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Physics of strongly magnetized neutron stars.
There has recently been growing evidence for the existence of neutronstars possessing magnetic fields with strengths that exceed the quantumcritical field strength of $4.4 \times 10^{13}$ G, at which thecyclotron energy equals the electron rest mass. Such evidence has beenprovided by new discoveries of radio pulsars having very high spin-downrates and by observations of bursting gamma-ray sources termedmagnetars. This article will discuss the exotic physics of thishigh-field regime, where a new array of processes becomes possible andeven dominant, and where familiar processes acquire unusual properties.We review the physical processes that are important in neutron starinteriors and magnetospheres, including the behavior of free particles,atoms, molecules, plasma and condensed matter in strong magnetic fields,photon propagation in magnetized plasmas, free-particle radiativeprocesses, the physics of neutron star interiors, and field evolutionand decay mechanisms. Application of such processes in astrophysicalsource models, including rotation-powered pulsars, soft gamma-rayrepeaters, anomalous X-ray pulsars and accreting X-ray pulsars will alsobe discussed. Throughout this review, we will highlight theobservational signatures of high magnetic field processes, as well asthe theoretical issues that remain to be understood.
| Measuring the Mass of 4U 0900-40 Dynamically
Accurate measurements of neutron star masses are needed to constrain theequation of state of neutron star matter-of importance to both particlephysics and the astrophysics of neutron stars-and to identify theevolutionary track of the progenitor stars that form neutron stars. Thebest measured values of the mass of 4U 0900-40 (=Vela XR-1), 1.86+/-0.16Msolar (Barziv et al.) and 1.93+/-0.20 Msolar(Abubekerov et al.), make it a leading candidate for the most massiveneutron star known. The direct relationship between the maximum mass ofneutron stars and the equation of state of ultradense matter makes 4U0900-40 an important neutron star mass to determine accurately. Theconfidence intervals on previous mass estimates obtained fromobservations that include parameters determined by nondynamical methodsare not small enough to significantly restrict possible equations ofstate. We describe here a purely dynamical method for determining themass of 4U 0900-40, an X-ray pulsar, using the reprocessed UV pulsesemitted by its B0.5 Ib companion. One can derive the instantaneousradial velocity of each component through simultaneous X-ray and UVobservations at the two quadratures of the system. The Doppler shiftcaused by the primary's rotational velocity and the illumination patternof the X-rays on the primary, two of the three principal contributors tothe uncertainty on the derived mass of the neutron star, almost exactlycancel out by symmetry in this method. A heuristic measurement of themass of 4U 0900-40, using observations obtained previously with the HighSpeed Photometer on the Hubble Space Telescope, is given in an appendix.
| On the metallicity dependence of high-mass X-ray binaries
It is commonly assumed that high-mass X-ray binary (HMXB) populationsare little affected by metallicity. However, the massive stars making uptheir progenitor systems depend on metallicity in a number of ways, notleast through their winds. We present simulations, well-matched to theobserved sample of Galactic HMXBs, which demonstrate that both thenumber and the mean period of HMXB progenitors can vary withmetallicity, with the number increasing by about a factor of 3 betweensolar and Small Magellanic Cloud (SMC) metallicity. However, the SMCpopulation itself cannot be explained simply by metallicity effects; itrequires both that the HMXBs observed therein primarily sample the olderend of the HMXB population and that the star formation rate at the timeof their formation was very large.
| The coherence of kilohertz quasi-periodic oscillations in the X-rays from accreting neutron stars
We study in a systematic way the quality factor of the lower and upperkilohertz quasi-periodic oscillations (kHz QPOs) in a sample oflow-luminosity neutron star X-ray binaries, showing both QPOs varyingover a wide frequency range. The sample includes 4U 1636-536, 4U1608-522, 4U 1735-44, 4U 1728-34, 4U 1820-303 and 4U 0614+09. We findthat all sources except 4U 0614+09 show evidence of a drop in thequality factor of their lower kHz QPOs at high frequency. For 4U 0614+09only the rising part of the quality factor versus frequency curve hasbeen sampled so far. At the same time, in all sources but 4U 1728-34,the quality factor of the upper kHz QPO increases all the way to thehighest detectable frequencies. We show that the high-frequencybehaviours of both the lower and the upper kHz QPO quality factors areconsistent with what is expected if the drop is produced by the approachof an active oscillating region to the innermost stable circular orbit:the existence of which is a key feature of general relativity in thestrong field regime. Within this interpretation, our results implygravitational masses around 2Msolar for the neutron stars inthose systems.
| New soft gamma-ray bursts in the BATSE records and spectral properties of X-ray rich bursts
A population of X-ray dominated gamma-ray bursts (GRBs) observed byGinga, BeppoSAX and the High Energy Transient Explorer (HETE-2) shouldbe represented in the Burst and Transient Source Experiment (BATSE) dataas presumably soft bursts. We have performed a search for soft GRBs inthe BATSE records in the 25-100 keV energy band. The softness of a burstspectrum could explain why it has been missed by the on-board procedureand by the previous searches for untriggered GRBs tuned to the 50-300keV range. We have found a surprisingly small number (~20yr-1 with fluxes down to 0.1 photoncm-2s-1) of soft GRBs where the count rate isdominated by the 25-50 keV energy channel. This fact, as well as theanalysis of HETE-2 and common BeppoSAX/BATSE GRBs, indicates that themajority of GRBs with a low Epeak have a relatively hard tailwith a high-energy power-law photon index β > -3. An exponentialcutoff in GRB spectra below 10-15 keV may be a distinguishing feature ofnon-GRB events.
| Super-orbital period in the high-mass X-ray binary 2S 0114+650
We report the detection of a stable super-orbital period in thehigh-mass X-ray binary 2S 0114+650. Analyses of data from the RossiX-ray Timing Explorer All-Sky Monitor from 1996 January 5 to 2004 August25 reveal a super-orbital period of 30.7+/-0.1 d, in addition toconfirming the previously reported neutron star spin period of 2.7 h andthe binary orbital period of 11.6 d. It is unclear if the super-orbitalperiod can be ascribed to the precession of a warped accretion disc inthe system.
| 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 pulsarOAO 1657-415. A short observation near mid-eclipse caught the source ina low-intensity state, with a weak continuum and iron emission dominatedby the 6.4-keV fluorescent line. A later, longer observation found thesource in a high-intensity state and covered the uneclipsed throughmid-eclipse phases. In the high-intensity state, the non-eclipsespectrum has an absorbed continuum component due to scattering bymaterial near the pulsar and 80 per cent of the fluorescent ironemission comes from less than 19 light-second away from the pulsar. Wefind a dust-scattered X-ray halo whose intensity decays through theeclipse. We use this halo to estimate the distance to the source as 7.1+/- 1.3kpc.
| INTEGRAL and XMM-Newton observations of the X-ray pulsar IGR J16320-4751/AX J1631.9-4752
We report on observations of the X-ray pulsar IGR J16320-4751 (alsoknown as AX J1631.9-4752) performed simultaneously with InternationalGamma-Ray Astrophysics Laboratory (INTEGRAL) and XMM-Newton. We refinethe source position and identify the most likely infrared counterpart.Our simultaneous coverage allows us to confirm the presence of X-raypulsations at ~1300 s, that we detect above 20 keV with INTEGRAL for thefirst time. The pulse fraction is consistent with being constant withenergy, which is compatible with a model of polar accretion by a pulsar.We study the spectral properties of IGR J16320-4751 during two majorperiods occurring during the simultaneous coverage with both satellites,namely a flare and a non-flare period. We detect the presence of anarrow 6.4 keV iron line in both periods. The presence of such a featureis typical of supergiant wind accretors such as Vela X-1 or GX 301-2. Weinspect the spectral variations with respect to the pulse phase duringthe non-flare period, and show that the pulse is solely due tovariations of the X-ray flux emitted by the source and not due tovariations of the spectral parameters. Our results are thereforecompatible with the source being a pulsar in a High Mass X-ray Binary.We detect a soft excess appearing in the spectra as a blackbody with atemperature of ~0.07 keV. We discuss the origin of the X-ray emission inIGR J16320-4751: while the hard X-rays are likely the result of Comptonemission produced in the close vicinity of the pulsar, based on energyargument we suggest that the soft excess is likely the emission by acollisionally energized cloud in which the compact object is embedded.
| Chandra observation of SAX J1818.6-1703: confirmation of optical counterpart & suggestion of non-transient nature
SAX J1818.6-1703, discovered in 1998 with the BeppoSAX Wide Field Cameras (in 't Zand et al. 1998, IAUC #6840), was found with INTEGRAL and RXTE to exhibit few-hours long outbursts similar to the Supergiant Fast X- ray Transients (Grebenev & Sunyaev 2005, AstL 31, 672;Sguera et al. 2005, A&A 444, 221). Negueruela & Smith (2006;ATel #831), on the basis of this analogy, identified within the 1.5arcmin radius error region determined by Grebenev & Sunyaev alikely optical/NIR counterpart whose I-band spectrum is typical for anOB supergiant.
| Outburst of IGR J11215-5952 Observed with RXTE
Sidoli et al. (2006, astro-ph/0203081) reported that the hard x-raytransient IGR J11215-5952 appeared to have a recurrence period of ~330days, based on three short outbursts observed by INTEGRAL, the mostrecent of which was actually the discovery observation (Lubinski et al., ATEL #469). This short outburst, and the presence in the INTEGRAL error circle of the blue supergiant HD 306414 (Negueruela et al.
| Vela X-1 in a highly active state
Recent routine INTEGRAL Galactic Plane Scan (GPS) observations includingthe Vela region have shown that Vela X-1 has entered a highly activestate, similar to the December 2003 flaring state (Staubert et al.,2004, Conf. Proc. 5th Integral Workshop). During the GPSobservations on 2006 January 16 and 2006 January 22, Vela X-1 was inthe field of view of ISGRI for 9 ksec and 12 ksec, respectively.
| Hard X-ray bursts detected by the IBIS telescope onboard the INTEGRAL observatory in 2003 2004
All of the observations performed with the IBIS telescope onboard theINTEGRAL observatory during the first one and a half years of itsin-orbit operation (from February 10, 2003, through July 2, 2004) havebeen analyzed to find X-ray bursts. The time history of the IBIS/ISGRItotal count rate in the energy range 15-25 keV revealed 1077 bursts ofdurations from 5 to 500 s with a high statistical significance(over the entire period of observations, only one event could bedetected by chance with a probability of 20%). A part from the eventsassociated with cosmic gamma-ray bursts (detected in the field of viewor passed through the IBIS shield), solar flares, and activity of thesoft gamma repeater SGR 1806-20, we were able to localize 105 burstsand, with one exception, to identify them with previously knownpersistent or transient X-ray sources (96 were identified with knownX-ray bursters). In one case, the burst source was a new burster in alow state that received the name IGR J17364-2711. Basic parameters ofthe localized bursts and their identifications are presented in thecatalog of bursts. Curiously enough, 61 bursts were detected from oneX-ray burster, GX 354-0. The statistical distributions of bursts induration, maximum flux, and recurrence time have been analyzed for thissource. Some of the bursts observed with the IBIS/ISGRI telescope werealso detected by the JEM-X telescope onboard the INTEGRAL observatory inthe standard X-ray energy range 3-20 keV.
| A Systematic Search for Periodicities in RXTE ASM Data
We present the results of a systematic search in 8.5 yr of Rossi X-RayTiming Explorer All-Sky Monitor data for evidence of periodicities. Thesearch was conducted by application of the Lomb-Scargle periodogram tothe light curves of each of 458 actually or potentially detected sourcesin each of four energy bands (1.5-3, 3-5, 5-12, and 1.5-12 keV). Awhitening technique was applied to the periodograms before evaluation ofthe statistical significance of the powers. We discuss individualdetections with a focus on relatively new findings.
| Exploration of the Ps-Porb Relation for Wind-fed X-Ray Pulsars
We have investigated the relation between the orbital periods(Porb) and the spin periods (Ps) of wind-fed X-raypulsars in high-mass X-ray binaries (HMXBs), based on populationsynthesis calculations of the spin evolution of neutron stars during thepre-HMXB stage. We show that most neutron stars either have steadyaccretion or still reside in the radio pulsar phase when the donor starstarts to evolve off the main sequence. In the former case, the spinperiod can be decelerated to ~102-103 s, dependingon the value of Porb. We briefly discuss possible origins ofthe Ps-Porb correlation in Be/X-ray binaries andthe existence of HMXBs with main-sequence donors. We also investigatethe evolution of the inclination angle between the magnetic and spinaxes of neutron stars in a massive binary, suggesting secular alignmentof the magnetic and spin axes during their evolution.
| X-Ray Variability in the Young Massive Triple θ2 Orionis A
Massive stars rarely show intrinsic X-ray variability. One exception isθ2 Ori A, which has shown strong variability over thelast 5 years. We observed a large outburst of the X-ray source with theHigh Energy Transmission Grating Spectrometer on board Chandra andcompare the emissivity and line properties in states of low and highflux. The low state indicates temperatures well above 25 MK. In the highstate we find high emissivities in the range from 3 to over 100 MK. Theoutburst event in stellar terms is one of the most powerful everobserved and the most energetic one in the ONC, with a lower totalenergy limit of 1.5×1037 ergs. The line diagnosticsindicate that the line-emitting regions in the low states are as closeas within 1-2 stellar radii from the O star's photosphere, whereas thehard states suggest a distance of 3-5 stellar radii. We discuss theresults in the context of stellar flares, magnetic confinement, andbinary interactions. By matching the dates of all observations with theorbital phases of the spectroscopy binary orbit, we find that outburstsoccur very close to the periastron passage of the stars. We argue thatthe high X-ray states are possibly the result of reconnection eventsfrom magnetic interactions of the primary and secondary stars of thespectroscopic binary. Effects from wind collisions seem unlikely forthis system. The line properties in the low state seem consistent withsome form of magnetic confinement. We also detect Fe fluorescenceindicative of the existence of substantial amounts of neutral Fe in thevicinity of the X-ray emission.
| Discovery of an Eclipsing X-Ray Binary with a 32.69 hr Period in M101: An Analog of Her X-1 or LMC X-4?
We report the discovery of an eclipsing X-ray binary in M101, the firstsuch system to be discovered outside the Local Group. Based on asequence of 25 Chandra observations that sample a wide range of orbitalphases, we find a period of 32.688+/-0.002 hr, which we interpret as anorbital period. The folded light curve exhibits an eclipse lasting about8 hr, suggesting a compact orbit in a nearly edge-on configuration. TheX-ray binary has an average luminosity of LX(0.3-8keV)~1.3×1038 ergs s-1, with only 1 out ofthe 25 observations significantly lower in flux than the average lightcurve. The presence of the eclipse and the ~1.4 day orbital periodsuggests that this source is an analog of the well-studied eclipsingX-ray binary pulsars Her X-1 or LMC X-4. Combining the Chandra data andthe HST ACS/WFC images, we have identified several possible opticalcounterparts, including an O5-O3 star with V=25.0 mag. Follow-up opticalmonitoring observations should be able to identify the donor and furtherconstrain the orbital properties.
| High-Energy Neutrino Astronomy Using Upward-going Muons in Super-Kamiokande I
We present the results from several studies used to search forastrophysical sources of high-energy neutrinos using theSuper-Kamiokande I (1996 April-2001 July) neutrino-induced upward-goingmuon data. The data set consists of 2359 events with minimum energy 1.6GeV, of which 1892 are through-going and 467 stop within the detector.The results of several independent analyses are presented, includingsearches for point sources using directional and temporal informationand a search for signatures of cosmic-ray interactions with theinterstellar medium in the upward-going muons. No statisticallysignificant evidence for point sources or any diffuse flux from theplane of the Galaxy was found, so specific limits on fluxes from likelypoint sources are calculated. The 90% confidence level (CL) upper limitson upward-going muon flux from astronomical sources that are located inthe southern hemisphere and always under the horizon forSuper-Kamiokande are ~(1-4)×10-15 cm-2s-1.
| X-Ray Spectral Study of the Photoionized Stellar Wind in Vela X-1
We present results from quantitative modeling and spectral analysis ofthe high-mass X-ray binary system Vela X-1 obtained with the ChandraHETGS. The spectra exhibit emission lines from H- and He-like ionsdriven by photoionization, as well as fluorescent emission lines fromseveral elements in lower charge states. The properties of these X-raylines are measured with the highest accuracy to date. In order tointerpret and make full use of the data, we have developed a simulator,which calculates the ionization structure of a stellar wind and performsMonte Carlo simulations of X-ray photons propagating through the wind.From comparisons of the observed spectra with results from thesimulator, we are able to find the ionization structure and thegeometrical distribution of material in the stellar wind that canreproduce the observed spectral line intensities and continuum shapesremarkably well. We find that the stellar wind profile can berepresented by a CAK model with a star mass-loss rate of(1.5-2.0)×10-6 Msolar yr-1,assuming a terminal velocity of 1100 km s-1. It is found thata large fraction of emission lines from highly ionized ions are formedin the region between the neutron star and the companion star. We alsofind that the fluorescent lines must be produced in at least threedistinct regions: the extended stellar wind, reflection off the stellarphotosphere, and in a distribution of dense material partially coveringand possibly trailing the neutron star, which may be associated with anaccretion wake. Finally, from detailed analysis of the emission-lineprofiles, we demonstrate that the stellar wind dynamics is affected byX-ray photoionization.
| Identification of the Infrared Counterpart to a Newly Discovered X-Ray Source in the Galactic Center
We present first results of a campaign to find and identify new compactobjects in the Galactic center. Selecting candidates from a combinationof Chandra and 2MASS survey data, we search for accretion disksignatures via infrared spectroscopy. We have found the infraredcounterpart to the Chandra source CXO J174536.1-285638, the spectrum ofwhich has strong Brγ and He I emission. The presence of C III, NIII, and He II indicate a binary system. We suspect that the system issome form of high-mass binary system, either a high-mass X-ray binary ora colliding-wind binary.
| Determining the Nature of the SS 433 Binary from an X-Ray Spectrum during Eclipse
We test the physical model of the relativistic jets in the GalacticX-ray binary SS 433 proposed in our previous paper using additionalobservations from the Chandra High Energy Transmission GratingSpectrometer. These observations sample two new orbital/precessionalphase combinations. In the observation near orbital phase zero, the H-and He-like Fe lines from both receding and approaching jets arecomparably strong and unocculted while the He-like Si line of thereceding jet is significantly weaker than that of the approaching jet.This condition may imply that the cooler parts of the receding jet areeclipsed by the companion. The X-ray spectrum from this observation hasbroader emission lines than obtained in Paper I that may arise from thedivergence of a conical outflow or from Doppler shift variations duringthe observation. Using recent optical results, along with the length ofthe unobscured portion of the receding jet assuming adiabatic cooling,we calculate the radius of the companion to be 9.6+/-1.0Rsolar, about one-third of the Roche lobe radius. For amain-sequence star, this corresponds to a companion mass of 35+/-7Msolar, giving a primary source mass of 20+/-5Msolar. If our model is correct, this calculation indicatesthat the compact object is a black hole, and accretion occurs through awind process. In a subsequent paper, we will examine the validity of theadiabatic cooling model of the jets and test the mode of linebroadening.
| Identifications of Four INTEGRAL Sources in the Galactic Plane via Chandra Localizations
Hard X-ray imaging of the Galactic plane by the INTEGRAL satellite isuncovering large numbers of 20-100 keV ``IGR'' sources. We presentresults from Chandra, INTEGRAL, optical, and IR observations of four IGRsources: three sources in the Norma region of the Galaxy (IGRJ16195-4945, IGR J16207-5129, and IGR J16167-4957) and one that iscloser to the Galactic center (IGR J17195-4100). In all four cases, onerelatively bright Chandra source is seen in the INTEGRAL error circle,and these are likely to be the soft X-ray counterparts of the IGRsources. They have hard 0.3-10 keV spectra with power-law photon indicesof Γ=0.5-1.1. While many previously studied IGR sources show highcolumn densities (NH~1023-1024cm-2), only IGR J16195-4945 has a column density that couldbe as high as 1023 cm-2. Using optical and IR skysurvey catalogs and our own photometry, we have obtained identificationsfor all four sources. The J-band magnitudes are in the range 14.9-10.4,and we have used the optical/IR spectral energy distributions (SEDs) toconstrain the nature of the sources. Blackbody components withtemperature lower limits of >9400 K for IGR J16195-4945 and>18,000 K for IGR J16207-5129 indicate that these are very likelyhigh-mass X-ray binaries (HMXBs). However, for IGR J16167-4957 and IGRJ17195-4100, low extinction and the SEDs indicate later spectral typesfor the putative companions, suggesting that these are not HMXBs.
| A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
| On the Spin-up/Spin-down Transitions in Accreting X-Ray Binaries
Accreting X-ray binaries display a wide range of behaviors. Some of themare observed to spin up steadily, others to alternate between spin-upand spin-down states, sometimes superimposed on a longer trend of eitherspin-up or spin-down. Here we interpret this rich phenomenology within anew, simple model of the disk-magnetosphere interaction. Our model,based on the simplest version of a purely material torque, accounts forthe fact that when a neutron star is in the propeller regime, a fractionof the ejected material does not receive enough energy to completelyunbind, and hence falls back into the disk. We show that the presence ofthis feedback mass component causes the occurrence of multiple statesavailable to the system for a given, constant value of the massaccretion rate M˙* from the companion star. If the angleχ of the magnetic dipole axis with respect to the perpendicular tothe disk is larger than a critical value χcrit, thesystem eventually settles in a cycle of spin-up/spin-down transitionsfor a constant value of M˙* and independent of theinitial conditions. No external perturbations are required to induce thetorque reversals. The transition from spin-up to spin-down is oftenaccompanied by a large drop in luminosity. The frequency range spannedin each cycle and the timescale for torque reversals depend onM˙*, the magnetic field of the star, the magneticcolatitude χ, and the degree of elasticity regulating themagnetosphere-disk interaction. The critical angle χcritranges from ~25° to 30° for a completely elastic interaction to~40°-45° for a totally anelastic one. Forχ<~χcrit, cycles are no longer possible and thelong-term evolution of the system is a pure spin-up. We specificallyillustrate our model in the cases of the X-ray binaries GX 1+4 and 4U1626-67.
| The Optical Counterpart to the Peculiar X-Ray Transient XTE J1739-302
The weak X-ray transient XTE J1739-302, characterized by extremely shortoutbursts, has recently been identified with a reddened star. Here wepresent spectroscopy and photometry of the counterpart, identifying itas a O8 Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 thusbecomes the prototype of the new class of supergiant fast X-raytransients (SFXTs). The optical and infrared spectra of the counterpartto XTE J1739-302 do not reveal any obvious characteristics setting itapart from other X-ray binaries with supergiant companions, whichdisplay a very different type of X-ray light curve.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO 73.D-0081).
| XTE J1739-302 as a Supergiant Fast X-Ray Transient
XTE J1739-302 is a transient X-ray source with unusually shortoutbursts, lasting on the order of hours. Here we give a summary ofX-ray observations we have made of this object in outburst with theRossi X-Ray Timing Explorer (RXTE) and at a low level of activity withthe Chandra X-Ray Observatory, as well as observations made by othergroups. Visible and infrared spectroscopy of the mass donor of XTEJ1739-302 are presented in a companion paper. The X-ray spectrum is hardboth at low levels and in outburst, but somewhat variable, and there isstrong variability in the absorption column from one outburst toanother. Although no pulsation has been observed, the outburst data frommultiple observatories show a characteristic timescale for variabilityon the order of 1500-2000 s. The Chandra localization(R.A.=17h39m11.58s,decl.=-30deg20'37.6", J2000.0) shows that despitebeing located less than 2° from the Galactic center and highlyabsorbed, XTE J1739-302 is actually a foreground object with a brightoptical counterpart. The combination of a very short outburst timescaleand a supergiant companion is shared with several other recentlydiscovered systems, forming a class we designate as supergiant fastX-ray transients (SFXTs). Three persistently bright X-ray binaries withsimilar supergiant companions have also produced extremely short, brightoutbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.
| A torque reversal of 4U 1907+09
Aims.We present an analysis of the accreting X-ray pulsar system 4U1907+09 based on INTEGRAL data. The main focus of this analysis is astudy of the timing behavior of this source. In addition we also show ananalysis of the 5-90 keV spectrum. Methods: .The data wereextracted using the official INTEGRAL software OSA 5.1. Timing analysiswas performed using epoch folding and pulsar pulse phasing.Results: .We have measured 12 individual pulse periods for the years2003 to 2005. We confirm earlier RXTE results that during 2003 the spindown became slower and show furthermore that after this phase 4U 1907+09started to spin up with dot{P}_pulse=-0.158 s yr-1 in 2004.The similarity of the pulse period histories of 4U 1907+09 and 4U1626-26 suggests that accretion onto an oblique rotator, as recentlyproposed by Perna et al., is a possible explanation for this change.
| The X-ray binary 2S0114+650=LSI+65 010. A slow pulsar or tidally-induced pulsations?
Context: .The X-ray source 2S0114+650=LSI+65 010 is a binary systemcontaining a B-type primary and a low mass companion believed to be aneutron star. The system has three reported periodicities: the orbitalperiod, P_orb11.6 days, X-ray flaring with P_flare2.7 h and a"superorbital" X-ray periodicity P_super=30.7 days. Aims: .Theobjective of this paper is to show that the puzzling periodicities inthe system may be explained in the context of scenarios in which tidalinteractions drive oscillations in the B-supergiant star. Methods: .We calculate the solution of the equations of motion for onelayer of small surface elements distributed along the equator of thestar, as they respond to the forces due to gas pressure, centrifugal,Coriolis, viscous forces, and the gravitational forces of both stars,which provides variability timescales that can be compared with thoseobserved for 2S0114+650. In addition, we use observational data obtainedat the Observatorio Astronómico Nacional en San PedroMártir (OAN/SPM) between 1993-2004 to determine whichperiodicities may be present in the optical region. Results: .Themodels for circular orbits predict "superorbital" periods while theeccentric orbit models predict strong variations on orbital timescales,associated with periastron passage. Both also predict oscillations ontimescales of ~2 h. We suggest that the tidal oscillations lead to astructured stellar wind which, when fed to the neutron star, producesthe X-ray modulations. The connection between the stellar oscillationsand the modulation of the mass ejection may lie in the shear energydissipation generated by the tangential motions that are produced by thetidal effects, particularly in the tidal bulge region. From anobservational standpoint, we find indications for variability in the HeI 5875 Å line on ~2 h timescale and, possibly, the "superorbital"timescale. However, the line profile variability exceeds that which ispredicted by the tidal interaction model and can be understood in termsof variable emission that is superposed on the photospheric absorption.This emission appears to be associated with the B-supergiant's stellarwind rather than the vicinity of the companion. Conclusions: .Themodel calculations lead us to conclude that the B-supergiant may be theorigin of the periodicities observed in the X-ray data, through acombination of a localized structured wind that is fed to the collapsedobject and, possibly, by production of X-ray emission on its ownsurface. This scenario weakens the case for 2S0114+650 containing amagnetar descendent.
| VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2
Model atmosphere fits to high-resolution optical spectra ofWray 977 confirm the B hypergiant classification ofthe massive companion to the X-ray pulsar GX301-2.The models give a radius of 62 Rȯ, an effectivetemperature of 18 100 K and a luminosity of 5 × 105Lȯ. These values are somewhat reduced compared to thestellar parameters of Wray 977 measured previously.The deduced mass-loss rate and terminal velocity of the stellar wind are10-5 Mȯ yr-1 and 305 kms-1, respectively. The interstellar Na I D absorptionindicates that Wray 977 is located behind the firstintersection with the Sagittarius-Carina spiral arm (1-2.5 kpc) andprobably belongs to the stellar population of the Norma spiral arm at adistance of 3-4 kpc. The luminosity derived from the model atmosphere isconsistent with this distance (3 kpc). The luminosity of the wind-fedX-ray pulsar (LX 1037 erg s-1) isin good accordance with the Bondi-Hoyle mass accretion rate. The spectraobtained with UVES on the Very Large Telescope (VLT) cover a full orbitof the system, including periastron passage, from which we derive theradial-velocity curve of the B hypergiant. The measured radial-velocityamplitude is 10 ± 3 km s-1 yielding a mass ratio q =M_X/M_opt = 0.046 ± 0.014. The absence of an X-ray eclipseresults in a lower limit to the mass of Wray 977 of39 Mȯ. An upper limit of 68 or 53 Mȯ isderived for the mass of Wray 977 adopting a maximumneutron star mass of 3.2 or 2.5 Mȯ, respectively. Thecorresponding lower limit to the system inclination is i > 44°,supporting the view that the dip in the X-ray lightcurve is due toabsorption by the dense stellar wind of Wray 977(Leahy 2002). The "spectroscopic" mass of Wray 977 is43 ± 10 Mȯ, consistent with the range in massderived from the binarity constraints. The mass of the neutron star is1.85 ± 0.6 Mȯ. Time series of spectral linesformed in the dense stellar wind (e.g. He I 5876 Å and H α)indicate the presence of a gas stream trailing the neutron star in itsorbit. The long-term behaviour of the H α equivalent widthexhibits strong variations in wind strength; the sampling of the data isinsufficient to conclude whether a relation exists between windmass-loss rate and pulsar spin period.
| IGR J17544-2619: a new supergiant fast X-ray transient revealed by optical/infrared observations
One of the most recent discoveries of the INTEGRAL observatory is theexistence of a previously unknown population of X-ray sources in theinner arms of the Galaxy. IGR J17544-2619,IGR J16465-4507 and XTE J1739-302are among these sources. Although the nature of these systems is stillunexplained, the investigations of the optical/NIR counterparts of thetwo last sources, combined with high energy data, have provided evidenceof them being highly absorbed high mass X-ray binaries with bluesupergiant secondaries and displaying fast X-ray transient behaviour. Inthis work we present our optical/NIR observations of IGRJ17544-2619, aimed at identifying and characterizing itscounterpart. We show that the source is a high mass X-ray binary at adistance of 2-4 kpc with a strongly absorbed O9Ib secondary, and discussthe nature of the system.
| XMM-Newton observation of the Be/neutron star system RX J0146.9+6121: a soft X-ray excess in a low luminosity accreting pulsar
We report on the XMM-Newton observation of the Be/neutron star X-raybinary system RX J0146.9+6121, a long period (~23 m) pulsar in the NGC663 open cluster. The X-ray luminosity decreased by a factor of twocompared to the last observation carried out in 1998, reaching a levelof 1×1034 erg s-1, the lowest everobserved in this source. The spectral analysis reveals the presence of asignificant excess at low energies over the main power-law spectralcomponent. The soft excess can be described by a black-body spectrumwith a temperature of about 1 keV and an emitting region with a radiusof ~140 m. Although the current data do not permit us to ascertainwhether the soft excess is pulsed or not, its properties are consistentwith emission from the neutron star polar cap. This is the thirddetection of a soft excess in a low luminosity(1×1034 erg s-1) pulsar, the othersbeing X Per and 3A 0535+262, suggesting that such a spectral component,observed to date in higher luminosity systems, is a common feature ofaccreting X-ray pulsars. The results of these three sources indicatethat, in low luminosity systems, the soft excess tends to have a highertemperature and a smaller surface area than in the high luminosity ones.
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