4-15 keV, in this case) study of the source since it wasdiscovered by the International Gamma-Ray Astrophysics Laboratorysatellite. The average energy spectrum is very similar to those ofneutron star HMXBs, being dominated by a highly absorbed power-lawcomponent with a photon index of Γ = 1.15+0.07-0.05. The spectrum also exhibits a soft excess below~2 keV and an iron Kα emission line at 6.39 ± 0.03 keV. Forthe primary power-law component, the column density is(1.19+0.06 -0.05) × 1023cm-2, indicating local absorption, likely from thestellar wind, and placing IGR J16207 - 5129 in the category ofobscured IGR HMXBs. The source exhibits a very high level of variabilitywith an rms noise level of 64% ± 21% in the10-4-0.05 Hz frequency range. Although the energyspectrum suggests that the system may harbor a neutron star, nopulsations are detected with a 90% confidence upper limit of ~2% in afrequency range from ~10-4 to 88 Hz. We discusssimilarities between IGR J16207 - 5129 and other apparentlynonpulsating HMXBs, including other IGR HMXBs as well as 4U 2206+54 and4U 1700 - 377.
| Long-Term X-Ray Monitoring of the TeV Binary LS I +61 303 With the Rossi X-Ray Timing Explorer
We report on the results of a long-term X-ray monitoring campaign of thegalactic binary LS I +61 303 performed by the Rossi X-ray TimingExplorer. This data set consists of 1 ks pointings taken every other daybetween 2007 August 28 and 2008 February 2. The observations covered sixfull cycles of the 26.496 day binary period and constitute the largestcontinuous X-ray monitoring data set on LS I +61 303 to date with thissensitivity. There is no statistically strong detection of modulation ofthe flux or the photon index with orbital phase; however, we do find astrong correlation between the flux and photon index, with the spectrumbecoming harder at higher fluxes. The data set contains three largeflaring episodes, the largest of these reaching a flux level of7.2+0.1 -0.2 × 10-11erg cm-2 s-1 in the 3-10 keV band,which is a factor 3 times larger than the flux levels typically seen inthe system. Analysis of these flares shows the X-ray emission from LS I+61 303 changing by up to a factor of 6 over timescales of severalhundred seconds as well as doubling times as fast as 2 s. This is thefastest variability ever observed from LS I +61 303 at this wavelengthand places constraints on the size of the X-ray emitting region.
| Superorbital variability in hard X-rays
We present the results of a study with the Swift Burst Alert Telescopein the 14-195 keV range of the long-term variability of five low-massX-ray binaries with reported or suspected superorbital periods - 4U1636-536, 4U 1820-303, 4U 1916-053, Cyg X-2 and Sco X-1. No significantpersistent periodic modulation was detected around the previouslyreported periods in the 4U 1916-053, Cyg X-2 or Sco X-1 light curves.The ~170-d period of 4U 1820-303 was detected up to 24 keV, consistentwith variable accretion due to the previously proposed triple systemmodel. The ~46-d period in 4U 1636-536 was detected up to 100 keV, withthe modulation in the low- and high-energy bands found to be phaseshifted by ~180° with respect to each other. This phase shift, whentaken together with the near-coincident onset of the ~46-d modulationand the low/hard X-ray state, leads us to speculate that the modulationcould herald transient jet formation.
| Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54
Context: The source 4U 2206+54 is one of the most enigmatic high-massX-ray binaries. In spite of intensive searches, X-ray pulsations havenot been detected in the time range 10-3-103 s. Acyclotron line at ~30 keV has been suggested by various authors butnever detected with significance. The stellar wind of the opticalcompanion is abnormally slow. The orbital period, initially reported tobe 9.6 days, disappeared and a new periodicity of 19.25 days emerged.Aims: The main objective of our RXTE monitoring of 4U 2206+54 isto study the X-ray orbital variability of the spectral and timingparameters. The new long and uninterrupted RXTE observations allow us tosearch for long (~1 h) pulsations for the first time. Methods:We divided the ~7-day observation into five intervals and obtainedtime-averaged energy spectra and power spectral density for eachobservation interval. We also searched for pulsations using variousalgorithms. Results: We have discovered 5560-s pulsations in thelight curve of 4U 2206+54. Initially detected in RXTE data, thesepulsations are also present in INTEGRAL and EXOSAT observations. Theaverage X-ray luminosity in the energy range 2-10 keV is 1.5 ×1035 erg s-1 with a ratio F_max/F_min ? 5. Thisratio implies an eccentricity of ~0.4, somewhat higher than previouslysuggested. The power spectrum is dominated by red noise that can befitted with a single power law whose index and strength decrease withX-ray flux. The source also shows a soft excess at low energies. If thesoft excess is modelled with a blackbody component, then the size andtemperature of the emitting region agrees with its interpretation interms of a hot spot on the neutron star surface. Conclusions: Thediscovery of X-ray pulsations in 4U 2206+54 confirms the neutron starnature of the compact companion and definitively rules out the presenceof a black hole. The source displays variability on time scales of days,presumably due to changes in the mass accretion rate as the neutron starmoves around the optical companion in a moderately eccentric orbit. Ifcurrent models for the spin evolution in X-ray pulsars are correct, thenthe magnetic field of 4U 2206+54 at birth must have been B ?1014 G.
| Quasi-Periodic Oscillations in Cen X-3 and the Long-Term Intensity Variations
We have investigated the properties of the quasi-periodic oscillation(QPO) features in the accretion-powered X-ray pulsar Cen X-3 over aperiod of about 4 years using observations carried out with theProportional Counter Array (PCA) of the Rossi X-ray Timing Explorer(RXTE). The observations cover a wide range of X-ray intensity of thesource in excess of the binary intensity modulation. In 11 of 81pointings with the PCA, we have detected QPOs with rms intensityfluctuation up to 10%. The QPO peak frequency shows clustering around 40and 90 mHz, with the QPO frequency having no dependence on X-rayintensity. This indicates that either (1) the observed X-ray luminosityof the source is not related to the mass accretion rate or inner radiusof the accretion disk, or (2) the QPO generation mechanism in Cen X-3 isdifferent from the beat frequency model or Keplerian frequency modelthat is believed to operate in most other transient and persistent X-raypulsars. We have also found that the rms variation in the 40 mHz QPOfeature is not dependent on the X-ray energy, indicating that the originof the QPO is unlikely to be related to disk absorption.
| INTEGRAL long-term monitoring of the supergiant fast X-ray transient XTE J1739-302
Context: In the past few years, a new class of high mass X-ray binaries(HMXRB) has been claimed to exist, the supergiant fast X-ray transients(SFXT). These are X-ray binary systems with a compact companion orbitinga supergiant star which show very short and bright outbursts in a seriesof activity periods overimposed on longer quiescent periods. Only veryrecently the first attempts to model the behaviour of these sources havebeen published, some of them within the framework of accretion fromclumpy stellar winds. Aims: Our goal is to analyze the propertiesof XTE J1739-302/IGR J17391-3021within the context of the clumpy structure of the supergiant wind. Methods: We have used INTEGRAL and RXTE/PCA observations to obtainbroad band (1-200 keV) spectra and light curves of XTEJ1739-302 and investigate its X-ray spectrum and temporalvariability. Results: We have found that XTEJ1739-302 follows a much more complex behaviour than expected.Far from presenting a regular variability pattern, XTEJ1739-302 shows periods of high, intermediate, and low flaringactivity.
| Superorbital variability of X-ray and radio emission of Cyg X-1 - II. Dependence of the orbital modulation and spectral hardness on the superorbital phase
We discover a pronounced dependence of the strength of the soft X-rayorbital modulation and the spectral hardness in Cyg X-1 in the hardstate on its superorbital phase. We find, our results can be wellmodelled as a combination of two effects: the precession of theaccretion disc (which appears to cause the superorbital flux modulation)and the orbital-phase dependent X-ray absorption in an accretion bulge,located at the accretion disc edge close to the supergiant companion butdisplaced from the line connecting the stars by about 25°. Ourfindings are supported by the distribution of the X-ray dips showingconcentration towards zero superorbital phase, which corresponds to thebulge passing through the line of sight. We Fourier analyse our model,and find it explains the previous finding of asymmetric beat (betweenthe orbital and superorbital modulations) frequencies in the observedpower spectrum, provided the disc precession is prograde. On the otherhand, we find no statistically significant changes of the orbitalmodulation with the superorbital phase in the 15-GHz radio data. Thisabsence is consistent with the radio being emitted by a jet in thesystem, in which case the orbital modulation is caused by windabsorption far away from the disc. We also find that both the X-ray andradio fluxes of Cyg X-1 in the hard state on time-scales >~104s have lognormal distributions, which complements aprevious finding of a lognormal flux distribution in the hard state on~1-s time-scales. We point out that the lognormal character of the fluxdistribution requires that flux logarithms rather than fluxes themselvesshould be used for averaging and error analysis. We also provide acorrect formula for the uncertainty of rms of a light curve for the casewhen the uncertainty is higher than the measurement.
| A detailed study of 2S 0114+650 with the Rossi X-ray Timing Explorer
We present the results of a detailed study of the high-mass X-ray binary2S 0114+650 made with the pointed instruments onboard the Rossi X-rayTiming Explorer. The spectral and temporal behaviour of this source wasexamined over the pulse, orbital and superorbital time-scales, covering~2 cycles of the 30.7 d superorbital modulation. Marginal evidence forvariability of the power-law photon index over the pulse period wasidentified, similar to that observed from other X-ray pulsars. If thisvariability is real it can be attributed to a varying viewing geometryof the accretion region with the spin of the neutron star. Variabilityof the neutral hydrogen column density over the orbital period wasobserved, which we attribute to the line-of-sight motion of the neutronstar through the dense circumstellar environment. A reduction in thepower-law photon index was observed during the orbital maximum, which wespeculate is due to the absorption effects as the neutron star passesbehind a heavily absorbing region near the base of the supergiantcompanion's wind. No significant variability of the column density wasobserved over the superorbital period, indicating that variableobscuration by a precessing warp in an accretion disc is not themechanism behind the superorbital modulation. In contrast, a significantincrease in the power-law photon index was observed during thesuperorbital minimum. We conclude that the observed superorbitalmodulation is tied to variability in the mass accretion rate due to someas yet unidentified mechanism.
| Are There Magnetars in High-Mass X-Ray Binaries? The Case of Supergiant Fast X-Ray Transients
In this paper we survey the theory of wind accretion in high-mass X-raybinaries hosting a magnetic neutron star and a supergiant companion. Weconcentrate on the different types of interaction between the inflowingwind matter and the neutron star magnetosphere that are relevant whenaccretion of matter onto the neutron star surface is largely inhibited;these include inhibition through the centrifugal and magnetic barriers.Expanding on earlier work, we calculate the expected luminosity for eachregime and derive the conditions under which transition from one regimeto another can take place. We show that very large luminosity swings(~104 or more on timescales as short as hours) can resultfrom transitions across different regimes. The activity displayed bysupergiant fast X-ray transients, a recently discovered class ofhigh-mass X-ray binaries in our galaxy, has often been interpreted interms of direct accretion onto a neutron star immersed in an extremelyclumpy stellar wind. We show here that the transitions across themagnetic and/or centrifugal barriers can explain the variabilityproperties of these sources as a result of relatively modest variationsin the stellar wind velocity and/or density. According to thisinterpretation we expect that supergiant fast X-ray transients whichdisplay very large luminosity swings and host a slowly spinning neutronstar are characterized by magnetar-like fields, irrespective of whetherthe magnetic or the centrifugal barrier applies. Supergiant fast X-raytransients might thus provide a new opportunity to detect and studymagnetars in binary systems.
| Long-term flux variations in Cen X-3: clues from flux-dependent orbital modulation and pulsed fraction
We have investigated the long-term flux variation in Cen X-3 usingorbital modulation and pulsed fraction in different flux states usingobservations made with the All-Sky Monitor and the Proportional CounterArray on board the Rossi X-ray Timing Explorer. In the high state, theeclipse ingress and egress are found to be sharp whereas in theintermediate state the transitions are more gradual. In the low state,instead of eclipse ingress and egress, the light curve shows a smoothflux variation with orbital phase. The orbital modulation of the X-raylight curve in the low state shows that the X-ray emission observed inthis state is from an extended object. The flux-dependent orbitalmodulations indicate that the different flux states of Cen X-3 areprimarily due to varying degree of obscuration. Measurement of thepulsed fraction in different flux states is consistent with the X-rayemission of Cen X-3 having one highly varying component with a constantpulsed fraction and an unpulsed component and in the low state, theunpulsed component becomes dominant. The observed X-ray emission in thelow state is likely to be due to scattering of X-rays from the stellarwind of the companion star. Though we cannot ascertain the origin andnature of the obscuring material that causes the aperiodic long-termflux variation, we point out that a precessing accretion disc driven byradiative forces is a distinct possibility.
| INTEGRAL SPI All-Sky View in Soft Gamma Rays: A Study of Point-Source and Galactic Diffuse Emission
We have processed the data accumulated with the INTEGRAL SPI instrumentover 4 years (~51 Ms) to study the morphology of the Galactic``diffuse'' emission in the 20 keV to 8 MeV energy range. To achievethis, we simultaneously derived an all-sky census of emitting sourcesand images of the Galactic ridge (GR) emission. In the central radian,the resolved point-source emission amounts to 88%, 91%, and 68% of thetotal in the 25-50, 50-100, and 100-300 keV domains, respectively. Wecompare the spatial distribution of the GR emission with thedistributions obtained from CO and near-IR maps and quantify our resultsthrough latitude and longitude profiles. Below 50 keV, the SPI data arebetter traced by the latter, supporting a stellar origin for thisemission. Furthermore, we find that the GR emission spectrum follows apower law with a photon index ~1.55 above 50 keV, while an additionalcomponent is required below that energy. This component shows a cutoffaround 30 keV, reinforcing a stellar origin, as proposed by Krivonos etal. The component of the diffuse emission due to e+/-annihilations is extracted simultaneously, leading to the determinationof the related parameters (positronium flux and fraction). Specificdiscussion is devoted to the annihilation-line distribution, sincesignificant emission is detected over a region as large as~80°×~10°, potentially associated with the disk or halosurrounding the central regions of our Galaxy.Based on observations with INTEGRAL, an ESA project with instruments andscience data center funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Spain, and Switzerland), theCzech Republic, and Poland, and with the participation of Russia and theUS.
| A Comparison of the Variability of the Symbiotic X-Ray Binaries GX 1+4, 4U 1954+31, and 4U 1700+24 from Swift BAT and RXTE ASM Observations
We present an analysis of the X-ray variability of three symbiotic X-raybinaries, GX 1+4, 4U 1700+24, and 4U 1954+31, using observations madewith the Swift Burst Alert Telescope (BAT) and the Rossi X-Ray TimingExplorer (RXTE) All-Sky Monitor (ASM). Observations of 4U 1954+31 withthe Swift BAT show modulation at a period near 5 hr. Models to explainthis modulation are discussed, including the presence of anexceptionally slow X-ray pulsar in the system and accretioninstabilities. We conclude that the most likely interpretation is that4U 1954+31 contains one of the slowest known X-ray pulsars. Unlike 4U1954+31, neither GX 1+4 nor 4U 1700+24 show any evidence for modulationon a timescale of hours. An analysis of the RXTE ASM light curves of GX1+4, 4U 1700+24, and 4U 1954+31 does not show the presence of periodicmodulation in any source, with the exception of a possible detection ofthe 5 hr period in 4U 1954+31, although there is considerablevariability on long timescales for all three sources. There is nomodulation in GX 1+4 on either the optical 1161 day orbital period or apreviously reported 304 day X-ray period. For 4U 1700+24 we do notconfirm the 404 day X-ray period previously proposed for this sourcefrom a shorter duration ASM light curve. We conclude that all threesources have substantial low-frequency noise in their power spectra thatmay give the appearance of periodic modulation if this noise is notproperly accounted for, particularly if short-duration light curves areexamined.
| Probing clumpy stellar winds with a neutron star
Context: INTEGRAL, the European Space Agency's γ-ray observatory,tripled the number of super-giant high-mass X-ray binaries (sgHMXB)known in the Galaxy by revealing absorbed and fast transient (SFXT)systems. Aims: In these sources, quantitative constraints on thewind clumping of the massive stars could be obtained from the study ofthe hard X-ray variability of the compact accreting object. Methods: Hard X-ray flares and quiescent emission of SFXT systems havebeen characterized and used to derive wind clump parameters. Results: A large fraction of the hard X-ray emission is emitted in theform of flares with a typical duration of 3 ks, frequency of 7 days andluminosity of 1036 erg/s. Such flares are most probablyemitted by the interaction of a compact object orbiting at 10Rast with wind clumps (1022-23 g) representing alarge fraction of the stellar mass-loss rate. The density ratio betweenthe clumps and the inter-clump medium is 102-4 in SFXTsystems. Conclusions: The parameters of the clumps and of theinter-clump medium, derived from the SFXT flaring behavior, are in goodagreement with macro-clumping scenario and line driven instabilitysimulations. SFXT have probably a larger orbital radius than classicalsgHMXB.
| INTEGRAL/IBIS all-sky survey in hard X-rays
We present results of an all-sky hard X-ray survey based on almost fouryears of observations with the IBIS telescope onboard the INTEGRALobservatory. The dead time-corrected exposure of the survey is ~33 Ms.Approximately 12% and 80% of the sky has been covered to limiting fluxeslower than 1 and 5 mCrab, respectively. Our catalog of detected sourcesincludes 403 objects, 316 of which exceed a 5σ detection thresholdon the time-averaged map of the sky, and the rest were detected invarious subsamples of exposures. Among the identified sources, 219 areGalactic (90 low-mass X-ray binaries, 76 high-mass X-ray binaries, 21cataclysmic variables, 6 coronally active stars, and other types) and137 are extragalactic, including 130 active galactic nuclei (AGNs) and 3galaxy clusters. We derived number-flux functions of AGNs and Galacticsources. The log N-log S relation of non-blazar AGNs is based on 68sources located at Galactic latitudes |b| > 5°, where the surveyis characterized by high identification completeness, with fluxes higherthan S_lim = 1.1 × 10-11 erg s-1cm-2(~0.8 mCrab) in the 17-60 keV energy band. The cumulativeAGN number-flux function can be described by a power law with a slope of1.62 ± 0.15 and normalization of (5.7±0.7) ×10-3 sources per deg2 at fluxes > 1.43 ×10-11 erg s-1 cm-2(>1 mCrab). ThoseAGNs with fluxes higher than S_lim make up ~1% of the cosmic X-raybackground at 17-60 keV. We present evidence of strong inhomogeneity inthe spatial distribution of nearby (⪉70 Mpc) AGNs, which reflectsthe large-scale structure in the local Universe.Based on observations with INTEGRAL, an ESA project with the instrumentsand science data center funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/475/775
| Radiative Column and Light Curve of X-Ray Binary Pulsars
We examine the published light curves (LCs) of 117 X-ray binary pulsars,while focusing on the dependence of their light curves on the observedenergy bands. It is found that the energy dependence of the LCs appearsonly when the X-ray luminosity is larger than ˜ 5 ×1036 erg s-1. Assuming that the behavior of lightcurve is related to the radiative accretion column on the neutron-starsurface, this energy threshold can be considered as observational proofof the accretion column formation proposed by Basko and Sunyaev. Once wecan grasp the existence of a radiative column, we can also obtainseveral useful pieces of information about the neutron-star properties.As an instance, we performed a statistical analysis of the orientationangle of the magnetic axis, and we found that the inclination angle ofthe magnetic axis should be small in order to explain the observedstatistics.
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