- ID:
- ivo://CDS.VizieR/J/AJ/150/94
- Title:
- Chandra Local Volume Survey. I.
- Short Name:
- J/AJ/150/94
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present comprehensive X-ray point source catalogs of NGC55, NGC2403, and NGC4214 as part of the Chandra Local Volume Survey. The combined archival observations have effective exposure times of 56.5ks, 190ks, and 79ks for NGC55, NGC2403, and NGC4214, respectively. When combined with our published catalogs for NGC300 and NGC404, our survey contains 629 X-ray sources total down to a limiting unabsorbed luminosity of ~5x10^35^erg/s in the 0.35-8keV band in each of the five galaxies. We present X-ray hardness ratios, spectral analysis, radial source distributions, and an analysis of the temporal variability for the X-ray sources detected at high significance. To constrain the nature of each X-ray source, we carried out cross-correlations with multi-wavelength data sets. We searched overlapping Hubble Space Telescope observations for optical counterparts to our X-ray detections to provide preliminary classifications for each X-ray source as a likely X-ray binary, background active galactic nucleus, supernova remnant, or foreground star.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/763/128
- Title:
- Chandra Local Volume Survey: NGC 404 sources
- Short Name:
- J/ApJ/763/128
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a comprehensive X-ray point-source catalog of NGC 404 obtained as part of the Chandra Local Volume Survey. A new 97ks Chandra ACIS-S observation of NGC 404 was combined with archival observations for a total exposure of ~123ks. Our survey yields 74 highly significant X-ray point sources and is sensitive to a limiting unabsorbed luminosity of ~6x10^35^erg/s in the 0.35-8keV band. To constrain the nature of each X-ray source, cross-correlations with multi-wavelength data were generated. We searched overlapping Hubble Space Telescope observations for optical counterparts to our X-ray detections, but find only two X-ray sources with candidate optical counterparts. We find 21 likely low-mass X-ray binaries (LMXBs), although this number is a lower limit due to the difficulties in separating LMXBs from background active galactic nuclei. The X-ray luminosity functions (XLFs) in both the soft and hard energy bands are presented. The XLFs in the soft band (0.5-2keV) and the hard band (2-8keV) have a limiting luminosity at the 90% completeness limit of 10^35^erg/s and 10^36^erg/s, respectively, significantly lower than previous X-ray studies of NGC 404. We find the XLFs to be consistent with those of other X-ray populations dominated by LMXBs. However, the number of luminous (>10^37^erg/s) X-ray sources per unit stellar mass in NGC 404 is lower than is observed for other galaxies. The relative lack of luminous XRBs may be due to a population of LMXBs with main-sequence companions formed during an epoch of elevated star formation ~0.5Gyr ago.
- ID:
- ivo://CDS.VizieR/J/ApJ/879/112
- Title:
- Chandra multi-epoch study of the spiral gal. NGC7331
- Short Name:
- J/ApJ/879/112
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- X-ray point sources in galaxies are dominated by X-ray binaries (XRBs) that are variables or transients, and whether their variability would alter the X-ray luminosity functions (XLFs) is still in debate. Here we report on NGC 7331 as an example to test this with seven Chandra observations. Their detection limit is 7x10^37^erg/s in the energy range 0.3-8.0keV by assuming a power-law (PL) spectral model with a photon index of 1.7. We detected 55 X-ray sources. Thirteen of them are variables, of which three are transients, and some of the sources possess a bimodal feature in their luminosity-hardness ratio, which is often observed among XRBs. Nine more ultraluminous X-ray sources are found in comparison with previous studies and eight are likely to be low-mass or high-mass XRBs. Twenty-one optical counterpart candidates are found based on the Hubble Space Telescope images, but we cannot rule out the possibility of positional coincidence. The spectral analysis of SN 2014C shows a trend of increasing soft photons and decreasing hydrogen column densities as its outer shell expands. We fit the seven incompleteness-corrected XLFs to both a PL and a PL with an exponential cut-off (PLC) model using the Bayesian method, which is used for the first time in XLF fitting. The hierarchical PLC model can describe the XLF of NGC 7331 best with a slope of ~0.5 and a luminosity cut-off around 8x10^38^erg/s. This study proves that multi-epoch observations decrease the deviation due to the variable luminous sources in XLFs.
- ID:
- ivo://CDS.VizieR/J/ApJ/836/50
- Title:
- Chandra observations of IC 10
- Short Name:
- J/ApJ/836/50
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have monitored the Cassiopeia dwarf galaxy (IC 10) in a series of 10 Chandra ACIS-S observations to capture its variable and transient X-ray source population, which is expected to be dominated by High Mass X-ray Binaries (HMXBs). We present a sample of 21 X-ray sources that are variable between observations at the 3{sigma} level, from a catalog of 110 unique point sources. We find four transients (flux variability ratio greater than 10) and a further eight objects with ratios >5. The observations span the years 2003-2010 and reach a limiting luminosity of >10^35^erg/s, providing sensitivity to X-ray binaries in IC 10 as well as flare stars in the foreground Milky Way. The nature of the variable sources is investigated from light curves, X-ray spectra, energy quantiles, and optical counterparts. The purpose of this study is to discover the composition of the X-ray binary population in a young starburst environment. IC 10 provides a sharp contrast in stellar population age (<10My) when compared to the Magellanic Clouds (40-200My) where most of the known HMXBs reside. We find 10 strong HMXB candidates, 2 probable background Active Galactic Nuclei, 4 foreground flare-stars or active binaries, and 5 not yet classifiable sources. Complete classification of the sample requires optical spectroscopy for radial velocity analysis and deeper X-ray observations to obtain higher S/N spectra and search for pulsations. A catalog and supporting data set are provided.
- ID:
- ivo://CDS.VizieR/J/ApJ/695/1094
- Title:
- Chandra observations of NGC 4636
- Short Name:
- J/ApJ/695/1094
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the X-ray point-source population in the nearby Virgo elliptical galaxy NGC 4636 from three Chandra X-ray observations. These observations, totaling ~193ks after time filtering, were taken with the Advanced CCD Imaging Camera (ACIS) over a three-year period. Using a wavelet decomposition detection algorithm, we detect 318 individual point sources. For our analysis, we use a subset of 277 detections with >=net 10 counts (a limiting luminosity of approximately 1.2x10^37^erg/s in the 0.5-2keV band, outside the central 1.5' bright galaxy core). We present a radial distribution of the point sources. Between 1.5' and 6' from the center, 25% of our sources are likely to be background sources (active galactic nuclei (AGNs)) and 75% are low-mass X-ray binaries (LMXBs) within the galaxy, while at radial distances greater than 6', background sources (AGN) will dominate the point sources. We find 77 matches between X-ray point sources and globular cluster (GC) candidates found in deep optical images of NGC 4636. In the annulus from 1.5' to 6' of the galaxy center, 48 of the 129 X-ray point sources (37%) with >=10 net counts are matched with GC candidates. Since we expect 25% of these sources to be background AGN, the percentage matched with GCs could be as high as 50%. Of these matched sources, we find that ~70% are associated with the redder GC candidates, those that are thought to have near-solar metal abundance.
- ID:
- ivo://CDS.VizieR/J/ApJS/243/3
- Title:
- Chandra observations of SINGS galaxies
- Short Name:
- J/ApJS/243/3
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new Chandra constraints on the X-ray luminosity functions (XLFs) of X-ray binary (XRB) populations, as well as their scaling relations, for a sample of 38 nearby galaxies (D=3.4-29Mpc). Our galaxy sample is drawn primarily from the Spitzer Infrared Nearby Galaxies Survey (SINGS) and contains a wealth of Chandra (5.8Ms total) and multiwavelength data, allowing for star formation rates (SFRs) and stellar masses (M_*_) to be measured on subgalactic scales. We divided the 2478 X-ray-detected sources into 21 subsamples in bins of specific SFR (sSFR=SFR/M_*_) and constructed XLFs. To model the XLF dependence on sSFR, we fit a global XLF model, containing contributions from high-mass XRBs (HMXBs), low-mass XRBs (LMXBs), and background sources from the cosmic X-ray background that respectively scale with SFR, M_*_, and sky area. We find an HMXB XLF that is more complex in shape than previously reported and an LMXB XLF that likely varies with sSFR, potentially due to an age dependence. When applying our global model to XLF data for each individual galaxy, we discover a few galaxy XLFs that significantly deviate from our model beyond statistical scatter. Most notably, relatively low-metallicity galaxies have an excess of HMXBs above ~10^38^erg/s, and elliptical galaxies that have relatively rich populations of globular clusters (GCs) show excesses of LMXBs compared to the global model. Additional modeling of how the XRB XLF depends on stellar age, metallicity, and GC specific frequency is required to sufficiently characterize the XLFs of galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/766/88
- Title:
- Chandra observations of X-ray binaries in Cen A
- Short Name:
- J/ApJ/766/88
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a spectral investigation of X-ray binaries (XBs) in NGC 5128 (Cen A), using six 100ks Chandra observations taken over two months in 2007. We divide our sample into thermally and non-thermally dominated states based on the behavior of the fitted absorption column N_H_, and present the spectral parameters of sources with L_x_>~2x10^37^erg/s. The majority of sources are consistent with being neutron star low-mass X-ray binaries (NS LMXBs) and we identify three transient black hole (BH) LMXB candidates coincident with the dust lane, which is the remnant of a small late-type galaxy. Our results also provide tentative support for the apparent "gap" in the mass distribution of compact objects between ~2-5M_{sun}_. We propose that BH LMXBs are preferentially found in the dust lane, and suggest this is because of the younger stellar population. The majority (~70%-80%) of potential Roche lobe filling donors in the Cen A halo are >~12Gyr old, while BH LMXBs require donors >~1M_{sun}_ to produce the observed peak luminosities. This requirement for more massive donors may also explain recent results that claim a steepening of the X-ray luminosity function with age at L_x_>=5x10^38^erg/s for the XB population of early-type galaxies; for older stellar populations, there are fewer stars >~1M_{sun}_, which are required to form the more luminous sources.
- ID:
- ivo://CDS.VizieR/J/ApJ/770/98
- Title:
- Chandra study of NGC 6791
- Short Name:
- J/ApJ/770/98
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first X-ray study of NGC 6791, one of the oldest open clusters known (8Gyr). Our Chandra observation is aimed at uncovering the population of close interacting binaries down to L_X_~1x10^30^erg/s (0.3-7keV). We detect 86 sources within 8' of the cluster center, including 59 inside the half-mass radius. We identify 20 sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy, we confirm the nature of one CV. We discover one new, X-ray variable candidate CV with Balmer and He II emission lines in its optical spectrum; this is the first X-ray-selected CV in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC 6791 are primordial. We compare the X-ray properties of NGC 6791 with those of a few old open (NGC 6819, M67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the number of ABs brighter than 1x10^30^erg/s normalized by cluster mass is lower in NGC 6791 than in M 67 by a factor ~3-7. CVs, ABs, and sub-subgiants brighter than 1x10^30^erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e., X-ray-emitting) binaries.
- ID:
- ivo://CDS.VizieR/J/ApJ/602/231
- Title:
- Chandra X-ray point sources in nearby galaxies
- Short Name:
- J/ApJ/602/231
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3-8.0keV) luminosity L_XP_ is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies and is well correlated with the B-band and K-band luminosities of elliptical galaxies. This suggests an intimate connection between L_XP_ and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star formation rate SFR.
- ID:
- ivo://CDS.VizieR/J/ApJ/591/138
- Title:
- Chandra X-ray sources of NGC 1068
- Short Name:
- J/ApJ/591/138
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analysis of the compact X-ray source population in the Seyfert 2 galaxy NGC 1068, imaged with a 50ks Chandra observation. We find a total of 84 compact sources on the S3 chip, of which 66 are located within the 25.0B-mag/arcsec^2^ isophote of the galactic disk of NGC 1068. Spectra have been obtained for the 21 sources with at least 50 counts and modeled with both multicolor disk blackbody and power-law models.
