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An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| XMM-Newton observations of the brightest ultraluminous X-ray sources
We present an analysis of 13 of the best quality ultraluminous X-raysource (ULX) data sets available from XMM-Newton European Photon ImagingCamera (EPIC) observations. We utilize the high signal-to-noise in theseULX spectra to investigate the best descriptions of their spectral shapein the 0.3-10keV range. Simple models of an absorbed power law ormulticolour disc blackbody prove inadequate at describing the spectra.Better fits are found using a combination of these two components, withboth variants of this model - a cool (~0.2keV) disc blackbody plus hardpower-law continuum, and a soft power-law continuum, dominant at lowenergies, plus a warm (~1.7keV) disc blackbody - providing good fits to8/13 ULX spectra. However, by examining the data above 2keV, we findevidence for curvature in the majority of data sets (8/13 with at leastmarginal detections), inconsistent with the dominance of a power law inthis regime. In fact, the most successful empirical description of thespectra proved to be a combination of a cool (~0.2keV) classic blackbodyspectrum, plus a warm disc blackbody that fits acceptably to 10/13 ULXs.The best overall fits are provided by a physically self-consistentaccretion disc plus Comptonized corona model (DISKPN + EQPAIR), whichfits acceptably to 11/13 ULXs. This model provides a physicalexplanation for the spectral curvature, namely that it originates in anoptically thick corona, though the accretion disc photons seeding thiscorona still originate in an apparently cool disc. We note similaritiesbetween this fit and models of Galactic black hole binaries at highaccretion rates, most notably the model of Done & Kubota. In thisscenario the inner disc and corona become energetically coupled at highaccretion rates, resulting in a cooled accretion disc and opticallythick corona. We conclude that this analysis of the best spectral datafor ULXs shows it to be plausible that the majority of the populationare high accretion rate stellar-mass (perhaps up to 80Msolar)black holes, though we cannot categorically rule out the presence oflarger, ~1000-Msolar intermediate-mass black holes (IMBHs) inindividual sources with the current X-ray data.
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| Radio Emission on Subparsec Scales from the Intermediate-Mass Black Hole in NGC 4395
The Seyfert 1 nucleus of NGC 4395 is energized by a black hole of mass3.6×105 Msolar, making it one of only twonuclear black holes of intermediate mass, 103-106Msolar, detected in the radio regime. Building on UV andX-ray evidence for outflows from this Seyfert nucleus, the VLBI HighSensitivity Array was used at 1.4 GHz to search for extended structureon scales greater than 5 mas (0.1 pc). Elongated emission wasdiscovered, extending over 15 mas (0.3 pc) and suggesting an outflow onsubparsec scales from this intermediate-mass black hole. The Seyfertnucleus is located at the center of an elliptical star cluster, and theelongation position angle of the subparsec radio structure is only19° from the star cluster's minor axis.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| Black Hole Mass of the Ultraluminous X-Ray Source M82 X-1
We report the first clear evidence for the simultaneous presence of alow-frequency break and a QPO in the fluctuation power spectrum of awell-known ultraluminous X-ray source (ULX) in M82 using long XMM-Newtonobservations. The break occurs at a frequency of34.2+6-3 mHz. The QPO has a centroid atνQPO=114.3+/-1.5 mHz, a coherenceQ≡νQPO/ΔνFWHM~=3.5, and anamplitude (rms) of 19% in the 2-10 keV band. The power spectrum isapproximately flat below the break frequency and then falls off abovethe break frequency as a power law with the QPO superposed. This form ofthe power spectrum is characteristic of the Galactic X-ray binaries(XRBs) in their high or intermediate states. M82 X-1 was likely in anintermediate state during the observation. The EPIC pn spectrum is welldescribed by a model comprising an absorbed power law (Γ~2) and aniron line at ~6.6 keV with a width σ~0.2 keV and an equivalentwidth of ~180 eV. Using the well-established correlations between thepower and energy spectral parameters for XRBs, we estimate a black holemass for M82 X-1 in the range of ~25-520 Msolar, includingsystematic errors that arise due to the uncertainty in the calibrationof the photon spectral index versus QPO frequency relation.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| Stellar clusters in dwarf galaxies
We present new observations in the Ks (2.2 μm) and L' (3.7μm) infrared bands of a sample of blue dwarf galaxies with the largeraim of studying the population of massive stellar clusters, theoccurrence of dust-embedded stellar clusters, and their properties. AllKs images show a rich population of clusters, but only asmall fraction of them is bright in L'. Most L' sources have radiocounterparts. We derived the luminosity function in Ks forthe galaxies IC 4661 and NGC 5408, finding both to be consistent withthose of similar galaxies. We also compared the number of clusters andtheir luminosities with the star-formation rate of the host galaxies andfound no compelling evidence of correlation. We conclude that youngclusters and embedded clusters are a common feature of blue dwarfgalaxies and possibly of galaxies in general, we suggest that theiroccurrence is due to purely statistical effects rather than a phenomenonrelated to specific physical conditions. In this sense we expect theseobjects to be abundant at high red-shift.
| XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby Galaxies
We examined X-ray spectral and timing properties of ultraluminous X-raysources (ULXs) in nearby galaxies in XMM-Newton archival data. Thereappear to be three distinct classes of spectra. One class shows emissionfrom hot, diffuse plasma. This thermal emission is similar to that seenfrom recent supernovae; the temperatures are in the range 0.6-0.8 keV,and the luminosities are the lowest in our sample, near 1039ergs s-1. Three sources have spectra that are strongly curvedat high energies and have the highest temperatures in our sample,1.0-1.4 keV. These spectra are well fitted with a power-law plusmulticolor disk blackbody model with the power law dominant at lowenergies or a Comptonization model. The remainder of the sources arebest fitted with a power-law plus multicolor disk blackbody model, as iscommonly used to describe the spectra of accreting black holes. Thesesources have the lowest thermal component temperatures, 0.1-0.4 keV, andextend to the highest luminosities, above 1040 ergss-1. The temperature of the thermal component is in threedistinct ranges for the three source classes. This diversity of spectralshapes and the fact that the sources lie in three distinct temperatureranges suggests that the ULXs are a diverse population. Two ULXs thatshow state transitions stay within a single class over the course of thetransition. However, we cannot conclude with certainty that the classesrepresent distinct types of objects rather than spectral states of asingle population of objects. More monitoring observations of ULXs withXMM-Newton are required. We also searched for timing noise from thesources and report detection of noise above the Poisson level from fivesources. In three of the sources, the power density spectrum increaseswith decreasing frequency as a power law down to the lowest frequenciesobserved, below 10-4 Hz.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| M74 X-1 (CXOU J013651.1+154547): An Extremely Variable Ultraluminous X-Ray Source
Ultraluminous X-ray sources (ULXs) have been described variously as themost luminous normal X-ray binaries, hypernovae, and``intermediate-mass'' black holes with masses of hundreds to thousandsof solar masses. We present results on M74 X-1 (CXOU J013651.1+154547),a ULX in the nearby spiral galaxy M74 (NGC 628), from observations byChandra and XMM-Newton. M74 X-1 is a persistent source that exhibitsextreme variability and changes in spectral state on timescales ofseveral thousand seconds. Its variability timescales and behaviorresemble some Galactic microquasars. This suggests that the emissionmechanism may be related to relativistically beamed jets and that M74X-1 could be an extragalactic ``microblazar''-a microquasar whose jetaxis is aligned with our line of sight. We also note that its spectrumis consistent with the presence of a low-temperature disk blackbodycomponent, which, assuming it is due to radiation from an accretiondisk, could indicate that M74 X-1 contains an intermediate-mass blackhole.
| Radio Emission Associated with the Ultraluminous X-Ray Source in Holmberg II
We report the detection of radio emission coincident with theultraluminous X-ray source (ULX) in Holmberg II. The radio emission isdiffuse and resolved, covering an area ~60×40 pc in extent andwell matched to the recently discovered He II nebula surrounding theX-ray source. Comparison of the radio and optical properties of thisextended radio emission argue against its association with either an HII region or a supernova remnant. This is additional evidence that thisULX is not powered by a stellar mass object whose emission isrelativistically beamed toward the observer, and thus is either asuper-Eddington source or an intermediate-mass black hole as suggestedby optical observations. Implications of this result to future andexisting radio studies of ULXs are discussed.
| A 2 Hour Quasi Period in an Ultraluminous X-Ray Source in NGC 628
Quasi-periodic oscillations and X-ray spectroscopy are powerful probesof black hole masses and accretion disks, and here we apply thesediagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxyNGC 628 (M74). This object was observed four times over 2 years with theChandra X-Ray Observatory and XMM-Newton, with three long observationsshowing dramatic variability, distinguished by a series of outburstswith a quasi period of 4000-7000 s. This is unique behavior among ULXsand Galactic X-ray binaries because of the combination of its burstlikepeaks and deep troughs, its long quasi periods, its high variationamplitudes of >90%, and its substantial variability betweenobservations. The X-ray spectra is fitted by an absorbed accretion diskplus a power-law component, suggesting the ULX was in a spectral stateanalogous to the low/hard state or the very high state of Galactic blackhole X-ray binaries. A black hole mass of ~(2-20)×103Msolar is estimated from the fb-M•scaling relation found in the Galactic X-ray binaries and activegalactic nuclei.
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| A radio monitoring survey of ultra-luminous X-ray sources
We present the results of a radio monitoring campaign to search forradio emission from nearby ultra-luminous X-ray sources (ULXs). Thesesources are bright off-nuclear X-ray point sources with luminositiesexceeding LX > 1039 erg s-1. Awell-defined sample of the 9 nearest ULXs has been monitored eight timesover 5 months with the Very Large Array in A and B configuration. Ourlimiting sensitivity is ≈0.15 mJy (4σ) for radio flares and≈60 μJy for continuous emission. In M 82 two ULXs seem to havecoincident compact radio sources, which are probably supernova remnants.No continuous or flaring radio emission has been detected from any otherULX. Thus, ULXs do not generally emit steady-state radio emission aboveradio powers of 1.5 × 1017 W/Hz. The non-detections ofthe continuous emission are consistent with beamed or unbeamed radioemission from accreting black holes of ≤ 103 Mȯ based on the radio/X-ray correlation. Other publishedradio detections (M 82, NGC 5408) are also discussed in this context.Both detections are significantly above our detection limit. If ULXshave flaring radio emission above 4 × 1017 W/Hz we cangive an upper limit on the duty cycle of the flares of 6%. This upperlimit is in agreement with the observed number of flares in Galacticradio transients. Additionally we present a yet unreported radio doublestructure in the nearby low-luminosity AGN NGC 4736.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.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/429/1125
| Ultraluminous X-ray Sources: an Observational Review
Ultraluminous X-ray Sources (ULXs) are, as suggested by their name,extremely luminous and rare X-ray emitting objects found in galaxies.Because of their luminosity, it has been suggested that they may bepowered by accretion onto a black hole (BH) of a few 100Mȯ, more massive than what one would expect to originatefrom normal stellar evolution. Alternative models include youngsupernova remnants (SNRs), beamed emission from normal BH X-ray binaries(XRB) with high accretion rates, and relativistically beamed XRBemission. The observational evidence on ULXs suggests that while most ofthem are likely to be compact accreting objects, there is no clearunique evidence pointing either to the beamed XRB model or to accretiononto a very massive BH. It is possible that what we call ULXs are aheterogeneous family of X-ray sources.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| Chandra observations of five ultraluminous X-ray sources in nearby galaxies
We report the results of a programme of dual-epoch Chandra ACIS-Sobservations of five ultraluminous X-ray sources (ULXs) in nearby spiralgalaxies. All five ULXs are detected as unresolved, point-like X-raysources by Chandra, though two have faded below the 1039 ergs-1 luminosity threshold used to first designate thesesources as ULXs. Using this same criterion, we detect three further ULXswithin the imaged regions of the galaxies. The ULXs appear to be relatedto the star-forming regions of the galaxies, indicating that even innormal spiral galaxies the ULX population is predominantly associatedwith young stellar populations. A detailed study of the Chandra ACIS-Sspectra of six of the ULXs shows that five are better described by apower-law continuum than a multicolour disc blackbody model, thoughthere is evidence for additional very soft components to two of thepower-law continua. The measured photon indices in four out of fivecases are consistent with the low/hard state in black hole binaries,contrary to the suggestion that power-law-dominated spectra of ULXsoriginate in the very high state. A simple interpretation of this isthat we are observing accretion on to intermediate-mass black holes,though we might also be observing a spectral state unique to very highmass accretion rates in stellar-mass black hole systems. Short-term fluxvariability is only detected in one of two epochs for two of the ULXs,with the lack of this characteristic arguing that the X-ray emission ofthis sample of ULXs is not dominated by relativistically beamed jets.The observational characteristics of this small sample suggest that ULXsare a distinctly heterogeneous source class.
| Is M82 X-1 Really an Intermediate-Mass Black Hole? X-Ray Spectral and Timing Evidence
Ultraluminous X-ray sources (ULXs) with apparent luminosities up tohundreds of times the Eddington luminosity for a neutron star have beendiscovered in external galaxies. The existence of intermediate-massblack holes has been proposed to explain these sources. We presentevidence for an intermediate-mass black hole in the ULX M82 X-1 based onthe spectral features and timing (quasi-periodic oscillation [QPO])properties of the X-radiation from this source. We revisited XMM-Newtonand Rossi X-Ray Timing Explorer (RXTE) data for M82 X-1 obtained in 2001and 1997 for XMM and RXTE, respectively. We show for these observationsthat the source is either in transition or in a high/soft state withphoton spectral indices 2.1 and 2.7, respectively. We confirm the earlydetermination of the QPO frequency ν~55 mHz in this source byStrohmayer & Mushotzky and identify this as the low-frequency QPOfor the source. We apply a new method to determine the black hole massof M82 X-1. The method uses the index-QPO low-frequency correlation thathas been recently established in Galactic black hole candidates GRS1915+105, XTE J1550-564, 4U 1630-47, and others. Using scaling argumentsand the correlation derived from the consideration of Galactic blackholes, we conclude that M82 X-1 is an intermediate black hole with amass of the order of 1000 Msolar.
| XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte Carlo Simulated Model
We present a systematic spectral analysis of six ultraluminous X-raysources (NGC 1313 X-1 and X-2, IC 342 X-1, Ho IX X-1, NGC 5408 X-1, andNGC 3628 X-1) observed with the XMM-Newton observatory. Theseextranuclear X-ray sources in nearby late-type galaxies have beenconsidered as intermediate-mass black hole candidates. We have performedMonte Carlo simulations of Comptonized multicolor blackbody accretiondisks. This unified and self-consistent spectral model assumes aspherically symmetric, thermal corona around each disk and accounts forthe radiation transfer in the Comptonization. We find that the modelprovides satisfactory fits to the XMM-Newton spectra of the sources. Thecharacteristic temperatures of the accretion disks (Tin), forexample, are in the range of ~0.05-0.3 keV, consistent with theintermediate-mass black hole interpretation. We find that the black holemass is typically about a few times 103 Msolar andhas an accretion rate of ~10-6 to 10-5 Msolar yr-1. For the spectra considered here, we find thatthe commonly used multicolor blackbody accretion disk model with anadditive power-law component, although not physical, provides a goodmathematical approximation to the Monte Carlo simulated model. However,the latter model provides additional constraints on the properties ofthe accretion systems, such as the disk inclination angles and coronaoptical depths.
| A Transition to a Low/Soft State in the Ultraluminous Compact X-Ray Source Holmberg II X-1
We present three XMM-Newton observations of the ultraluminous compactX-ray source Holmberg II X-1 in its historical brightest and fainteststates. The source was in its brightest state in 2002 April with anisotropic X-ray luminosity of ~2×1040 ergss-1 but changed to a peculiar low/soft state in 2002September in which the X-ray flux dropped by a factor of ~4 and thespectrum softened. In all cases, a soft excess component, which can bedescribed by a simple or multicolor disk blackbody (kT~120-170 eV), isstatistically required in addition to a power-law continuum(Γ~2.4-2.9). Both spectral components became weaker and softer inthe low/soft state; however, the dramatic variability is seen in thepower-law component. This spectral transition is opposite to the``canonical'' high/soft-low/hard transitions seen in many Galactic blackhole binaries. There is a possible contribution from an optically thinthermal plasma. When this component is taken into account, the spectraltransition appears to be normal-a drop of the power-law flux and aslightly softer blackbody component in the low state.
| Revealing a Cool Accretion Disk in the Ultraluminous X-Ray Source M81 X-9 (Holmberg IX X-1): Evidence for an Intermediate-Mass Black Hole
We report the results of an analysis of two XMM-Newton EPIC-pn spectraof the bright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1)obtained in snapshot observations. Soft thermal emission is clearlyrevealed in spectra dominated by hard power-law components. Depending onthe model used, M81 X-9 was observed at a luminosity ofLX=(1.0-1.6)×1040ergss-1(0.3-10.0 keV). The variability previously observed in this sourcesignals that it is an accreting source that likely harbors a black hole.Remarkably, accretion disk models for the soft thermal emission yieldvery low inner disk temperatures (kT=0.17-0.29 keV, including 90%confidence errors and variations between observations and disk models)and improve the fit statistic over any single-component continuum modelat the 6 σ level of confidence. This represents much strongerevidence for a cool disk than prior evidence that combined spectra fromdifferent observatories, and the strongest evidence of a cool disk in anultraluminous X-ray source apart from that for NGC 1313 X-1. In commonwith NGC 1313 X-1, scaling the temperatures measured in M81 X-9 to thosecommonly seen in stellar-mass Galactic black holes at their highestobserved fluxes (kT~=1 keV) may imply that M81 X-9 harbors a black holewith a mass on the order of 103 Msolar themeasured disk component normalization and broadband luminosity implyblack hole masses on the order of 102 Msolar. Itis therefore possible that these sources harbor 103Msolar black holes accreting atLX~=0.1×LEdd. While these results do notrepresent proof that M81 X-9 harbors an intermediate-mass black hole,radio and optical observations suggest that beaming and anisotropicemission from a stellar-mass black hole are unlikely to account for theimplied luminosity. We further argue that the strength of the hardemission in these sources and well-established phenomena frequentlyobserved in stellar-mass black holes near to the Eddington limit suggestthat optically thick photospheres are unlikely to be the origin of thecool thermal emission in bright ultraluminous X-ray sources. Forcomparison to M81 X-9, we have also analyzed the previously unpublishedEPIC-pn spectrum of NGC 1313 X-1 cool disk emission is again observed,and refined spectral fit parameters and mass estimates are reported.
| The Ultraluminous X-Ray Source NGC 1313 X-2 (MS 0317.7-6647) and Its Environment
We present new optical and Chandra observations of the field containingthe ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 m image, theChandra error box embraces an R=21.6 pointlike object and excludes apreviously proposed optical counterpart. The resulting X-ray/opticalflux ratio of NGC 1313 X-2 is ~500. The value offX/fopt, the X-ray variability history, and thespectral distribution derived from a reanalysis of the ROSAT, ASCA, andXMM-Newton data indicate a luminous X-ray binary in NGC 1313 as a likelyexplanation for NGC 1313 X-2. If the X-ray soft component observed inthe XMM-Newton EPIC spectrum originates from an accretion disk, theinferred mass of the compact remnant is ~100 Msolar, makingit an intermediate-mass black hole. The derived optical luminosity(L~105Lsolar) is consistent with that of a ~15-20Msolar companion. The properties of the environment of NGC1313 X-2 are briefly discussed.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB -11 toMB -18 (or with increasing rotation velocity fromVrot 10 km s-1 to Vrot 100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org}
| X-ray flares from the ultra-luminous X-ray source in NGC 5408
We have studied an ultra-luminous X-ray source (ULX) in the dwarf galaxyNGC 5408 with a series of XMM-Newton observations, between 2001 July and2003 January. We find that its X-ray spectrum is best fitted with apower law of photon index Γ ≈ 2.6-2.9 and a thermal componentwith blackbody temperature kTbb ≈ 0.12-0.14 keV. Thesespectral features, and the inferred luminosity ≈ 1040 ergs-1 in the 0.3-12 keV band, are typical of bright ULXs innearby dwarf galaxies. The blackbody plus power-law model is asignificantly better fit than either a simple power law or a brokenpower law (although the latter model is also acceptable at some epochs).Doppler-boosted emission from a relativistic jet is not required,although we cannot rule out this scenario. Our preliminary timinganalysis shows flaring behaviour which we interpret as variability inthe power-law component, on timescales of 102 s. Thehard component is suppressed during the dips, while the soft thermalcomponent is consistent with being constant. The power density spectrumis flat at low frequencies, has a break at νb ≈ 2.5mHz, and has a slope ≈ -1 at higher frequencies. A comparison withthe power spectra of Cyg X-1 and of a sample of other BH candidates andAGN suggests a mass of 102 Mȯ. It isalso possible that the BH is at the upper end of the stellar-mass class(M 50 Mȯ), in a phase of moderatelysuper-Eddington accretion. The formation of such a massive BH via normalstellar evolution may have been favoured by the very metal-poorenvironment of NGC 5408.
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