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Jets in neutron star X-ray binaries: a comparison with black holes
We present a comprehensive study of the relation between radio and X-rayemission in neutron star (NS) X-ray binaries, use this to infer thegeneral properties of the disc-jet coupling in such systems and comparethe results quantitatively with those already established for black hole(BH) systems. There are clear qualitative similarities between the twoclasses of object: hard states below about 1 per cent of the Eddingtonluminosity produce steady jets, while transient jets are associated withoutbursting and variable sources at the highest luminosities. However,there are important quantitative differences: the NSs are less radioloud for a given X-ray luminosity (regardless of mass corrections) andthey do not appear to show the strong suppression of radio emission insteady soft states that we observe in BH systems. Furthermore, in thehard states, the correlation between radio and X-ray luminosities of theNS systems is steeper than the relation observed in BHs by about afactor of 2. This result strongly suggests that the X-ray emission inthe BH systems is radiatively inefficient, with an approximate relationof the form , consistent with both advection-dominated models and thejet-dominated scenario. In contrast, the jet power in both classes ofobject scales linearly with accretion rate. This constitutes some of thefirst observational evidence for the radiatively inefficient scaling ofX-ray luminosity with accretion rate in accreting BH systems. Moreover,based on simultaneous radio/X-ray observations of Z-type NSs (thebrightest of our Galaxy, always near or at the Eddington accretionrate), we draw a model that can describe the disc-jet coupling in suchsources, finding a possible association between a particular X-ray statetransition [horizontal branch to normal branch] and the emission oftransient jets.
| 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.
| X-Ray Absorption Spectroscopy of the Multiphase Interstellar Medium: Oxygen and Neon Abundances
X-ray absorption spectroscopy provides a powerful tool in determiningthe metal abundances in various phases of the interstellar medium (ISM).We present a case study of the sight line toward 4U 1820-303, based onChandra grating observations. The detection of O I, O II, O III, O VII,O VIII, and Ne IX Kα absorption lines allows us to measure theatomic column densities of the neutral, warm ionized, and hot phases ofthe ISM through much of the Galactic disk. By comparing thesemeasurements with the 21 cm hydrogen emission and with the pulsardispersion measure, we estimate the mean oxygen abundances in theneutral and total ionized phases as 0.3(0.2, 0.6) and 2.2(1.1, 3.5) inunits of Anders & Greversse's solar value (90% confidenceintervals). This significant oxygen abundance difference is apparently aresult of molecule/dust grain destruction and recent metal enrichment inthe warm ionized and hot phases. We also measure the column density ofneon from its absorption edge and obtain a solar value of the Ne/O ratioaccounting for the expected oxygen contained in molecules and dustgrains. From a joint analysis of the O VII, O VIII, and Ne IX lines, weobtain the Ne/O abundance ratio of the hot phase as 1.4(0.9, 2.1) solar,which is not sensitive to the exact hot gas temperature distributionassumed. These comparable ISM Ne/O ratios for the different phases arethus considerably less than the value recently inferred from coronaemission of solar-like stars.
| On the Nature of the Ultraluminous X-Ray Transient in Cen A (NGC 5128)
We combine nine ROSAT, nine Chandra, and two XMM-Newton observations ofthe Cen A galaxy to obtain the X-ray light curve of 1RXHJ132519.8-430312 (=CXOU J132519.9-430317) spanning 1990-2003. The sourcereached a peak 0.1-2.4 keV flux FX>10-12 ergscm-2 s-1 during a 10 day span in 1995 July. Theinferred peak isotropic luminosity of the source therefore exceeded3×1039 ergs s-1, which places the source inthe class of ultraluminous X-ray sources. Coherent pulsations at 13.264Hz are detected during a second bright episode(FX>3×10-13 ergs cm-2s-1) in 1999 December. The source is detected and variessignificantly within three additional observations but is below thedetection threshold in seven observations. The X-ray spectrum in 1999December is best described as a cutoff power law or a disk blackbody(multicolored disk). We also detect an optical source,mF555W~24.1 mag, within the Chandra error circle of 1RXHJ132519.8-430312 in Hubble Space Telescope images taken 195 days beforethe nearest X-ray observation. The optical brightness of this source isconsistent with a late O or early B star at the distance of Cen A. Ifthe optical source is the counterpart, then the X-ray and opticalbehavior of 1RXH J132519.8-430312 is similar to the transient Be/X-raypulsar A0538-66.
| Isolated, Massive Supergiants near the Galactic Center
We have carried out a pilot project to assess the feasibility of usingradio, infrared, and X-ray emission to identify young, massive starslocated between 1 and 25 pc from the Galactic center. We first comparedcatalogs compiled from the VLA, Chandra, and 2MASS. We identified twomassive, young stars: the previously identified star that is associatedwith the radio H II region H2 and a newly identified star that we referto as CXOGC J174516.1-290315. The infrared spectra of both stars exhibitvery strong Brγ and He I lines and resemble those of massivesupergiants that have evolved off of the main sequence but not yetreached the Wolf-Rayet phase. We estimate that each star has abolometric luminosity >~106 Lsolar. These twostars are also associated with bright mid-infrared sources from the MSXsurvey, although the origin of this emission is uncertain. Likewise, thedetection of these two sources in X-rays is surprising because stars atsimilar evolutionary states are not uniformly bright X-ray sources.Therefore, we suggest that both stars are in binary systems that containeither OB stars whose winds collide with those of the luminoussupergiants or compact objects that are accreting from the winds of thesupergiants. We also identify X-ray emission from a nitrogen-typeWolf-Rayet star and place upper limits on the X-ray luminosities ofthree more evolved, massive stars that previously have been identifiedbetween 1 and 25 pc from Sgr A*. Finally, we briefly discuss theimplications that future searches for young stars will have for ourunderstanding of the recent history of star formation near the Galacticcenter.
| A Neutron Star with a Massive Progenitor in Westerlund 1
We report the discovery of an X-ray pulsar in the young, massiveGalactic star cluster Westerlund 1. We detected a coherent signal fromthe brightest X-ray source in the cluster, CXO J164710.2-455216, duringtwo Chandra observations on 2005 May 22 and June 18. The period of thepulsar is 10.6107(1) s. We place an upper limit to the period derivativeof P˙<2×10-10 s s-1, which impliesthat the spin-down luminosity is E˙<=3×1033 ergss-1. The X-ray luminosity of the pulsar isLX~3+10-2×1033(D/5kpc)2 ergs s-1, and the spectrum can be describedby a kT=0.61+0.02-0.02 keV blackbody with a radiusof Rbb=0.27+/-0.03(D/5 kpc) km. Deep infrared observationsreveal no counterpart with K<18.5, which rules out a binary companionwith M>1 Msolar. Taken together, the properties of thepulsar indicate that it is a magnetar. The rarity of slow X-ray pulsarsand the position of CXO J164710.2-455216 only 1.6 arcmin from the core ofWesterlund 1 indicates that it is a member of the cluster with>99.97% confidence. Westerlund 1 contains 07 V stars with initialmasses Mi~35 Msolar and >50 post-main-sequencestars that indicate the cluster is 4+/-1 Myr old. Therefore, theprogenitor to this pulsar had an initial mass Mi>40Msolar. This is the most secure result among a handful ofobservational limits to the masses of the progenitors to neutron stars.
| New γ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103
A growing number of early Be stars exhibit X-ray luminosities that areintermediate between those typical of early type stars and those emittedby most Be/X-ray binaries in quiescence. We report on XMM-Newtonobservations of two such Be stars, SAO 49725 andHD 161103, which were originally discovered in asystematic cross-correlation between the ROSAT all-sky survey andSIMBAD. The new observations confirm the X-ray luminosity excess(LX ~ 1032-33 erg s-1) and the hardnessof their X-ray spectra, which are both unusual for normal early typestars. An iron Kα complex is clearly detected in HD161103 in which the H-like, He-like, and fluorescentcomponents are resolved, while strong evidences also exist for thepresence of similar features in SAO 49725. X-rayspectra can be equally well-fitted by a thermal plasma (mekal) with T ~108 K and solar abundances or by a power law + iron linemodel with photon index ~1.5-1.8, both with a soft thermal componentwith T ~ 107 K. The intensity of the fluorescence 6.4 keVline reflects the presence of large amounts of cold material close tothe X-ray sources and strongly argues against accretion onto a companionneutron star in a large orbit. On the other hand, the probable thermalorigin of the X-ray emission, as supported by the ionised iron lines,disagrees with those observed in all known Be/X-ray binaries, in which anon-thermal component is always required. Remarkably, the X-ray featuresare similar to those of white dwarves in several cataclysmic variables.There is no evidence of high frequency pulsations in neither of the twosystems. However, a large oscillation in the light curve of HD161103 with P ~ 3200 s is readily detected. The X-ray lightcurve of SAO 49725 exhibits clear variability by ~80%on time scales as short as ~1000 s. New optical observations provideupdated spectral types (B0.5 III-Ve) and disclose a dense, large, andapparently stable circumstellar disc for both stars. The nature of theexcess X-ray emission is discussed in light of the models proposed forγ Cas, magnetic disc-star interaction, oraccretion onto a compact companion object - whether neutron star orwhite dwarf. These two new objects, added to similar cases discovered inXMM-Newton surveys, point to the emergence of a new class ofγ Cas analogs.
| Study of the cyclotron feature in MXB 0656-072
We have monitored a type II outburst of the Be/X-ray binary MXB0656-072 in a series of pointed RXTE observations duringOctober through December 2003. The source spectrum shows a cyclotronresonance scattering feature at 32.8+0.5-0.4 keV,corresponding to a magnetic field strength of3.67+0.06-0.04× 1012 G and isstable through the outburst and over the pulsar spin phase. The pulsar,with an average pulse period of 160.4±0.4 s, shows a spin-up of0.45 s over the duration of the outburst. From optical data, the sourcedistance is estimated to be 3.9±0.1 kpc and this is used toestimate the X-ray luminosity and a theoretical prediction of the pulsarspin-up during the outburst.
| Unveiling the nature of INTEGRAL objects through optical spectroscopy. III. Observations of seven southern sources
Context: . The detection, thanks to the INTEGRAL satellite, of a numberof new hard X-ray sources calls for the identification of their natureby means of observations at longer wavelengths. Aims. In our continuingcampaign to identify longer-wavelength counterparts of thesenewly-discovered hard X-ray sources detected by INTEGRAL, we observedthe putative optical counterparts of seven southern sources at the SouthAfrican Astronomical Observatory and at the European SouthernObservatory. Methods. Optical spectroscopy was performed on allcandidates of these X-ray sources. For two of these objects, opticalphotometry was also acquired. Results. These observations firmlyestablished the nature of four of these sources: we found that IGRJ10404-4625 (=LEDA 93974), 4U 1344-60 and IGR J16482-3036 are ActiveGalactic Nuclei (AGNs) at redshifts z = 0.0237, 0.013 and 0.0313,respectively, and that 2RXP J130159.6-635806 is a Galactic High-MassX-ray Binary (HMXB). We also give possible optical identifications forthree further objects, namely IGR J11215-5952, IGR J11305-6256 and IGRJ16207-5129, which are consistent with being Galactic HMXBs. Physicalparameters for these objects are also evaluated by collecting anddiscussing the available multiwavelength information. Conclusions. Thedetection of four definite or likely HMXBs out of seven objects in oursample further stresses INTEGRAL's crucial contribution in hunting thisclass of object. Also, the determination of the extragalactic nature ofa substantial fraction of the INTEGRAL survey sources underlines theimportance of hard X-ray observations for the study of background AGNslocated beyond the "Zone of Avoidance" of the Galactic Plane.
| An additional Hα emission component in LS I +61°303: Further evidence for the Hα emission related to the neutron star
A long-term program has been established since 1992 to monitor more thana dozen of observable X-ray binaries. We present the long-termspectroscopic results of the Be/X-ray binary, LS I +61°303. TheHα emission line, usually observed as a double-peaked profile witha dominant red peak, varies noticeably during our program. While theblue peak of the Hα emission line decreases gradually, the redpeak increases considerably in 2002 October observational run andremains relatively unchanged in 2004 September run. We attribute thesevariations to the asymmetric perturbation in the Be envelope. Finally,we report on the discovery of a two-component structure in the Hαemission line when close to the apastron. In addition to the originaldouble-peaked Hα emission component, an additional emissioncomponent emerges at the center part of the emission line, which couldoriginate in the accretion disc around the neutron star.
| On the feasibility of detection of neutron star companions to OB runaways using Gaia astrometry
For an illustrative sample of classical OB runaway stars, we examine thecapability of the upcoming Gaia satellite to detect compact companionsby the use of astrometric techniques. For the OB runaway stars in oursample, we estimate initial system parameters and consider the modifyingevolutionary effects of mass transfer and supernova explosion of theprimary. The possible system configurations that follow from this, andthe expected Gaia accuracy, determine the likelihood of detecting amovement of the photocentre due to an unseen companion. As the size ofthe natal kick imparted to the core of the exploding star is increasedthe overall probability of detecting a neutron star companion decreasesas more systems become disrupted. The overall detection probabilitiesfor our illustrative sample range from 2% to 27%, which imply thatwithin a distance of approximately 5 kpc from the Sun around 48detections of compact companions to runaway stars can be expected. Forcomparison, around 15% of High Mass X-ray Binaries would exhibit wobblesdetectable with Gaia.
| Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction
We relate the equivalent widths of 11 diffuse interstellar bands,measured in the spectra of 49 stars, to different colour excesses in theultraviolet. We find that most of the observed bands correlatepositively with the extinction in the neighbourhood of the2175-Åbump. Correlation with colour excesses in other parts of theextinction curve is more variable from one diffuse interstellar band toanother; we find that some diffuse bands (5797, 5850 and 6376 Å)correlate positively with the overall slope of the extinction curve,while others (5780 and 6284 Å) exhibit negative correlation. Wediscuss the implications of these results on the links between thediffuse interstellar band carriers and the properties of theinterstellar grains.
| Hard Spectra of X-ray Pulsars from INTEGRAL Data
We present spectra for 34 accretion-powers X-ray pulsars and onemillisecond pulsar that were within the field of view of the INTEGRALobservatory over two years (December 2002 January 2005) of its in-orbitoperation and that were detected by its instruments at a statisticallysignificant level (> 8σ in the energy range 18 60 keV). Thereare seven recently discovered objects of this class among the pulsarsstudied: 2RXP J130159.6-635806, IGR/AX J16320-4751, IGR J16358-4726, AXJ163904-4642, IGR J16465-4507, SAX/IGR J18027-2017, and AX J1841.0-0535.We have also obtained hard X-ray (>20 keV) spectra for theaccretion-powered pulsars RX J0146.9+6121, AX J1820.5-1434, and AXJ1841.0-0535 for the first time. We analyze the evolution of spectralparameters as a function of the intensity of the sources and comparethese with the results of previous studies.
| Constraining the Nature of the Galactic Center X-Ray Source Population
We searched for infrared counterparts to the cluster of X-ray pointsources discovered by Chandra in the Galactic center region (GCR). Whilethe sources could be white dwarfs, neutron stars, or black holesaccreting from stellar companions, their X-ray properties are consistentwith magnetic cataclysmic variables (CVs), or high-mass X-ray binaries(HMXBs) at low accretion rates. A direct way to decide between thesepossibilities, and hence between alternative formation scenarios, is tomeasure or constrain the luminosity distribution of the companions.Using infrared (J, H, K, Brγ) imaging, we searched forcounterparts corresponding to typical HMXB secondaries: spectral type B0V with K<15 at the GCR. We found no significant excess of brightstars in Chandra error circles, indicating that HMXBs are not thedominant X-ray source population and that they account for fewer than10% of the hardest X-ray sources.
| Chandra Localization of XTE J1906+090 and Discovery of its Optical and Infrared Counterparts
We present the Chandra identification and localization of the transientX-ray source XTE J1906+090 and the discovery of its optical and infraredcounterparts. Our analysis of archival Chandra ACIS-I observations ofthe field found the source approximately 8' away from the positiondetermined earlier with the RXTE PCA. We have confirmed the sourceidentification with timing analysis of the X-ray data, which detectedthe source spin period of 89.6 s. The best Chandra position for thesource is R.A.=19h04m47.491s,decl.=+09deg02'41.40". Subsequently, we performedoptical observations of the field around the new location and discovereda coincident optical source with R-band magnitude of 18.7. A search inthe Two Micron All Sky Survey catalog revealed an infrared point sourcewith J=15.2, H=14.2, and K=13.5, whose location is also coincident withour Chandra and optical positions. Our results add fresh evidence for aBe/X-ray transient nature for XTE J1906+090.
| Spectral Formation in X-Ray Pulsars: Bulk Comptonization in the Accretion Shock
Accretion-powered X-ray pulsars are among the most luminous X-raysources in the Galaxy. However, despite decades of theoretical andobservational work since their discovery, no satisfactory model for theformation of the observed X-ray spectra has emerged. In particular, thepreviously available theories are unable to reproduce the power-lawvariation observed at high energies in many sources. In this paper wepresent the first self-consistent calculation of the spectrum emergingfrom a pulsar accretion column that includes an explicit treatment ofthe energization occurring in the shock. Using a rigorous eigenfunctionexpansion method based on the exact dynamical solution for the velocityprofile in the column, we obtain a closed-form expression for theGreen's function describing the upscattering of radiation injected intothe column from a monochromatic source located at the top of the thermalmound, near the base of the flow. The Green's function is convolved witha Planck distribution to calculate the radiation spectrum resulting fromthe reprocessing of blackbody photons emitted by the thermal mound. Wedemonstrate that the energization of the photons in the shock naturallyproduces an X-ray spectrum with a power-law shape at high energies and ablackbody shape at low energies, in agreement with many observations ofaccreting X-ray pulsars.
| A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-Ray Binaries?
We present results from a 34 ks Chandra ACIS-S observation of thelow-mass X-ray binary (LMXB) population and the hot interstellar medium(ISM) in the nearby (d=3.1 Mpc) lenticular galaxy NGC 5102, previouslyshown to have an unusually low X-ray luminosity. We detect 11 X-raypoint sources within the D25 optical boundary of the galaxy(93% of the light), one-third to one-half of which are likely to bebackground active galactic nuclei (AGNs). One of the X-ray sources iscoincident with the optical nucleus and may be a low-luminosity AGN.Only two sources with an X-ray luminosity greater than 1037ergs s-1 in the 0.5-5.0 keV band were detected, one of whichis statistically likely to be a background AGN. We expected to detectseven or five such luminous sources if the X-ray binary (XRB) populationscales linearly with the B-band or J-band magnitudes, respectively, ofthe host galaxy. By this measure, NGC 5102 has an unusually low numberof XRBs. The deficit of LMXBs is even more striking, because some ofthese sources may in fact be high-mass X-ray binaries (HMXBs). NGC 5102is unusually blue for its morphological type and has undergone at leasttwo recent bursts of star formation only ~1.5×107 and~3×108 yr ago. We present the results of optical/UVspectral synthesis analysis and demonstrate that a significant fraction(>50%) of the stars in this galaxy are comparatively young(<3×109 yr old). We discuss the relationship betweenthe XRB population, the globular cluster (GC) population, and therelative youth of the majority of stars in this galaxy. If the lack ofX-ray binaries is related to the relative youth of most of the stars,this would support models of LMXB formation and evolution that requirewide binaries to shed angular momentum on a timescale of Gyr. We havealso analyzed archival Hubble Space Telescope (HST) images of NGC 5102and find that it has an unusually low specific frequency of GCs(SN~0.4). The lack of LMXBs could also be explained by thesmall number of GCs. We have also detected diffuse X-ray emission in thecentral ~1 kpc of the galaxy with an X-ray luminosity of4.1×1037 ergs s-1 in the 0.1-2.0 keV band.This hot gas is most likely a superbubble created by multiple supernovaeof massive stars born during the most recent star burst and is drivingthe shock into the ISM, which was inferred from previous [O III]λ5007 and Hα observations.
| Spectral Formation in X-Ray Pulsar Accretion Columns
We present the first self-consistent model for the dynamics and theradiative transfer occurring in bright X-ray pulsar accretion columns,with a special focus on the role of the shock in energizing the emergingX-rays. The pressure inside the accretion column of a luminous X-raypulsar is dominated by the photons, and consequently the equationsdescribing the coupled radiative-dynamical structure must be solvedsimultaneously. Spectral formation in these sources is therefore acomplex, nonlinear phenomenon. We obtain the analytical solution for theGreen's function describing the upscattering of monochromatic radiationinjected into the column from the thermal mound located near the base ofthe flow. The Green's function is convolved with a Planck distributionto model the X-ray spectrum resulting from the reprocessing of blackbodyphotons produced in the thermal mound. These photons diffuse through theinfalling gas and eventually escape out the walls of the column, formingthe observed X-ray spectrum. We show that the resultingcolumn-integrated, phase-averaged spectrum has a power-law shape at highenergies and a blackbody shape at low energies, in agreement with theobservational data for many X-ray pulsars.
| Study of the Galactic Interstellar Medium from High-Resolution X-Ray Spectroscopy: X-Ray Absorption Fine Structure and Abundances of O, Mg, Si, S, and Fe
We study the composition of the Galactic interstellar medium (ISM)toward the Galactic center region(5deg<|l|<20deg) by utilizing X-rayabsorption features of three bright low-mass X-ray binaries (LMXBs), GX13+1, GX 5-1, and GX 340+0, observed with the Chandra HETGS. We detectX-ray absorption fine structure (XAFS) of the Si K edge, characterizedby a narrow and a broad absorption feature at 1846 and ~1865 eV,respectively. Comparison with ground experimental data indicates thatmost of the ISM Si exists in the form of silicates, although acomposition of ``pure'' forsterite is ruled out. The XAFS spectra of thesulfur K edge indicate that a significant fraction of S exists in thegas phase. From each source, we derive the column densities of Mg, S,Si, and Fe from the K-edge depth and that of O (or H) from theabsorption of the continuum. The elemental abundance ratios are found tobe consistent between the three targets: the mean values of O/Si, Mg/Si,S/Si, and Fe/Si are determined to be 0.63+/-0.17, 1.14+/-0.13,1.03+/-0.12, and 0.97+/-0.31 solar, respectively (90% error in the meanvalue). We discuss the origins of the overabundances of the heavy metalsrelative to O in the Galactic ISM by comparison with the abundancepattern of the intracluster medium in clusters of galaxies. Assumingthat most of the Mg and Si atoms are depleted into silicates of eitherthe proxine or olivine family, we estimate that the number ratio of Mgto Fe in olivine is >~1.2 and that 15%-37% of the total O atoms inthe ISM must be contained in silicate grains.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| Unresolved Hα Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced Hα emission at|b|>10deg subtending approximately 1° or less on theWisconsin Hα Mapper (WHAM) sky survey. These high-latitude ``WHAMpoint sources'' have Hα fluxes of 10-11-10-9ergs cm-2 s-1, radial velocities within about 70km s-1 of the LSR, and line widths that range from less than20 to about 80 km s-1 (FWHM). Twenty-nine of theseenhancements are not identified with either cataloged nebulae or hotstars and appear to have kinematic properties that differ from thoseobserved for planetary nebulae. Another 14 enhancements are near hotevolved low-mass stars that had no previously reported detections ofassociated nebulosity. The remainder of the enhancements are catalogedplanetary nebulae and small, high-latitude H II regions surroundingmassive O and early B stars.
| Phosphorus in the diffuse interstellar medium
We present FUSE and HST/STIS measurements of the P ii column densitytoward Galactic stars. We analyzed P ii through the profile fitting ofthe unsaturated λ1125 and λ1533 lines and derived columndensities integrated along the sightlines as well as in individualresolved components. We find that phosphorus is not depleted along thosesightlines sampling the diffuse neutral gas. We also investigate thecorrelation existing between P ii and O i column densities and find thatthere is no differential depletion between these two specie.Furthermore, the ratio N(P ii)/N(O i) is consistent with the solar P/Ovalue, implying that P ii and O i coexist in the same gaseous phase andare likely to evolve in parallel. We argue that phosphorus, as traced byP ii, is an excellent neutral oxygen tracer in various physicalenvironments, except when ionization corrections are a significantissue. Thus, P ii lines (observable with FUSE, HST/STIS, or withVLT/UVES for the QSO sightlines) are particularly useful as a proxy forO i lines when these are saturated or blended.
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431
We present the first long-term study of the optical counterpart to theX-ray pulsar RX J0440.9+4431/LS V +4417. The data consist of optical spectroscopic and infraredphotometric observations taken during the period 1995-2005. The infraredobservations are the first published for this source. The results of ourphotometric and spectroscopic analysis show that RXJ0440.9+4431/LS V +44 17 contains amoderately reddened, E(B-V)=0.65±0.05, B0.2V star located atabout 3.3 kpc. The Hα line consistently shows a double-peakprofile varying from symmetric shape to completely distorted on one side(V/R phases). A correlation between the equivalent width of the Hαline and the infrared magnitudes is seen: as the EW(Hα) decreasesthe IR magnitudes become fainter. This long-term optical/IR variabilityis attributed to structural changes in the Be star's circumstellar disc.The observations include a recent decline in the circumstellar disc andsubsequent recovery. We have witnessed the cessation of a globaloscillation due to the decline of the circumstellar disc. If the presentdisc growth rate continues we predict the onset of another episode ofV/R variability by the end of 2006. We have investigated the typicaltime scales for disc variability of various Be/X-ray binaries and founda correlation with the orbital period. This correlation is hard toestablish due to the difficulty in defining the exact duration of thevarious activity states, but it is seen both in the duration of the discgrowth/dissipation phase and the value of the Hα equivalent widthprior to the appearance of asymmetric profiles. These relationshipsprovide further evidence for the interaction of the neutron star withthe circumstellar disc of the Be star's companion and confirms the needof a fully developed disc for the V/R variability to be observed.
| A Be star with a low nitrogen abundance in the SMC cluster NGC 330
High-resolution UVES/VLT spectra of B 12, an extreme pole-on Be star inthe SMC cluster NGC 330, have been analysed using non-LTE modelatmospheres to obtain its chemical composition relative to the SMCstandard star AV 304. We find a general underabundance of metals whichcan be understood in terms of an extra contribution to the stellarcontinuum due to emission from a disk which we estimate to be at the~25% level. When this is corrected for, the nitrogen abundance for B 12shows no evidence of enhancement by rotational mixing as has been foundin other non-Be B-type stars in NGC 330, and is inconsistent withevolutionary models which include the effects of rotational mixing. Asecond Be star, NGC 330-B 17, is also shown to have no detectablenitrogen lines. Possible explanations for the lack of rotational mixingin these rapidly rotating stars are discussed, one promising solutionbeing the possibility that magnetic fields might inhibit rotationalmixing.
| X-ray properties of the transient pulsar 3A 0535+262 in quiescence
We present the timing and spectral properties of the transient Be/X-raybinary pulsar 3A 0535+262 during quiescence using three observationswith the narrow field imaging instruments (NFI) of BeppoSAX. Assuming adistance of 2 kpc for this system, the 2-10 keV X-ray luminositiesmeasured from the three observations are in the range of 1.5-4.0 ×1033 erg s-1, indicating a very low rate ofaccretion. We report the detection of pulsations at a very lowluminosity of 2 × 1033 erg s-1 during one ofthe three observations, though at this accretion rate the system isexpected to be in the centrifugally inhibited regime. The X-ray spectrafor the unpulsed observations are best modeled as power law type while acombined model of power law and black-body is required to fit the pulsedspectrum.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Binaries with Compact Components: Theoretical and Observational Challenges
We report on recent progress in our theoretical understanding of X-raybinaries, which has largely been driven by new observations, andillustrate the interplay between theory and observations considering asexamples intermediate-mass X-ray binaries, irradiation-driven evolution,ultraluminous X-ray sources and neutron stars with low-velocity kicks.
| Populations of Massive X-ray binaries
The population of massive X-ray binaries in a galaxy can provideimportant constraints on its recent history of star formation. However,further knowledge of the Galactic population and formation mechanisms isneeded before deeper inferences can be made for other galaxies.
| Correlations between diffuse interstellar bands and atomic lines
We present and discuss correlations between strengths of the well-known,strong interstellar atomic lines of KI and CaII, and four selected,strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850and 6614. In order to analyse a homogeneous sample of echellehigh-resolution spectra it has been chosen to use measurements fromTerskol Observatory in Northern Caucasus plus a selected number ofhigher resolution observations performed using other instruments. Wedemonstrate that the strength of certain DIBs correlate well withneutral potassium lines and to a much lower degree with ionized calciumlines. This fact suggests that the degree of irradiation of a cloud withUV photons, capable to ionize interstellar atoms, plays a crucial rolein the formation/maintenance of certain molecular species: possiblecarriers of DIBs.
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