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Near-infrared survey of high mass X-ray binary candidates
Context. The INTEGRAL satellite is detecting a large population of newX-ray sources that were missed by previous missions because of highobscuration and, in some cases, very short duty cycles. The nature ofthese sources must be addressed by characterizing their optical and/orinfrared counterparts. Aims: We investigate the nature of theoptical counterparts to five of these newly discovered X-ray sources. Methods: We combine infrared spectra in the I, J,H, and K bandswith JHK photometry to characterize the spectral type, luminosity class,and distance to the infrared counterparts to these systems. For IGRJ19140+0951, we present spectroscopy from the red to the K band and newred and infrared photometry. For SAX J18186-1703 and IGR J18483-0311, wepresent the first intermediate-resolution spectroscopy to be published.Finally, for IGR J18027-2016, we present new I and K band spectra. Results: We find that four systems harbour early-type B supergiants.All of them are heavily obscured, with E(B-V) ranging between 3 and 5,implying visual extinctions of ~ 9 to 15 mag. We refine the publishedclassifications of IGR J18027-2016 and IGR J19140+0951 by constrainingtheir luminosity class. In the first case, we confirm the supergiantnature but exclude a class III. In the second case, we propose aslightly higher luminosity class (Ia instead of Iab) and provide animproved value of the distance based on new optical photometry. Twoother systems, SAX J18186-1703 and IGR J18483-0311 are classified assupergiant fast X-ray transients (SFXTs). XTE J1901+014, on the otherhand, contains no bright infrared source in its error circle. Conclusions: Owing to their infrared and X-ray characteristics, IGRJ18027-2016 and IGR J19140+0951, emerge as supergiant X-ray binarieswith X-ray luminosities of the order of L_X˜ [1-2]×1036 erg s-1, while SAX J1818.6-1703 and IGRJ18483-0311, are found to be SFXTs at 2 and 3 kpc, respectively.Finally, XTE J1901+014 emerges as a puzzling source: its X-ray behaviouris strongly reminiscent of SFXTs, but a supergiant nature is firmlyexcluded for the counterpart. We discuss several alternative scenariosto explain its behaviour.
| Supergiant fast X-ray transients: a review
We review the status of our knowledge on supergiant fast X-raytransients (SFXTs), a new hot topic in multi wavelength studies ofbinaries. We discuss the mechanisms believed to power these transientsand then highlight the unique contribution Swift is giving to thisfield, and how new technology complements and sometimes changes the viewof things.
| Supergiant Fast X-ray Transients in outburst: new Swift observations of XTE J1739-302, IGR J17544-2619 and IGR J08408-4503
We report on new X-ray outbursts observed with Swift from threeSupergiant Fast X-ray Transients (SFXTs): XTE J1739-302, IGR J17544-2619and IGR J08408-4503. XTE J1739-302 underwent a new outburst on 2008August 13, IGR J17544-2619 on 2008 September 4 and IGR J08408-4503 on2008 September 21. While the XTE J1739-302 and IGR J08408-4503 brightemission triggered the Swift/Burst Alert Telescope, IGR J17544-2619 didnot, thus we could perform a spectral investigation only of the spectrumbelow 10 keV. The broad-band spectra from XTE J1739-302 and IGRJ08408-4503 were compatible with the X-ray spectral shape displayedduring the previous flares. A variable absorbing column density duringthe flare was observed in XTE J1739-302 for the first time. Thebroad-band spectrum of IGR J08408-4503 requires the presence of twodistinct photon populations, a cold one (~0.3keV) most likely from athermal halo around the neutron star and a hotter one (1.4-1.8keV) fromthe accreting column. The outburst from XTE J1739-302 could be monitoredwith a very good sampling, thus revealing a shape which can be explainedwith a second wind component in this SFXT, in analogy to what we havesuggested in the periodic SFXT IGR J11215-5952. The outburst recurrencetime-scale in IGR J17544-2619 during our monitoring campaign with Swiftsuggests a long orbital period of ~150 d (in a highly eccentric orbit),compatible with what previously observed with INTEGRAL.
| Multiple flaring activity in the supergiant fast X-ray transient IGR J08408-4503 observed with Swift
IGR J08408-4503 is a supergiant fast X-ray transient discovered in 2006with a confirmed association with a O8.5Ib(f) supergiant star, HD 74194.We report on the analysis of two outbursts caught by Swift/Burst AlertTelescope (BAT) on 2006 October 4 and 2008 July 5, and followed up atsofter energies with Swift/X-ray Telescope (XRT). The 2008 XRT lightcurve shows a multiple-peaked structure with an initial bright flarethat reached a flux of ~10-9 ergcm-2s-1(2-10keV), followed by two equally bright flares within 75ks. Thespectral characteristics of the flares differ dramatically, with most ofthe difference, as derived via time-resolved spectroscopy, being due toabsorbing column variations. We observe a gradual decrease in theNH, derived with a fit using absorbed power-law model, astime passes. We interpret these NH variations as due to anionization effect produced by the first flare, resulting in asignificant decrease in the measured column density towards the source.The durations of the flares as well as the times of the outburstssuggest that the orbital period is ~35 d, if the flaring activity isinterpreted within the framework of the Sidoli et al. model with theoutbursts triggered by the neutron star passage inside an equatorialwind inclined with respect to the orbital plane.
| Discovery of an eccentric 30 day period in the supergiant X-ray binary SAX J1818.6-1703 with INTEGRAL
Context: SAX J1818.6-1703 is a flaring transient X-ray sourceserendipitously discovered by BeppoSAX in 1998 during an observation ofthe Galactic centre. The source was identified as a high-mass X-raybinary with an OB supergiant companion (SGXB). Displaying short andbright flares and an unusually very low quiescent level implying anintensity dynamical range as large as 103-4, the source wasclassified as a supergiant fast X-ray transient (SFXT). Aims: Themechanism triggering the different temporal behaviour observed betweenthe classical SGXBs and the recently discovered class of SFXTs is stilldebated. The discovery of long orbits (>15 d) should help todiscriminate between emission models and bring constraints. Methods: We analysed archival INTEGRAL data on SAX J1818.6-1703. Webuilt short- and long-term light curves and performed a timing analysisin order to study the temporal behaviour of SAX J1818.6-1703 ondifferent time scales. Results: INTEGRAL revealed an unusuallylong orbital period of 30.0 ± 0.2 d and an elapsed accretionphase of ~6 d in the transient SGXB SAX J1818.6-1703. This implies anelliptical orbit and constraints the possible supergiant spectral typebetween B0.5-1I with eccentricities e ~ 0.3-0.4 (for the averagefundamental parameters of supergiant stars). During the accretion phase,the source behaved like a classical SGXB. The huge variations of theobserved X-ray flux can be explained through accretion of macro-clumpsformed within the stellar wind. Our analysis strengthens the model whichpredicts that SFXTs behave as SGXBs but with different orbitalparameters, thus different temporal behaviour.Discovery first reported at the 7th INTEGRAL workshop (Zurita Heras etal., in press). Based on observations with INTEGRAL, an ESA project withinstruments and science data centre funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), Czech Republic and Poland, and with theparticipation of Russia and the USA.
| High variability in Vela X-1: giant flares and off states
Aims: We investigate the spectral and temporal behavior of the high massX-ray binary Vela X-1 during a phase of high activity, with specialfocus on the observed giant flares and off states. Methods:INTEGRAL observed Vela X-1 in a long almost uninterrupted observationfor two weeks in 2003 Nov/Dec. The data were analyzed with OSA 7.0 andFTOOLS 6.2. We derive the pulse period, light curves, spectra, hardnessratios, and hardness intensity diagrams, and study the eclipse. Results: In addition to an already high activity level, Vela X-1exhibited several intense flares, the brightest ones reaching a maximumintensity of more than 5 Crab in the 20-40 keV band and several offstates where the source was no longer detected by INTEGRAL. We determinethe pulse period to be 283.5320±0.0002 s, which is stablethroughout the entire observation. Analyzing the eclipses provided animprovement in the ephemeris. Spectral analysis of the flares indicatesthat there appear to be two types of flares: relatively brief flares,which can be extremely intense and show spectral softening, in contrastto high intensity states, which are longer and show no softening. Conclusions: Both flares and off states are interpreted as being dueto a strongly structured wind of the optical companion. When Vela X-1encounters a cavity with strongly reduced density, the flux will droptriggering the onset of the propeller effect, which inhibits furtheraccretion, giving rise to off states. The sudden decrease in the densityof the material required to trigger the propeller effect in Vela X-1 isof the same order as predicted by theoretical papers about the densitiesin OB star winds. A similarly structured wind can produce giant flareswhen Vela X-1 encounters a dense blob in the wind.
| Monitoring Supergiant Fast X-Ray Transients with Swift. I. Behavior outside Outbursts
Supergiant fast X-ray transients (SFXTs) are a new class of high-massX-ray binaries (HMXBs) discovered thanks to the monitoring of theGalactic plane performed with the INTEGRAL satellite in the last 5years. These sources display short outbursts (significantly shorter thantypical Be/X-ray binaries) with a peak luminosity of a few1036 erg s-1. The quiescent level, measured onlyin a few sources, is around 1032 erg s-1. TheX-ray spectral properties are reminiscent of those of accreting pulsars;thus, it is likely that all the members of the new class are indeedHMXBs hosting a neutron star, although only two SFXTs have a measuredpulse period, IGR J11215-5952 (~187 s) and IGR J18410-0535 (~4.7 s).Several competing mechanisms have been proposed to explain the shortnessof these outbursts, mostly involving the structure of the wind from thesupergiant companion. To characterize the properties of these sources ontimescales of months (e.g., the quiescent level and the outburstrecurrence), we are performing a monitoring campaign with Swift of fourSFXTs (IGR J16479-4514, XTE J1739-302, IGR J17544-2619, and AXJ1841.0-0536/IGR J18410-0535). We report on the first 4 months of Swiftobservations, which started on 2007 October 26. We detect low-levelX-ray activity in all four SFXTs, which demonstrates that thesetransient sources accrete matter even outside their outbursts. Thisfainter X-ray activity is composed of many flares with a large fluxvariability, on timescales of thousands of seconds. The light-curvevariability is also evident on larger timescales of days, weeks, andmonths, with a dynamic range of more than 1 order of magnitude in allfour SFXTs. The X-ray spectra are typically hard, with an average 2-10keV luminosity during this monitoring of about1033-1034 erg s-1. We detectedpulsations from the pulsar AX J1841.0-0536/IGR J18410-0535, with aperiod of 4.7008+/-0.0004 s. This monitoring demonstrates that thesetransients spend most of the time accreting matter, although at a muchlower level (~100-1000 times lower) than during the bright outbursts,and that the ``true quiescence,'' characterized by a soft spectrum and aluminosity of a few 1032 erg s-1, observed in thepast in only a couple of members of this class, is probably a very rarestate.
| INTEGRAL, XMM-Newton and ESO/NTT identification of AX J1749.1-2733: an obscured and probably distant Be/X-ray binary
Context: AX J1749.1-2733 is an unclassified transient X-ray sourcediscovered during surveys by ASCA in 1993-1999. The transient behaviourand the short and bright flares of the source have led to the idea thatit is part of the recently revealed subclass of supergiant fast X-raytransients. Aims: A multi-wavelength study in NIR, optical,X-rays, and hard X-rays of AX J1749.1-2733 is undertaken in order todetermine its nature. Methods: Public INTEGRAL data and ourtarget of opportunity observation with XMM-Newton were used to study thehigh-energy source through timing and spectral analysis.Multi-wavelength observations in optical and NIR with the ESO/NTTtelescope were also performed to search for the counterpart. Results: AX J1749.1-2733 is a new high-mass X-ray binary pulsar withan orbital period of 185.5±1.1 days (or 185.5/f with f=2,3 or 4)and a spin period of 66 s, parameters typical of a Be/X-ray binary. Theoutbursts last 12 d. A spin-down of dot{P}=0.08 ± 0.02 s yr-1 is also observed, very likely due to the propeller effect.The most accurate X-ray position is RA (2000)=17h49m06.8s and Dec =-27°32arcmin32.5 arcsec (uncertainty 2 arcsec). The high-energy broad-bandspectrum is well-fitted with an absorbed powerlaw and a high-energycutoff with valuesNH=20.1-1.3+1.5×1022cm-2, ?=1.0-0.3+0.1, andEcut=21-3+5 keV. The only optical/NIRcandidate counterpart within the X-ray error circle has magnitudes ofR=21.9±0.1, I=20.92±0.09, J=17.42±0.03,H=16.71±0.02, and Ks=15.75±0.07, which pointstowards a Be star located far away (>8.5 kpc) and highly absorbed(NH˜ 1.7×1022 cm-2). Theaverage 22-50 keV luminosity is 0.4-0.9×1036 ergs-1 during the long outbursts and 3×1036 ergs-1 during the bright flare that occurred on MJD 52891 for anassumed distance of 8.5 kpc.Based on observations made with 1) INTEGRAL, an ESA project withinstruments and science data centre funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), Czech Republic and Poland, and with theparticipation of Russia and the USA; 2) XMM-Newton, an ESA sciencemission with instruments and contributions directly funded by ESA MemberStates and NASA; and 3) ESO Telescopes at the La Silla or ParanalObservatories under programme ID 079.D-0432(A).
| The Far Ultraviolet Spectroscopic Explorer Survey of O VI Absorption in the Disk of the Milky Way
To probe the distribution and physical characteristics of interstellargas at temperatures T~3×105 K in the disk of the MilkyWay, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) toobserve absorption lines of O VI λ1032 toward 148 early-typestars situated at distances >1 kpc. After subtracting off a mildexcess of O VI arising from the Local Bubble, combining our new resultswith earlier surveys of O VI, and eliminating stars that showconspicuous localized X-ray emission, we find an average O VI midplanedensity n0=1.3×10-8 cm-3. Thedensity decreases away from the plane of the Galaxy in a way that isconsistent with an exponential scale height of 3.2 kpc at negativelatitudes or 4.6 kpc at positive latitudes. Average volume densities ofO VI along different sight lines exhibit a dispersion of about 0.26 dex,irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size anddensity, but instead is distributed in regions that have a very broadrange of column densities, with the more strongly absorbing cloudshaving a lower space density. Line widths and centroid velocities aremuch larger than those expected from differential Galactic rotation, butthey are nevertheless correlated with distance and N(O VI), whichreinforces our picture of a diverse population of hot plasma regionsthat are ubiquitous over the entire Galactic disk. The velocity extremesof the O VI profiles show a loose correlation with those of very stronglines of less ionized species, supporting a picture of a turbulent,multiphase medium churned by shock-heated gas from multiple supernovaexplosions.
| Investigation of the conspicuous infrared star cluster and star-forming region ``RCW 38 IR Cluster''
The infrared star cluster RCW 38 IR Cluster, which is also a massivestar-forming region, is investigated. The results of observations withthe SEST (Cerro La Silla, Chile) telescope on the 2.6-mm 12COspectral line and with SIMBA on the 1.2-mm continuum are given. The12CO observations revealed the existence of several molecularclouds, two of which (clouds 1 and 2) are connected with the object RCW38 IR Cluster. Cloud 1 is a massive cloud, which has a depression inwhich the investigated object is embedded. It is not excluded that thedepression was formed by the wind and/or emission from the young brightstars belonging to the star cluster. Rotation of cloud 2, around theaxis having SE-NW direction, with an angular velocity ? = 4.6· 10?14 s?1 is also found. Ared-shifted outflow with velocity ˜+5.6 km/s, in the SE directionand perpendicular to the elongation of cloud 2 has also been found. Theinvestigated cluster is associated with an IR point source IRAS08573-4718, which has IR colors typical for a non-evolved embedded (inthe cloud) stellar object. The cluster is also connected with a watermaser. The SIMBA image shows the existence of a central brightcondensation, coinciding with the cluster itself, and two extensions.One of these extensions (the one with SW-NE direction) coincides, bothin place and shape, with cloud 2, so that the possibility that thisextension might also be rotating like cloud 2 is not excluded. In thevicinity of these extensions there are condensations resembling HHobjects.
| Trigger 324561: IGR J08408-4503 detected by Swift.
Not Available
| Refined analysis of Swift trigger 316063, is IGR J08408-4503.
Not Available
| Trigger 316063: Swift detection of a transient.
Not Available
| 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
| The Third IBIS/ISGRI Soft Gamma-Ray Survey Catalog
In this paper we report on the third soft gamma-ray source catalogobtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRALsatellite. The scientific data set is based on more than 40 Ms ofhigh-quality observations performed during the first 3.5 yr of CoreProgram and public IBIS/ISGRI observations. Compared to previousIBIS/ISGRI surveys, this catalog includes a substantially increasedcoverage of extragalactic fields, and comprises more than 400high-energy sources detected in the energy range 17-100 keV, includingboth transients and faint persistent objects that can only be revealedwith longer exposure times.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, Switzerland, Spain), CzechRepublic, and Poland, and with the participation of Russia and the USA.
| A description of sources detected by INTEGRAL during the first 4 years of observations
Context: In its first 4 years of observing the sky above 20 keV,INTEGRAL-ISGRI has detected 500 sources, around half of which are new orunknown at these energies. Follow-up observations at other wavelengthsrevealed that some of these sources feature unusually large columndensities, long pulsations, and other interesting characteristics. Aims: We investigate where new and previously-known sources detected byISGRI fit in the parameter space of high-energy objects, and we use theparameters to test correlations expected from theoretical predictions.For example, the influence of the local absorbing matter on periodicmodulations is studied for Galactic High-Mass X-ray Binaries (HMXBs)with OB supergiant and Be companions. We examine the spatialdistribution of different types of sources in the Milky Way usingvarious projections of the Galactic plane, in order to highlightsignatures of stellar evolution and to speculate on the origin of thegroup of sources whose classifications are still uncertain. Methods:Parameters that are available in the literature, such as positions,photoelectric absorption (NH), spin and orbital periods, anddistances or redshifts, were collected for all sources detected byISGRI. These values and their references are provided online. Results:ISGRI has detected similar numbers of X-ray Binaries and Active GalacticNuclei (AGN). The former group contains new members of the class ofHMXBs with supergiant stellar companions. Usually, this type of objectpresents strong intrinsic absorption which leads to a peak emission inan energy range that ISGRI is ideally suited to detect. Thanks to theseadditional systems, we are able to show that HMXBs are generallysegregated in plots of intrinsic NH versus the orbital periodof the system and versus the spin period of the pulsar, based on whetherthe companion is a Be or an OB supergiant star. We also find a tentativebut expected anti-correlation between NH and the orbitalperiod, and a possible and unexpected correlation between theNH and the spin period. While only a handful of new Low-MassX-ray Binaries (LMXBs) have been discovered, there are many sources thatremain unclassified and they appear to follow a spatial distributiontypical of Galactic sources (especially LMXBs) rather than extragalacticsources.
| Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds
Context: A 1000-s flare from a new hard X-ray transient, IGRJ08408-4503, was observed by INTEGRAL on May 15, 2006 duringthe real-time routine monitoring of IBIS/ISGRI images performed at theINTEGRAL Science Data Centre. The flare, detected during a singleone-hour long pointing, peaked at 250 mCrab in the 20-40 keV energyrange. Aims: Multi-wavelength observations, combininghigh-energy and optical data, were used to unveil the nature ofIGR J08408-4503. Methods: A search in allINTEGRAL public data for other bursts from IGRJ08408-4503 was performed, and the detailed analysis ofanother major flare is presented. The results of two Swift Target ofOpportunity observations are also described. Finally, a study of thelikely optical counterpart, HD 74194, is provided. Results: IGR J08408-4503 is very likely asupergiant fast X-ray transient (SFXT) system. The system parametersindicate that the X-ray flares are probably related to the accretion ofwind clumps on a compact object orbiting about 1013 cm fromthe supergiant HD 74194. The clump mass loss rate isof the order of 10-6 M? yr-1. Conclusions: Hard X-ray flares from SFXTs allow to probe thestellar winds of massive stars, and could possibly be associated withwind perturbations due to line-driven instabilities.
| IGR J08408-4503: A New Recurrent Supergiant Fast X-Ray Transient
The supergiant fast X-ray transient IGR J08408-4503 was discovered byINTEGRAL on 2006 May 15 during a bright flare. The source shows sporadicrecurrent short bright flares, reaching a peak luminosity of1036 ergs s-1 within less than 1 hr. The companionstar is HD 74194, an Ob5Ib(f) supergiant star located at 3 kpc in theVela region. We report the light curves and broadband spectra (0.1-200keV) of all the three flares of IGR J08408-4503 detected up to now,based on INTEGRAL and Swift data. The flare spectra are well describedby a power-law model with a high-energy cutoff at ~15 keV. Theabsorption column density during the flares was found to be~1021 cm-2, indicating a very low matter densityaround the compact object. Using the supergiant donor star parameters,the wind accretion conditions imply an orbital period of the order of 1yr, a spin period of the order of hours, and a magnetic field of theorder of 1013 G.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| HD 74194, a new binary supergiant fast X-ray transient?, possible optical counterpart of INTEGRAL hard X-ray source IGR J08408-4503
HD 74194 is an O-type supergiant, classified as O8.5 Ib (f) (Walborn1973, AJ 78, 1067), also suspected as single-lined binary (see MaizApellaniz et al. 2004, ApJS 151, 103). This star is beingspectroscopically monitored as part of our program of study of massivebinaries. We have obtained high-resolution spectra of HD 74194 with theEchelle spectrograph attached to the du Pont 2.5-m telescope at LasCampanas Observatory in 2006 May 18.00, 20.96, 22.00, and 22.97.
| Swift/XRT observation of IGR J08408-4503
At 10:46UT on May 22nd, 2006, Swift/XRT began a Target of Opportunityobservation of the newly discovered super-fast X-ray Transient (SF-XRT)IGR J08408-4503 (e.g. ATEL #814). In 2ks of XRT photon counting modedata we find a faint point source inside the INTEGRAL JEM-X error circlereported by Brandt et al. (ATEL #817) at the following coordinates:RA(J2000) = 08:40:47.97, Dec(J2000) = -45:03:29.8, with an estimateduncertainty of 5.4 arcseconds radius (90% containment).
| Refined position of IGR J08408-4503 by JEM-X on INTEGRAL
We report a refined position of IGR J08408-4503 based on data from theJEM-X X-ray monitor on INTEGRAL taken during the outburst on May 15th2006 that led to the discovery of the source (ATEL #813). The bestestimate of the position is RA = 08h 40m 48.7s, Dec = -45o03' 41'' (J2000, +-46'' at 90% confidence). This strengthens theassociation of IGR J08408-4503 with the supergiant HD 74194 (ATELs #813,#814, #815), still contained in this smaller error box.
| Emission-line early-type stars as possible optical counterparts of INTEGRAL hard X-ray sources IGR J08408-4503 and IGR J15539-6142
We retrieved and analyzed ESO archival spectra of the blue supergiantstar HD 74194, putatively associated with the hard X-ray source IGRJ08408-4503 (Gotz et al., ATel #813; Mereghetti et al., ATel #814). Thespectra were acquired on 2003 Jan 9, for a total exposure time of 10 s,under programme 70.C-0396(A) at the ESO 3.6m equipped with EFOSC, andcover the 6000-10000 Angstrom range. The analysis show the Halphaline in emission (with EW ~ 1 Angstrom) and HeI absorption lines at 6675and 7065 Angstrom.
| IGR J08408-4503 is a recurrent transient
Analysis of data in the INTEGRAL archive shows that an outburst from therecently discovered source IGR J08408-4503 (Atel #813) occurred on 1stJuly 2003. The outburst lasted approximately two hours. During itsbrightest part (around 20 UT) an average flux of about6x10-10 erg/cm2/s was detected in the20-60 keV range with the IBIS/ISGRI instrument. The derived position(RA=130.1900 deg, Dec=-45.07037 deg (J2000), uncertainty of 2.6 arcmin)is consistent with that reported in Atel #813 and with the position ofthe supergiant star HD 74194.
| Outburst of a new source IGR J08408-4503 detected by INTEGRAL
We report the discovery of a new source, IGR J08408-4503, with the ISGRIcamera of the IBIS telescope on board the INTEGRAL satellite, during thedeep observation of the Vela region on May 15th 2006. The source isdetected in a single 1 hour long pointing and inspection of the sourcelight curve on a 100 s time scale reveals a short outburst of 900 sduration starting at 18:25:33 UTC with a peak flux of 250 mCrab in the20-40 keV energy band.
| IGR J08408-4503 (= v* LM vel): Swift detection of a transient.
Not Available
| Line-Profile Variations in Pulsating Sdb Stars as a Pulsation Mode Diagnostic
In previous attempts to perform seismic modeling of pulsating subdwarf-Bstars, various mode identification techniques are used with uncertainresults. We investigated a method so far neglected in sdB stars, butvery successful for main-sequence pulsators, i.e., mode identificationfrom the line-profile variations caused by stellar pulsation. We reportthe calculation of time-resolved synthetic spectra for sdB starspulsating with various combinations of pulsation modes; thesecalculations were carried out over appropriate ranges of effectivetemperature, surface gravity and helium abundances. Preliminary testsusing these synthetic line-profile variations demonstrated theirpotential for mode identification by comparison with observations.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
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