All-Sky Survey Bright Source Catalog Research Articles

Far-infrared Properties of Infrared-bright Dust-obscured Galaxies Selected with IRAS and AKARI Far-infrared All-sky Survey

Abstract We investigate the star-forming activity of a sample of infrared (IR)-bright dust-obscured galaxies (DOGs) that show an extreme red color in the optical and IR regime, . Combining an IR-bright DOG sample with the flux at 22 μm > 3.8 mJy discovered by Toba & Nagao with the IRAS faint source catalog version 2 and AKARI far-IR (FIR) all-sky survey bright source catalog version 2, we selected 109 DOGs with FIR data. For a subsample of seven IR-bright DOGs with spectroscopic redshifts ( ) that were obtained from the literature, we estimated their IR luminosity, star formation rate (SFR), and stellar mass based on the spectral energy distribution fitting. We found that (1) the WISE 22 μm luminosity at the observed frame is a good indicator of IR luminosity for IR-bright DOGs and (2) the contribution of the active galactic nucleus to IR luminosity increases with IR luminosity. By comparing the stellar mass and SFR relation for our DOG sample and the literature, we found that most of the IR-bright DOGs lie significantly above the main sequence of star-forming galaxies at similar redshift, indicating that the majority of IRAS- or AKARI-detected IR-bright DOGs are starburst galaxies.

X-RAY SOURCES WITH NO COUNTERPARTS IN OTHER WAVELENGTH BANDS

In order to identify peculiar X-ray sources, we select 442 sources with no counterparts in other wavelength bands (as of the year 1999) from the ROSAT All-Sky Survey Bright Source Catalog. We cross-correlate this initial list with the NASA/IPAC Extragalactic Database, the USNO and WISE catalogs, and the HEASARC XRAY Master Catalog. Eventually, we are left with four unidentified sources with no counterparts in other wavelength bands. We present these four sources and their X-ray properties in this paper.

Radio emission from dusty galaxies observed by AKARI

We probe radio–infrared correlation for two samples of extragalactic sources from the local Universe from the AKARI All-Sky Catalogue. The first, smaller sample (1053 objects) was constructed by the cross-correlation of the AKARI/FIS All-Sky Survey Bright Source Catalogue, the AKARI IRC All-Sky Survey Point Source Catalogue and the NRAO VLA Sky Survey, i.e. it consists of sources detected in the mid- and far-infrared by AKARI, and at the 1.4GHz radio frequency by NRAO. The second, larger sample (13,324 objects) was constructed by the cross-correlation of only the AKARI/FIS All-Sky Survey Bright Source Catalogue and the NRAO VLA Sky Survey, i.e. it consists of sources detected in the far-infrared and radio, without a condition to be detected in the mid-infrared. Additionally, all objects in both samples were identified as galaxies in the NED and/or SIMBAD databases, and a part of them is known to host active galactic nuclei (AGNs). For the present analysis, we have restricted our samples only to sources with known redshift z. In this paper, we analyse the far-infrared–radio correlation for both of these samples. We compare the ratio of infrared and radio emission from normal star-forming dusty galaxies and AGNs in both samples. For the smaller sample we obtained 〈qAGN〉=2.14 for AGNs and 〈qn〉=2.27 for normal galaxies, while for the larger sample 〈qAGN〉=2.15 for AGNs and 〈qn〉=2.22 for normal galaxies. An average value of the slope in both samples is 〈q〉~2.2, which is consistent with the previous measurements from the literature.

BL Lacertae identifications in a ROSAT-selected sample ofFermiunidentified objects

The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.

RADIO EMISSION FROM AKARI GALAXIES

It is a long known fact that there exists a tight correlation between far-infrared and radio emission both for galaxies hosting active galactic nuclei and for star forming galaxies. We probe the radio - infrared correlation for a sample of extragalactic sources constructed by the cross-correlation of the AKARI/IRC All-Sky Survey Point Source Catalogue, the AKARI/FIS All-Sky Survey Bright Source Catalogue, and the NRAO VLA Sky Survey. Additionally, all objects of our sample were identified as galaxies in NED and SIMBAD databases, and a part of them is known to host active galactic nuclei (AGNs). After remeasuring all the fluxes, in order to avoid small aperture effects, we compare the ratio of radio to infrared emission from different types of extragalactic sources, and discuss the FIR/radio correlation as seen by AKARI and make a comparison to the previous results obtained thanks to IRAS.

BAT X‐Ray Survey. I. Methodology and X‐Ray Identification

We applied the maximum likelihood (ML) method, as an image reconstruction algorithm, to the BAT X-Ray Survey (BXS). This method was specifically designed to preserve the full statistical information in the data and to avoid mosaicking of many exposures with different pointing directions, thus reducing systematic errors when co-adding images. We reconstructed, in the 14–170 keV energy band, the image of a 90 x 90 deg^2 sky region, centered on (R.A., decl.)=(105°,-25°), which BAT surveyed with an exposure time of ~1 Ms (in 2005 November). The best sensitivity in our image is ~0.85 mcrab or 2.0 x 10-11 ergs cm−2. We detect 49 hard X-ray sources above the 4.5 σ level; of these, only 12 were previously known as hard X-ray sources (>15 keV). Swift XRT observations allowed us to firmly identify the counterparts for 15 objects, while 2 objects have Einstein IPC counterparts (Harris et al. 1990); in addition to those, we found a likely counterpart for 13 objects by correlating our sample with the ROSAT All-Sky Survey Bright Source Catalog (Voges et al. 1999). Seven objects remain unidentified. Analysis of the noise properties of our image shows that ~75% of the area is surveyed to a flux limit of ~1 mcrab. This study shows that the coupling of the ML method to the most sensitive, all-sky surveying, hard X-ray instrument, BAT, is able to probe for the first time the hard X-ray sky to the millicrab flux level. The successful application of this method to BAT demonstrates that it could also be applied with advantage to similar instruments such as INTEGRAL IBIS.

Discovery of an Isolated Compact Object at High Galactic Latitude

We report the discovery of a at high Galactic latitude. The was initially identified as a ROSAT All-Sky Survey Bright Source Catalog X-ray source, 1RXS J141256.0+792204, statistically likely to possess a high X-ray to optical flux ratio. Further observations using Swift, Gemini-North, and the Chandra X-Ray Observatory refined the source position and confirmed the absence of any optical counterpart to an X-ray to optical flux ratio of FX(0.1–2.4 keV)/FV > 8700 (3 σ). Interpretation of 1RXS J141256.0+792204—which we have dubbed Calvera—as a typical X-ray-dim isolated neutron star would place it at z ≈ 5.1 kpc above the Galactic disk—in the Galactic halo—implying that it either has an extreme space velocity (vz 5100 km s−1) or has failed to cool according to theoretical predictions. Interpretations as a persistent anomalous X-ray pulsar or a compact central object present conflicts with these classes' typical properties. We conclude that the properties of Calvera are most consistent with those of a nearby (80-260 pc) radio pulsar, similar to the radio millisecond pulsars of 47 Tucanae, with further observations required to confirm this classification. If it is a millisecond pulsar, it is has an X-ray flux equal to the X-ray brightest millisecond pulsar (and so is tied for highest flux); the closest northern hemisphere millisecond pulsar; and potentially the closest known millisecond pulsar in the sky, making it an interesting target for X-ray study, a radio pulsar timing array, and LIGO.

Spectral Classification of Optical Counterparts toROSATAll-Sky Survey X-Ray Sources

Previous work statistically identified 5492 optical counterparts, with 90% confidence, from among the ≈18,000 X-ray sources appearing in the ROSAT All-Sky Survey Bright Source Catalog. Using low-resolution spectra in the wavelength range 3700-7900 A, we present spectroscopic classifications for 195 of these counterparts which have not previously been classified. Of these 195, we find 168 individual stars of F, G, K, or M type, 6 individual stars of unknown type, 6 double stars, 6 AGNs or galaxies, and 7 unclassifiable objects; the spectra of the 2 remaining objects were saturated.

RXTE all-sky slew survey. Catalog of X-ray sources at $\mathsf{|\textit{b}|>10^\circ}$

We report results of a serendipitous hard X-ray (3−20 keV), nearly all-sky (|b| > 10 ◦ ) survey based on RXTE/PCA observations performed during satellite reorientations in 1996−2002. The survey is 80% (90%) complete to a 4σ limiting flux of ≈1.8 (2.5) × 10 −11 erg s −1 cm −2 in the 3−20 keV band. The achieved sensitivity in the 3−8 keV and 8−20 keV subbands is similar to and an order of magnitude higher than that of the previously record HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined 7 × 10 3 sq deg area of the sky is sampled to flux levels below 10 −11 erg s −1 cm −2 (3−20 keV). In total 294 sources are detected and localized to better than 1 deg. 236 (80%) of these can be confidently associated with a known astrophysical object; another 22 likely result from the superposition of 2 or 3 closely located known sources. 35 detected sources remain unidentified, although for 12 of these we report a likely soft X-ray counterpart from the ROSAT all-sky survey bright source catalog. Of the reliably identified sources, 63 have local origin (Milky Way, LMC or SMC), 64 are clusters of galaxies and 100 are active galactic nuclei (AGN). The fact that the unidentified X-ray sources have hard spectra suggests that the majority of them are AGN, including highly obscured ones (NH > 10 23 cm −2 ). For the fi rst time we present a logN− logS diagram for extragalactic sources above 4 × 10 −12 erg s −1 cm −2 at 8−20 keV.

The Rate of Visual Binaries among the Brightest X-Ray Stars

Using the Tycho Double Star Catalogue and the Hipparcos Catalogue, I compare the rates of occurrence of resolved visual doubles among the brightest X-ray stars from the ROSAT All-Sky Survey Bright Source Catalog (RASS-BSC) and generally in these two catalogs. A strong correlation between visual binarity and brightness in X-rays is found. The rate of binarity, quantified as the fraction of resolved components, is 7 times higher for the RASS-BSC stars than generally for all Tycho-2 stars. In order to eliminate the distance bias in the sample, I compare the rates in a distance-limited sample of Hipparcos stars within 50 pc. The comparison has shown that the fraction of visual and unresolved orbiting binaries among stars with counterparts in the RASS-BSC, hence bright in X-rays, is 2.4 times higher than among fainter X-ray stars, i.e., the ones whose X-ray brightness is below the limit of the RASS-BSC. This difference is not caused by the enhancement of the observed flux due to the combined component's radiation. Previously found in the nearby open clusters, the correlation between binarity and X-ray luminosity exists for nearby field stars as well. Possible implications of the discrepancy are discussed, including the binarity-age and binarity-rotation velocity relations. The H-R diagram of components of nearby visual binaries indicates that the red clump giants luminous in X-rays may be unresolved binaries.

Post–T Tauri Stars in the Nearest OB Association

We present results of a spectroscopic survey of X-ray– and proper-motion–selected samples of late-type stars in the Lower Centaurus–Crux (LCC) and Upper Centaurus–Lupus (UCL) subgroups of the nearest OB association: Scorpius-Centaurus. The primary goals of the survey are to determine the star formation history of the OB subgroups and to assess the frequency of accreting stars in a sample dominated by post–T pre–main-sequence (PMS) stars. We investigate two samples: (1) proper-motion candidates from the ACT Catalog and Tycho Reference Catalog (TRC) with X-ray counterparts in the ROSAT All-Sky Survey (RASS) Bright Source Catalog and (2) G- and K-type stars in the Hipparcos catalog found to be candidate members by de Zeeuw et al. We obtained optical spectra of 130 candidates with the Siding Spring 2.3 m dual-beam spectrograph. PMS stars were identified by (1) strong Li λ6707 absorption, (2) subgiant surface gravities, (3) proper motions consistent with Sco-Cen membership, and (4) H-R diagram positions consistent with being PMS. We find 93% of the RASS-ACT/TRC stars to be probable PMS members, compared with 73% of the Hipparcos candidates. We demonstrate that measuring the gravity-sensitive band ratio of Sr II λ4077 to Fe I λ4071 is a valuable means of discriminating PMS and zero-age main-sequence (ZAMS) stars. Using secular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Survey photometry, we construct an H-R diagram. Depending on the choice of published evolutionary tracks, we find the mean ages of the PMS populations to range between 17 and 23 Myr for LCC and 15 and 22 Myr for UCL. Taking into account observational errors, it appears that 95% of the low-mass star formation in each subgroup must have occurred in less than 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, we find main-sequence turnoff ages for Hipparcos B-type members to be 16 ± 1 Myr for LCC and 17 ± 1 Myr for UCL. Contrary to previous findings, it appears that LCC is coeval with, or slightly older than, UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of 85–215 pc, with 12 of the LCC members lying within 100 pc of the Sun. Only one out of 110 (0.9%; 1 σ) PMS solar-type stars in the sample with ages of 13 ± 1 (s.e.) ± 6 (1 σ) Myr and masses of 1.3 ± 0.2 (1 σ) M⊙ shows both enhanced Hα emission and a K-band excess indicative of accretion from a truncated circumstellar disk: the nearby (d 86 pc) classical T Tauri star PDS 66.

Galaxy identifications in the ROSAT All-Sky Survey

In a correlation study of the ROSAT All-Sky Survey Bright Source Catalogue (RASS-BSC) with the Catalogue of Principal Galaxies (PGC) 904 X-ray sources were found that possess possible extragalactic counterparts within a search radius of 100 arcsec. A visual screening process was applied to classify the reliability of the correlations. 547 correlations have been quoted as reliable identifications. From these, 349 sources are known as active galaxies. Although for the other sources no hints for activity were found in the literature, 69% of those for which we have distances show X-ray luminosities exceeding those of normal galaxies, a clear sign that these galaxies also own hitherto unreported X-ray active components. Some objects are located inside or in the direction of a known group or cluster of galaxies. Their X-ray flux may therefore be in part affected by hot gas emission. Luminosity and $\log N{-}\log S$ distributions are used to characterize different subsamples. Nuclei that are both optically and X-ray active are found predominantly in spirals. Two special source samples are defined, one with candidates for X-ray emission from hitherto unknown groups or clusters of galaxies, and one with high X-ray luminosity sources, that are likely candidates to possess hitherto unreported active galactic nuclei. Besides a compilation of X-ray and optical parameters, also X-ray overlays on optical images for all the objects are supplied as part of this work.

The angular correlation function of the ROSAT All-Sky Survey Bright Source Catalogue

We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All-Sky Survey (RASS) Bright Source Catalogue, in order to investigate the clustering properties of active galactic nuclei (AGN) in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30° and declination δ<-30° are also rejected owing to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue suggests that the vast majority of our sources are indeed AGN. A 4.1σ correlation signal between 0° and 8° was detected with w(θ<8°)=2.5±0.6×10−2. Assuming a two-point correlation function of the form w(θ)=(θθ0)−0.8, we find θ0=0.°062. Deprojection on three dimensions, using Limber's equation, yields a spatial correlation length of r0≈6.0±1.6 h−1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.

X-ray luminous star-forming galaxies

We present results from the cross-correlation of the spectroscopic atlas of Ho etal (1995) with the ROSAT All-Sky Survey Bright Source Catalogue, in an attempt to understand the X-ray emission mechanisms in nearby galaxies. The resulting sample of 45 galaxies consists predominantly of AGN. However, there are several starforming galaxies spanning a wide range of X-ray luminosities (~10^{38} - 10^{42} erg s^{-1}). We have analyzed ROSAT and ASCA data for the two most luminous star-forming galaxies, namely NGC3310 and NGC3690. We find that their 0.1-10 keV X-ray spectra can be fitted by a soft thermal plasma of kT$\sim0.8$ keV and a harder component with kT~10-15 keV or a power-law with Gamma~1.6. These are very similar to the spectra of the archetypal star-forming galaxies NGC253 and M82.

Nine newly discovered bright low-redshift quasars from RASS

By using the CCD detector and Cassegrain spectrograph of the 2.16 m telescope at Beijing Astronomical Observatory, we are carrying out optical identifications of the X-ray sources selected from the ROSAT All-Sky Survey Bright Source Catalogue. We present here the discovery of 9 bright low-redshift quasars.

Newly discovered candidate weak-line T Tauri stars in the surrounding area of the Taurus-Auriga region

We present results of an extensive search for weak-line T Tauri stars (WTTS) in the outskirts of the Taurus-Auriga molecular cloud on the basis of the ROSAT All-Sky Survey Bright Source Catalog. Our surveyed region extends from 2h 40m to 5h 40m in right ascension and from 10° to 40° in declination, with the central part of Taurus-Auriga (4 5h , 15 34°), accomplished by Wichmann et al. (1996), excluded. Within a sky coverage of about 103 square degrees, 219 X-ray sources fulfil the criteria for selecting program sources suggested by Neuhauser et al. (1995a), and 164 of these X-ray sources were found to have at least one optical counterpart with E magnitude brighter than 16. Low-resolution spectroscopic observation has been carried out in order to discard early type stars and galaxies from the sample, additional intermediate-resolution spectra of a sub-sample of 156 late type optical counterparts were obtained for spectral classification and for the calculation of the equivalent width of H emission and LiI line absorption at 6707 A. Excluding 2 previously identified WTTS, a total of 75 new candidate WTTS and one possible classical T Tauri star have been discovered in our study. The majority of the newly found Li-rich optical counterparts are believed to be PMS stars rather than ZAMS stars as those of the Pleiades.

Searching for old neutron stars with ROSAT

This is the second in a series of three papers constraining the number of detectable old neutron stars in the Galaxy. Here, I present the statistical analysis of a sample of X-ray sources coincident with areas of dark clouds in the Galactic plane. I compare this sample with all sources in the ROSAT All-Sky Survey bright source catalog within 20 of the Galactic plane. I present the re- sults of an identication program of a subset of sources that are compatible with a soft, thermal X-ray spectrum and an eective source temperature of less than 70 eV. The three brightest sources in this sample form an intriguing subgroup. One of them is a previously identied candi- date for an accreting neutron star. I identify the other two sources with hot white dwarf stars. I nd no new ac- creting neutron star candidate in this sample. Based on this result, I derive an upper limit to the space density of accreting neutron stars in elds of Galactic dark clouds of 2s r 1 at a count rate > 0:05 s 1 .