Variability-selected active galactic nuclei from supernova search in the Chandra deep field south

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically-identified and observed samples, especially when rare subclasses are considered, or when subsets are isolated to search for evolution or correlations between wavebands. We describe initial results of a new program aimed to ultimately yield ∼10 4 fully characterized X-ray source identifications-a sample -2about an order of magnitude larger than earlier efforts. The technique is detailed, and employs X-ray data from the ROSAT All-Sky Survey (RASS), and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of f x /f opt for various classes of X-ray emitters at typical RASS fluxes of ∼ 10 −13 erg cm −2 s −1 . SDSS photometric parameters for optical morphology, magnitude, colors, plus FIRST radio information, serve as proxies for object class.

Symposium - International Astronomical Union, 1999

We present low resolution spectrophotometric and imaging ISO observations of a sample of 58…

Monthly Notices of the Royal Astronomical Society, 2010

We investigate the far-infrared (FIR) properties of X-ray sources detected in the Chandra Deep Field-South (CDF-S) survey using the ultradeep 70 and 24 μm Spitzer observations taken in this field. Since only 30 (i.e. ≈10 per cent) of the 266 X-ray sources in the region of the 70 μm observations are detected at 70 μm, we rely on stacking analyses of the 70 μm data to characterize the average 70 μm properties of the X-ray sources as a function of redshift, X-ray luminosity and X-ray absorption. Using Spitzer-IRS data of the Swift-Burst Alert Telescope (BAT) sample of z ≈ 0 active galactic nuclei (AGNs), we show that the 70/24 μm flux ratio can distinguish between AGN-dominated and starburst-dominated systems out to z ≈ 1.5. Among the X-ray sources detected at 70 μm, we note a large scatter in the observed 70/24 μm flux ratios, spanning almost a factor of 10 at similar redshifts, irrespective of object classification, suggesting a range of AGN:starburst ratios. From stacking analyses we find that the average observed 70/24 μm flux ratios of AGNs out to an average redshift of 1.5 are similar to z ≈ 0 AGNs with similar X-ray luminosities (L X = 10 42−44 erg s −1) and absorbing column densities (N H ≤ 10 23 cm −2). Furthermore, both high-redshift and z ≈ 0 AGNs follow the same tendency towards warmer 70/24 μm colours with increasing X-ray luminosity (L X). From analyses of the Swift-BAT sample of z ≈ 0 AGNs, we note that the 70 μm flux can be used to determine the IR (8-1000 μm) luminosities of high-redshift AGNs. We use this information to show that L X = 10 42−43 erg s −1 AGNs at high redshifts (z = 1-2) have IR to X-ray luminosity ratios (L IR /L X) that are, on average, 4.7 +10.2 −2.0 and 12.7 +7.1 −2.6 times higher than AGNs with similar X-ray luminosities at z = 0.5-1 and ≈0, respectively. By comparison, we find that the L IR /L X ratios of L X = 10 43−44 erg s −1 AGNs remain largely unchanged across this same redshift interval. We explore the consequences that these results may have on the identification of distant, potentially Compton-thick AGNs using L IR /L X ratios. In addition, we discuss possible scenarios for the observed increase in the L IR /L X ratio with redshift, including changes in the dust covering factor of AGNs and/or the star formation rates of their host galaxies. Finally, we show how deep observations to be undertaken by the Herschel Space Observatory will enable us to discriminate between these proposed scenarios and also identify Compton-thick AGNs at high redshifts.

2021

In this study, we use the Swift/BAT AGN sample, which has received extensive multiwavelength follow-up analysis as a result of the BAT AGN Spectroscopic Survey (BASS), to develop a diagnostic for nuclear obscuration by examining the relationship between the line-of-sight column densities (NH), the 2–10 keV-to-12μm luminosity ratio, and WISE mid-infrared colors. We demonstrate that heavily obscured AGNs tend to exhibit both preferentially “redder” mid-infrared colors and lower values of LX,Obs./L12μm than less obscured AGNs, and we derive expressions relating NH to the LX,Obs./L12μm and L22μm/L4.6μm luminosity ratios as well as develop diagnostic criteria using these ratios. Our diagnostic regions yield samples that are & 80% complete and & 60% pure for AGNs with log(NH/cm) ≥ 24, as well as & 85% pure for AGNs with log(NH/cm) & 23.5. We find that these diagnostics cannot be used to differentiate between optically star forming galaxies and active galaxies. Further, mid-IR contribution...

2010

\Ve compare mid-IR emission-lines properties, from high-resolution Spitzer IRS spectra of a statistically-complete hard X-ray (14 195 keV) selected sample of nearby (z < 0.05) AGN detected by the Burst Alert Telescope (BAT) aboard Swift. The luminosity distribution for the mid-infrared emission-lines, [0 IV] 25.89 /-Lm, [Ne II] 12.81 /-Lm, [Ne III] 15.56 /-Lm and [Ne V] 14.32 /-Lm, and hard Xray continuum show no differences between Seyfert 1 and Seyfert 2 populations, although six newly discovered BAT AGNs are shown to be under-luminous in [0 IV], most likely the result of dust extinction in the host galaxy. The overall tightness of the mid-infrared correlations and BAT luminosities suggests that the emission lines primarily arise in gas ionized by the AGN. We also compared the mid-IR emission-lines in the BAT AGNs with those from published studies of star-forming galaxies and LINERs. We found that the BAT AGN fall into a distinctive region when comparing the [Ne III]/[Ne II] and the [0 IVl![Ne III] quantities. From this we found that sources that have been previously classified in the mid-infrared/optical as AGN have smaller emission line ratios than those found for the BAT AGNs, suggesting that, in our X-ray selected sample, the AGN represents the main contribution to the observed line emission. Overall, we present a different set of emission line diagnostics to distinguish between AGN and star forming galaxies that can be used as a tool to find new AGN.

2012

We utilize the W2R sample to quantify biases and incompleteness in the VCV catalog. We find it is highly complete for bright (J < 14), northern AGN, but the completeness drops below 50% for fainter, southern samples and for sources near the Galactic plane. This approach also led to the spectroscopic identification of 10 new AGN in the Kepler field, more than doubling the number of AGN being monitored by Kepler. This has identified ~1 bright AGN every 10 square degrees, permitting construction of AGN samples in any sufficiently large region of sky.

2009

From stacking analysis we find that both high redshift and z~0 AGNs follow the same tendency toward warmer 70um/24um colours with increasing X-ray luminosity (L_X). We also show that the 70um flux can be used to determine the infrared (8-1000um) luminosities of high redshift AGNs. We use this information to show that L_X=10^{42-43} erg/s AGNs at high redshifts (z=1-2) have infrared to X-ray luminosity ratios (hereafter, L_IR/L_X) that are, on average, 4.7_{-2.0}^{+10.2} and 12.7^{+7.1}_{-2.6} times higher than AGNs with similar X-ray luminosities at z=0.5-1 and z~0, respectively. By comparison, we find that the L_IR/L_X ratios of L_X=10^{43-44} erg/s AGNs remain largely unchanged across this same redshift interval. We explore the consequences that these results may have on the identification of distant, potentially Compton thick AGNs using L_IR/L_X ratios. We discuss possible scenarios for the observed increase in the L_IR/L_X ratio with redshift, including changes in the dust covering factor of AGNs and/or the star formation rates of their host galaxies. Finally, we show how deep observations to be undertaken by the Herschel Space Observatory will enable us to discriminate between these proposed scenarios and also identify Compton-thick AGNs at high redshifts.

Revista Mexicana De Astronomia Y Astrofisica, 2008

Resumen en: AGN are identified in deep HST surveys via a number of selection techniques and wavelengths. This work compares the samples of AGN selected via optical ...

Monthly Notices of the Royal Astronomical Society, 2013

To find the spectral properties of AGNs in optical spectral band (around the Hb line) we constructed an Fe II template, that is covering Fe II emission from 4100 A to 5600 A. Using the new Fe II template we explored spectral properties of 302 type 1 AGNs. The most interesting results we found is the correlation of the Hb Full Width at Half Maximum (FWHM) and luminosity for a subsample of type 1 AGNs where the ratio of narrow lines indicates a significant starburst contribution.