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Register a new userThe Chandra Local Volume Survey: The X-ray Point Source Population of NGC 404
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We present a comprehensive X-ray point source catalog of NGC 404 obtained as part of the Chandra Local Volume Survey. A new, 97 ks Chandra ACIS-S observation of NGC 404 was combined with archival observations for a total exposure of ~123 ks. Our survey yields 74 highly significant X-ray point sources and is sensitive to a limiting unabsorbed luminosity of ~6x10^35 erg s^-1 in the 0.35-8 keV band. To constrain the nature of each X-ray source, cross-correlations with multi-wavelength data were generated. We searched overlapping HST 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 AGN. The X-ray luminosity functions (XLFs) in both the soft and hard energy bands are presented. The XLFs in the soft band (0.5-2 keV) and the hard band (2-8 keV) have a limiting luminosity at the 90% completeness limit of 10^35 erg s^-1 and 10^36 erg s^-1, 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^-1) 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 ~0.5 Gyr ago.
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We present the X-ray point-source catalog produced from the Chandra Advanced CCD Imaging Spectrometer (ACIS-I) observations of the combined sim3.2 deg2 DEEP2 (XDEEP2) survey fields, which consist of four ~0.7-1.1 deg2 fields. The combined total exposures across all four XDEEP2 fields range from ~10ks-1.1Ms. We detect X-ray point-sources in both the individual ACIS-I observations and the overlapping regions in the merged (stacked) images. We find a total of 2976 unique X-ray sources within the survey area with an expected false-source contamination of ~30 sources (~1%). We present the combined logN-logS distribution of sources detected across the XDEEP2 survey fields and find good agreement with the Extended Chandra Deep Field and Chandra-COSMOS fields to f_{X,0.5-2keV}sim2x10^{-16} erg/cm^2/s. Given the large survey area of XDEEP2, we additionally place relatively strong constraints on the logN-logS distribution at high fluxes (f_{X,0.5-2keV}sim3x10^{-14} erg/cm^2/s), and find a small systematic offset (a factor ~1.5) towards lower source numbers in this regime, when compared to smaller area surveys. The number counts observed in XDEEP2 are in close agreement with those predicted by X-ray background synthesis models. Additionally, we present a Bayesian-style method for associating the X-ray sources with optical photometric counterparts in the DEEP2 catalog (complete to R_AB < 25.2) and find that 2126 (~71.4pm2.8%) of the 2976 X-ray sources presented here have a secure optical counterpart with a <6% contamination fraction. We provide the DEEP2 optical source properties (e.g., magnitude, redshift) as part of the X-ray-optical counterpart catalog.
We present the results of a 100 ks {it Chandra} observation of the NGC 404 nuclear region. The long exposure and excellent spatial resolution of {it Chandra} has enabled us to critically examine the nuclear environment of NGC 404, which is known to host a nuclear star cluster and potentially an intermediate-mass black hole (on the order of a few times $10^5$ Msun). We find two distinct X-ray sources: a hard, central point source coincident with the optical and radio centers of the galaxy, and a soft extended region that is coincident with areas of high H$alpha$ emission and likely recent star formation. When we fit the 0.3-8 keV spectra of each region separately, we find the hard nuclear point source to be dominated by a power law (PL = 1.88), while the soft off-nuclear region is best fit by a thermal plasma model ($kT$ = 0.67 keV). We therefore find evidence for both a power law component and hot gas in the nuclear region of NGC 404. We estimate the 2-10 keV luminosity to be 1.3$^{+0.8}_{-0.5}times10^{37}$ erg s$^{-1}$. A low-level of diffuse X-ray emission is detected out to $sim$15as ($sim$0.2 kpc) from the nucleus. We compare our results to the observed relationships between power law photon index and Eddington ratio for both X-ray binaries and low luminosity active galaxies and find NGC 404 to be consistent with other low luminosity active galaxies. We therefore favor the conclusion that NGC 404 harbors an intermediate-mass black hole accreting at a very low level.
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centered at Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning (l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of observations for each line of sight in r, i and H{alpha} filters. It is complete down to r = 20.2 and i = 19.2 mag; the mean 5{sigma} depth is r = 22.5 and i = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with the 1640 unique X-ray sources detected in Chandra observations of the GBS area, and find candidate optical counterparts to 1480 X-ray sources. We use a false alarm probability analysis to estimate the contamination by interlopers, and expect ~ 10 per cent of optical counterparts to be chance alignments. To determine the most likely counterpart for each X-ray source, we compute the likelihood ratio for all optical sources within the 4{sigma} X-ray error circle. This analysis yields 1480 potential counterparts (~ 90 per cent of the sample). 584 counterparts have saturated photometry (r<17, i<16), indicating these objects are likely foreground sources and the real counterparts. 171 candidate counterparts are detected only in the i-band. These sources are good qLMXB and CV candidates as they are X-ray bright and likely located in the Bulge.
We present the X-ray point source population of NGC 7457 based on 124 ks of Chandra observations. Previous deep Chandra observations of low mass X-ray binaries (LMXBs) in early-type galaxies have typically targeted the large populations of massive galaxies. NGC 7457 is a nearby, early-type galaxy with a stellar luminosity of $1.7times10^{10} L_{Kodot}$, allowing us to investigate the populations in a relatively low mass galaxy. We classify the detected X-ray sources into field LMXBs, globular cluster LMXBs, and background AGN based on identifying optical counterparts in new HST/ACS images. We detect 10 field LMXBs within the $r_{ext}$ ellipse of NGC 7457 (with semi-major axis $sim$ 9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with $L_{x}>2times10^{37}erg/s$ per stellar luminosity is consistent with that observed in more massive galaxies, $sim 7$ per $10^{10} L_{Kodot}$. We detect a small globular cluster population in these HST data and show that its colour distribution is likely bimodal and that its specific frequency is similar to that of other early type galaxies. However, no X-ray emission is detected from any of these clusters. Using published data for other galaxies, we show that this non-detection is consistent with the small stellar mass of these clusters. We estimate that 0.11 (and 0.03) LMXBs are expected per $10^{6}M_{odot}$ in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per $10^{10} L_{Kodot}$, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected with $L_{x}$ varying from $2.8-6.8times10^{38}erg/s$. Combining this $L_{x}$ with a published dynamical mass estimate for the central SMBH in NGC 7457, we find that $L_{x}/L_{Edd}$ varies from $0.5-1.3times10^{-6}$.
The Extended Chandra Deep Field-South (ECDFS) survey consists of 4 Chandra ACIS-I pointings and covers $approx$ 1100 square arcminutes ($approx$ 0.3 deg$^2$) centered on the original CDF-S field to a depth of approximately 228 ks. This is the largest Chandra survey ever conducted at such depth, and only one XMM-Newton survey reaches a lower flux limit in the hard 2.0--8.0 keV band. We detect 651 unique sources -- 587 using a conservative source detection threshold and 64 using a lower source detection threshold. These are presented as two separate catalogs. Of the 651 total sources, 561 are detected in the full 0.5--8.0 keV band, 529 in the soft 0.5--2.0 keV band, and 335 in the hard 2.0--8.0 keV band. For point sources near the aim point, the limiting fluxes are approximately $1.7 times 10^{-16}$ $rm{erg cm^{-2} s^{-1}}$ and $3.9 times 10^{-16}$ $rm{erg cm^{-2} s^{-1}}$ in the 0.5--2.0 keV and 2.0--8.0 keV bands, respectively. Using simulations, we determine the catalog completeness as a function of flux and assess uncertainties in the derived fluxes due to incomplete spectral information. We present the differential and cumulative flux distributions, which are in good agreement with the number counts from previous deep X-ray surveys and with the predictions from an AGN population synthesis model that can explain the X-ray background. In general, fainter sources have harder X-ray spectra, consistent with the hypothesis that these sources are mainly obscured AGN.
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