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Measuring neutron star mass and radius with three mass-radius relations
We propose to determine the mass and radius of a neutron star (NS) usingthree measurable mass-radius relationships, namely the `apparent' radiusinferred from the NS thermal emission, the gravitational redshiftinferred from the absorption lines, as well as the averaged stellar massdensity inferred from the orbital Keplerian frequency derived from thekilohertz quasi-periodic oscillation data. We apply the method toconstrain the NS mass and radius of the X-ray sources, 1E 1207.4-5209,Aql X-1 and EXO 0748-676.
| A new symbiotic low mass X-ray binary system: 4U 1954+319
Aims.4U 1954+319 was discovered 25 years ago, but only recently has aclear picture of its nature begun to emerge. We present for the firsttime a broad-band spectrum of the source and a detailed timing studyusing more than one year of monitoring data. Methods: .The timingand spectral analysis was done using publicly available Swift, INTEGRAL,BeppoSAX, and RXTE/ASM data in the 0.7 to 150 keV energy band.Results: .The source spectrum is described well by a highly absorbed(NH 1023 cm-2) power law with ahigh-energy exponential cutoff around 15 keV. An additional black bodycomponent is needed below 3 keV to account for a soft excess. Thederived ~5 h periodicity, with a spin-up timescale of ~25 years, couldbe identified as the neutron star spin period. The spectral and timingcharacteristics indicate that we are dealing both with the slowestestablished wind-accreting X-ray pulsar and with the second confirmedmember of the emerging class dubbed "symbiotic low mass X-ray binaries"to host a neutron star.
| UBV(RI)C photometric sequences for symbiotic stars. III
We present accurate UBV(RI)C photometric sequences andastrometric positions for a final set of 41 symbiotic stars. In asimilar manner to the 40 targets of Papers I and II, these sequencesextend over wide brightness and color ranges and are suited to coveringboth quiescence and outburst phases. They are intended to assist boththe CCD photometric monitoring of current variability and exploitationof old photographic plates from historical archives.
| ChaMPlane Discovery of Candidate Symbiotic Binaries in Baade's and Stanek's Windows
We have searched the OGLE-II archive for candidate counterparts of X-raysources detected in two low-extinction windows included in our Galacticbulge Chandra-HST survey. We find that a significant number-that is, inexcess of the expected level of random associations-can be matched withprobable M giants. Their X-ray properties can be understood if thesesources are symbiotic binaries in which the X-rays are typically, eitherdirectly or indirectly, the result of a white dwarf accreting from thewind of a cool giant. Optical and near-infrared properties of selectedsources are consistent with a symbiotic nature, although none of thespectra collected for eight out of 13 candidate counterparts show thehigh-ionization nebular emission lines observed for many symbiotics. Thehard X-ray emission for several sources (power-law photon indices-1.5<~Γ<~1.5) suggests that our sample includes systemssimilar to the symbiotics recently detected with INTEGRAL and Swift.
| M-type giants as optical counterparts of X-ray sources 4U 1700+24 and 4U 1954+319
We observed with Chandra two peculiar galactic X-ray sources, 4U 1700+24and 4U 1954+319, which are suspected to have a M-type giant star asoptical counterpart, in order to get a high-precision astrometricposition for both of them. The peculiarity of these sources lies in thefact that these are the only two cases among low-mass X-ray binaries(LMXBs), besides the confirmed case of GX 1+4, for which the companioncan possibly be a M-type giant. We found that in both cases the fieldM-type giant star is indeed the counterpart of these X-ray sources. Wealso determined the distance to 4U 1954+319 to be 1.7 kpc. This resultsuggests that a number of faint (LX 1032{-}1034 erg s-1) Galactic X-raysources are "symbiotic X-ray binaries", that is, wide-orbit LMXBscomposed of a compact object, most likely a neutron star, accreting fromthe wind of a M-type giant.
| Evidence for strange stars from joint observation of harmonic absorption bands and of redshift
From recent reports on terrestrial heavy ion collision experiments itappears that one may not obtain information about the existence ofasymptotic freedom (AF) and chiral symmetry restoration (CSR) for quarksof QCD at high density. This information may still be obtained fromcompact stars - if they are made up of strange quark matter (SQM).Very high gravitational redshift lines (GRL), seen from some compactstars, seem to suggest high ratios of mass and radius (M/R) for them.This is suggestive of strange stars (SS) and can in fact be fitted verywell with SQM equation of state (EOS) deduced with built in AF and CSR.In some other stars broad absorption bands (BAB) appear at about ~0.3keVand multiples thereof, that may fit in very well with resonance withharmonic compressional breathing mode frequencies of these SS. Emissionat these frequencies are also observed in six stars.If these two features of large GRL and BAB were observed together in asingle star, it would strengthen the possibility for the existence of SSin nature and would vindicate the current dogma of AF and CSR that webelieve in QCD. Recently, in 4U 1700 - 24, both features appear to bedetected, which may well be interpreted as observation of SS - althoughthe group that analyzed the data did not observe this possibility. Wepredict that if the shifted lines, that has been observed, are from neonwith GRL shift z= 0.4- then the compact object emitting it is a SS ofmass 1.2Msolar and radius 7km. In addition the fit to thespectrum leaves a residual with broad dips at 0.35keV and multiplesthereof, as in 1E 1207 - 5209 which is again suggestive of SS.
| A Compton reflection dominated spectrum in a peculiar accreting neutron star
We report on a puzzling event that occurred during a long BeppoSAXobservation of the slowly rotating binary pulsar GX1+4. During thisevent, lasting about 1 d, the source X-ray flux was over a factor 10lower than normal. The low-energy pulsations disappeared while at higherenergies they were shifted in phase by ~0.25. The continuum spectrumtaken outside this low-intensity event was well fitted by an absorbedcut-off power law and exhibited a broad iron line at ~6.5keV probablydue to the blending of the neutral (6.4keV) and ionized (6.7keV)Kα iron lines. The spectrum during the event was Comptonreflection dominated and it showed two narrow iron lines at ~6.4 and~7.0keV, the latter never revealed before in this source. We alsopresent a possible model for this event in which a variation of theaccretion rate thickens a torus-like accretion disc that hides for awhile the direct neutron star emission from our line of sight. In thisscenario, the Compton reflected emission observed during the event iswell explained in terms of emission reflected by the side of the torusfacing our line of sight.
| Discovery of a redshifted X-ray emission line in the symbiotic neutron star binary 4U 1700+24
We present the spectral analysis of an XMM-Newton observation of theX-ray binary 4U 1700+24, performed during an outburst in August 2002.The EPIC-PN spectrum above 1 keV can be modeled by a blackbody plusComptonization model, as in previous observations. At lower energies,however, we detect a prominent soft excess, which we model with a broadGaussian centered at 0.5 keV. In the high resolution RGS spectrumwe detect a single emission line, centered at19.19+0.05-0.09 Å. We discuss two possibleinterpretations for this line: O VIII at redshift z=0.012+0.002-0.004 or Ne IX at redshift z 0.4.
| Further evidence for the presence of a neutron star in 4U 2206+54. INTEGRAL and VLA observations
The majority of High Mass X-ray Binaries (HMXBs) behave like X-raypulsars, revealing that they contain a magnetised neutron star. Amongthe four HMXBs showing neither pulsations nor the characteristics ofaccreting black holes, there is the unusual HMXB 4U2206+54. Here we present contemporaneous high-energy and radioobservations of this system conducted with INTEGRAL and the VLA, inorder to unveil its nature. The high-energy spectra show clearindications of the presence of an absorption feature at ~32 keV. This isthe third high-energy observatory to reveal marginal evidence of thisfeature, giving strong support to the existence of a cyclotron resonancescattering feature, which implies a magnetic field of 3.6×1012 G. On the other hand, the source is not detected atcentimetre radio wavelengths with a 3σ upper limit of 0.039 mJy.The expected radio emission for an accreting black hole in the low/hardstate, inferred from X-ray flux measurements, would be at least 60 timesgreater than the measured upper limit. Both results firmly indicatethat, in spite of the absence of pulsations, 4U2206+54 hosts a magnetic accreting neutron star, the first onenot to be observed as an X-ray pulsar.
| High-Resolution X-Ray Spectrum of the Accreting Binary X-Ray Pulsar GX 1+4
We present here the high-resolution X-ray spectrum of the accretingbinary X-ray pulsar GX 1+4, obtained with the High Energy TransmissionGrating (HETG) instrument of the Chandra X-Ray Observatory. This wassupplemented by a simultaneous observation with the Proportional CounterArray (PCA) of the Rossi X-Ray Timing Explorer (RXTE). During thisobservation, the source was in a somewhat low intensity state, and thepulse profile with both Chandra and RXTE shows a narrow dip,characteristic of GX 1+4 in medium- and low-intensity states. Thecontinuum X-ray spectrum obtained with HETG and PCA can be fitted wellwith a high-energy cutoff power-law model with line-of-sight absorption.Interestingly, we find that this low state is accompanied by arelatively small absorption column density. A 6.4 keV narrow emissionline with an equivalent width of 70 eV is clearly detected in the HETGspectrum. The fluorescence iron line, or at least part of it, isproduced in the neutral or lowly ionized iron in the circumstellarmaterial that also causes most of the line-of-sight absorption. In theHETG spectrum, we have found evidence for a weak (equivalent width ~30eV) emission line at 6.95 keV. This line is identified as Lyαemission line from hydrogen-like iron, and the spectrum does not showemission lines from helium-like iron. We discuss various emissionregions for the hydrogen-like iron emission line, such as gas diffusedinto the Alfvén sphere or an accretion curtain flowing from theinner accretion disk to the magnetic poles.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.Table 2 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/418/927
| Photometry of symbiotic stars. XI. EG And, Z And, BF Cyg, CH Cyg, CI Cyg, V1329 Cyg, TX CVn, AG Dra, RW Hya, AR Pav, AG Peg, AX Per, QW Sge, IV Vir and the LMXB V934 Her
We present new photometric observations of EG And, Z And, BF Cyg, CHCyg, CI Cyg, V1329 Cyg, TX CVn, AG Dra, RW Hya, AG Peg, AX Per, IV Virand the peculiar M giant V934 Her, which were made in the standardJohnson UBV(R) system. QW Sge was measured in the Kron-Cousin B, V,RC, IC system and for AR Pav we present its newvisual estimates. The current issue gathers observations of theseobjects to December 2003. The main results can be summarized as follows:EG And: The primary minimum in the U light curve (LC) occurred at theend of 2002. A 0.2 -- 0.3 mag brightening in U was detected in theautumn of 2003. Z And: At around August 2002 we detected for the firsttime a minimum, which is due to eclipse of the active object by the redgiant. Measurements from 2003.3 are close to those of a quiescent phase.BF Cyg: In February 2003 a short-term flare developed in the LC. Adifference in the depth of recent minima was detected. CH Cyg: This starwas in a quiescent phase at a rather bright state. A shallow minimumoccurred at JD 2 452 730, close to the position of the inferiorconjunction of the giant in the inner binary of the triple-star model ofCH Cyg. CI Cyg: Our observations cover the descending branch of a broadminimum. TX CVn: At/around the beginning of 2003 the star entered abright stage containing a minimum at JD 2 452 660. AG Dra: Newobservations revealed two eruptions, which peaked in October 2002 and2003 at 9.3 in U. AR Pav: Our new visual estimates showed atransient disappearance of a wave-like modulation in the star'sbrightness between the minima at epochs E = 66 and E = 68 and itsreappearance. AG Peg: Our measurements from the end of 2001 showedrather complex profile of the LC. RW Hya: Observations follow behaviourof the wave-like variability of quiet symbiotics. AX Per: In May 2003 a0.5 mag flare was detected following a rapid decrease of the light to aminimum. QW Sge: CCD observations in B, V, RC, ICbands cover a period from 1994.5 to 2003.5. An increase in the star'sbrightness by about 1 mag was observed in all passbands in 1997. Lesspronounced brightening was detected in 1999/2000. V934 Her: Ourobservations did not show any larger variation in the optical as areaction to its X-ray activity.
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Search for TeV Emissions from Pulsars in Binary Systems
A survey of binary systems containing pulsars was conducted, with theintention of detecting Galactic sources of very high energy γ-rayemission. Observations were carried out with the Whipple 10 m imagingatmospheric Cerenkov telescope. Standard analysis techniques wereapplied to these sources to search for steady, unpulsed emission.Periodic tests were also performed to search for emission correlatedwith both the orbital and spin phases, where appropriate. Analysesindicate that the binaries in this study do not emit detectable levelsof very high energy photons within the sensitivity of our instrument.The flux upper limits presented here fail to seriously constrainemission models.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Correlated Radial Velocity and X-Ray Variations in HD 154791/4U 1700+24
We present evidence for approximately 400 day variations in the radialvelocity of HD 154791 (V934 Her), the suggested optical counterpart of4U 1700+24. The variations are correlated with the previously reported~400 day variations in the X-ray flux of 4U 1700+24, which supports theassociation of these two objects, as well as the identification of thissystem as the second known X-ray binary in which a neutron star accretesfrom the wind of a red giant. The HD 154791 radial velocity variationscan be fitted with an eccentric orbit with period 404+/-3 days,amplitude K=0.75+/-0.12kms-1, and eccentricity e=0.26+/-0.15.There are also indications of variations on longer timescales >~2000days. We have reexamined all available All-Sky Monitor (ASM) datafollowing an unusually large X-ray outburst in 1997-1998 and confirmthat the 1 day averaged 2-10 keV X-ray flux from 4U 1700+24 is modulatedwith a period of 400+/-20 days. The mean profile of the persistent X-rayvariations was approximately sinusoidal, with an amplitude of0.108+/-0.012 ASM counts s-1 (corresponding to 31% rms). Theepoch of X-ray maximum was approximately 40 days after the time ofperiastron, according to the eccentric orbital fit. If the 400 dayoscillations from HD 154791/4U 1700+24 are due to orbital motion, thenthe system parameters are probably close to those of the only otherneutron star symbiotic-like binary, GX 1+4. We discuss the similaritiesand differences between these two systems.
| Search for point sources of gamma radiation above 15 TeV with the HEGRA AIROBICC array
A search for potential point sources of very high energy gamma rays hasbeen carried out on the data taken simultaneously by the HEGRA AIROBICCand Scintillator arrays from August 1994 to March 2000. The list ofsought sources includes supernova remnants, pulsars, AGNs and binarysystems. The energy threshold is around 15 TeV. For the Crab Nebula, amodest excess of 2.5 standard deviations above the cosmic ray backgroundhas been observed. Flux upper limits (at 90% c.l.) of around 1.3 timesthe flux of the Crab Nebula are obtained, in average, for the candidatesources. A different search procedure has been used for an all-skysearch which yields absolute flux upper limits between 4 and 9 crabsdepending on declination, in the band from delta = 0 to delta =60o. The full versions of Tables 1 and 2, including thecoordinates of the sources, are available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/39
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her
We report on ASCA and BeppoSAX X-ray broad band observations of thegalactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and1998, respectively, and on (quasi-)simultaneous ground observations ofits optical counterpart, V934 Her, from the Loiano 1.5-meter telescope.In order to better understand the nature of the source we also analyzepublic archival ROSAT and RXTE data as well as the RXTE ASM light curveof 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The opticalspectra are typical of a M2 III star; this allows us to determine arevised distance to the object of ~ 400 pc. While these spectra do notshow either any spectral change between the two epochs or any peculiarfeature apart from those observed in normal red giants, thespectroscopic measurements carried out in X-rays reveal a complex andlong-term variable spectrum, with a clear soft excess. The X-rayspectral properties of the source are best described by a thermalComptonization spectrum plus a soft energy (<1 keV) excess, which canbe modeled in the form of a blackbody emission with kT_BB ~ 1 keV; thelatter component is not detected at the lowest source flux levels. Theratio between the two components varies substantially with the sourceflux. The X-ray emission from the object appears to become harder as itsluminosity increases: indeed, the RXTE data acquired during an outburstoccurred in October-November 1997 display a hard tail, detected up to100 keV and modeled with a comptonizing cloud which is hotter and lessopaque than that seen in the low intensity state. Apart from erraticshot-noise variability on timescales of tens to thousands of seconds, nosignificant properties (such as pulsations or QPOs) are found from thetiming analysis of the X-ray light curves extracted from theobservations presented here. With the new distance determination, the2-10 keV X-ray luminosity range spanned in the considered observationslies between ~ 2x1032 and ~ 1x1034 ergs-1. All this information, combined with the findings byother authors, allows us to suggest that the scenario which bestdescribes the object consists of a wide binary system in which a neutronstar accretes matter from the wind of a M-type giant star. Implicationsof such a model are discussed. Partly based on observations collected atthe Bologna Astronomical Observatory in Loiano, Italy.
| Position and Variability of 2A 1704+241
We present results of analyses of observations of the X-ray source 2A1704+241 with the ROSAT Position Sensitive Proportional Counter (PSPC)and the High Resolution Imager (HRI). The source 2A 1704+241 was firstassociated with the M giant star HD 154791 based on observations withthe HEAO 1 scanning modulation collimator and the Einstein IPC andanalysis of a spectrum of HD 154791 obtained with the InternationalUltraviolet Explorer. This identification was unusual because there arefew bright X-ray binaries associated with an M giant star. We observed2A 1704+241 with the PSPC and the HRI in order to determine moreaccurately the position of the X-ray source and in order to study thepreviously seen 900 s variability in the Einstein data. Based on theprevious identification and determination of the position of MS1703.7+2417, an active galactic nucleus in the field, and the positionof three previously unreported X-ray sources that we have associatedwith stars in the USNO-A2.0 catalog, we have greatly reduced the X-raypositional error of 2A 1704+241. HD 154791 remains the prime candidateas the optical counterpart of the X-ray source. While the 50% modulationin the X-ray flux seen by the Einstein IPC is apparent in the ROSATdata, it appears to be at a slightly different frequency.
| A search for rapid photometric variability in symbiotic binaries
We report on our survey for rapid (time-scale of minutes) photometricvariability in symbiotic binaries. These binaries are becoming anincreasingly important place to study accretion on to white dwarfs,since they are candidate Type Ia supernova progenitors. Unlike in mostcataclysmic variables, the white dwarfs in symbiotics typically accretefrom a wind, at rates greater than or equal to10-9Msolaryr-1. In order to elucidatethe differences between symbiotics and other white dwarf accretors, aswell as to search for magnetism in symbiotic white dwarfs, we havestudied 35 symbiotic binaries via differential optical photometry.Included in our sample are all but one of the symbiotics from the listsof Kenyon and Downes & Keyes with published V magnitudes less than14 and declinations greater than -20°. Our study is the mostcomprehensive to date of rapid variability in symbiotic binaries. Wehave found one magnetic accretor, Z And, previously reported bySokoloski & Bildsten. In four systems (EG And, BX Mon, CM Aql and BFCyg), some evidence for flickering at a low level (roughly 10mmag) isseen for the first time. These detections are, however, marginal. For 25systems, we place tight upper limits on both aperiodic variability(flickering) and periodic variability, highlighting a major differencebetween symbiotics and cataclysmic variables. The remaining five of theobjects included in our sample (the two recurrent novae RS Oph and TCrB, plus CH Cyg, o Ceti and MWC 560) had previous detections of opticalflickering. We discuss our extensive observations of these previouslyknown flickering systems in a separate paper. Five new variable starswere discovered serendipitously in the fields of the survey objects, andthe observations of these stars are also presented elsewhere. We discussthe impact of our results on the `standard' picture of wind-fedaccretion, and speculate on the possibility that light from quasi-steadynuclear burning on the surface of the white dwarf hides the fluctuatingemission from accretion.
| X-Ray Observations of the New Pulsar-Supernova Remnant System PSR J1119-6127 and Supernova Remnant G292.2-0.5
PSR J1119-6127 is a recently discovered 1600-yr-old radio pulsar thathas a very high inferred surface dipolar magnetic field. We present adetailed analysis of a pointed ASCA observation and archival ROSAT dataof PSR J1119-6127 and its surroundings. Both data sets reveal extendedemission coincident with the newly discovered radio supernova remnantG292.2-0.5, which is reported in a companion paper by Crawford et al. Ahard point source, offset ~1.5′ from the position of the radiopulsar, is seen with the ASCA Gas Imaging Spectrometer (GIS). Nopulsations are detected at the radio period with a pulsed fraction upperlimit of 61% (95% confidence). The limited statistics prevent a detailedspectral analysis, although a power-law model with photon indexΓ~1-2 describes the data well. Both the spectral model and derivedX-ray luminosity are consistent with those measured for other youngradio pulsars, although the spatial offset renders an identification ofthe source as the X-ray counterpart of the pulsar uncertain.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| A catalogue of low-mass X-ray binaries
We present a catalogue of low-mass X-ray binaries. The catalogue is anupdated version of the catalogue of van Paradijs (\cite{vanpar95}). Thisnew catalogue contains 150 sources, 31 new low-mass X-ray binaries inaddition to the 119 sources listed in van Paradijs' catalogue. The aimof this catalogue is to help the reader gain easy access to the recentliterature (up to about August 2000) on individual sources, and toprovide of some basic information on the X-ray sources and theircounterparts in other wavelength ranges (UV, optical, IR, and radio). Incases where there is some doubt about the low-mass nature of the X-raybinary, this is mentioned. In an appendix we list the Anomalous X-rayPulsars (AXPs), which nowadays are no longer thought to be low-massX-ray binaries. Table 1 is also available in electronically at the CDSvia anonymous ftp (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/368/1021
| A catalogue of symbiotic stars
We present a new catalogue of symbiotic stars. In our list we include188 symbiotic stars as well as 30 objects suspected of being symbiotic.For each star, we present basic observational material: coordinates, Vand K magnitudes, ultraviolet (UV), infrared (IR), X-ray and radioobservations. We also list the spectral type of the cool component, themaximum ionization potential observed, references to finding charts,spectra, classifications and recent papers discussing the physicalparameters and nature of each object. Moreover, we present the orbitalphotometric ephemerides and orbital elements of known symbioticbinaries, pulsational periods for symbiotic Miras, Hipparcos parallaxesand information about outbursts and flickering.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| The peculiar M giant HD 154791. Optical counterpart of the X-ray source 4U1700+24
HD 154791 is the only optical counterpart of an hard X-ray source(4U1700+24) with an apparently normal M giant spectrum. We present herenew high S/N spectroscopic observations of this interesting and poorlystudied object, pointing out a number of peculiarities never noticedbefore; therefore we derive a possible scenario involving a wide binarysystem, where an old, slowly rotating, neutron star is weeklyinteracting with its late optical companion. We foresee that this systemcould evolve, in a short time, into a normal LMXB. Based on datacollected at the Loiano Observatory
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
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