arXiv subjects

W. N. Brandt

Publications and source records attributed to W. N. Brandt.

At least 253 records · Page 14Linked to original sources

arXiv2016

A New Population of Compton-Thick AGN Identified Using the Spectral Curvature Above 10 keV

We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick AGN in low-quality Swift BAT X-ray data. Using NuSTAR, we observe nine high SC-selected AGN. We find that high-sensitivity spectra show the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (NH~5-8x10^23 cm^-2). We find the SC_bat and SC_nustar measurements are consistent, suggesting this technique can be applied to future telescopes. We tested the SC method on well-known Compton-thick AGN and find it is much more effective than broad band ratios (e.g. 100% using SC vs. 20% using 8-24/3-8 keV). Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the OIII to X-ray emission ratio (F_OIII/F_2-10 keV>1) and WISE colors do not identify most of them as AGN. Based on this small sample, we find that a higher fraction of these AGN are in the final merger stage than typical BAT AGN. Additionally, these nine obscured AGN have, on average, four times higher accretion rates than other BAT-detected AGN (Edd ratio=0.068 compared to 0.016). The robustness of SC at identifying Compton-thick AGN implies a higher fraction of nearby AGN may be Compton-thick (~22%) and the sum of black hole growth in Compton-thick AGN (Eddington ratio times population percentage), is nearly as large as mildly obscured and unobscured AGN.

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arXiv2016

CIV Broad Absorption Line Acceleration in Sloan Digital Sky Survey Quasars

We present results from the largest systematic investigation of broad absorption line (BAL) acceleration to date. We use spectra of 140 quasars from three Sloan Digital Sky Survey programs to search for global velocity offsets in BALs over timescales of ~2.5-5.5 years in the quasar rest frame. We carefully select acceleration candidates by requiring monolithic velocity shifts over the entire BAL trough, avoiding BALs with velocity shifts that might be caused by profile variability. The CIV BALs of two quasars show velocity shifts consistent with the expected signatures of BAL acceleration, and the BAL of one quasar shows a velocity-shift signature of deceleration. In our two acceleration candidates, we see evidence that the magnitude of the acceleration is not constant over time; the magnitudes of the change in acceleration for both acceleration candidates are difficult to produce with a standard disk-wind model or via geometric projection effects. We measure upper limits to acceleration and deceleration for 76 additional BAL troughs and find that the majority of BALs are stable to within about 3% of their mean velocities. The lack of widespread acceleration/deceleration could indicate that the gas producing most BALs is located at large radii from the central black hole and/or is not currently strongly interacting with ambient material within the host galaxy along our line of sight.

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arXiv2016

The Evolution of Normal Galaxy X-ray Emission Through Cosmic History: Constraints from the 6 Ms Chandra Deep Field-South

We present measurements of the evolution of normal-galaxy X-ray emission from $z \approx$ 0-7 using local galaxies and galaxy samples in the 6 Ms Chandra Deep Field-South (CDF-S) survey. The majority of the CDF-S galaxies are observed at rest-frame energies above 2 keV, where the emission is expected to be dominated by X-ray binary (XRB) populations; however, hot gas is expected to provide small contributions to the observed- frame < 1 keV emission at $z < 1$. We show that a single scaling relation between X-ray luminosity ($L_{\rm X}$) and star-formation rate (SFR) is insufficient for characterizing the average X-ray emission at all redshifts. We establish that scaling relations involving not only SFR, but also stellar mass ($M_\star$) and redshift, provide significantly improved characterizations of the average X-ray emission from normal galaxy populations at $z \approx$ 0-7. We further provide the first empirical constraints on the redshift evolution of X-ray emission from both low-mass XRB (LMXB) and high-mass XRB (HMXB) populations and their scalings with $M_\star$ and SFR, respectively. We find $L_{\rm 2-10 keV}({\rm LMXB})/M_\star \propto (1 + z)^{2-3}$ and $L_{\rm 2-10 keV}({\rm HMXB})$/SFR $\propto (1 + z)$, and show that these relations are consistent with XRB population-synthesis model predictions, which attribute the increase in LMXB and HMXB scaling relations with redshift as being due to declining host galaxy stellar ages and metallicities, respectively. We discuss how emission from XRBs could provide an important source of heating to the intergalactic medium in the early Universe, exceeding that of active galactic nuclei.

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arXiv2016

NuSTAR reveals the extreme properties of the super-Eddington accreting SMBH in PG 1247+267

PG1247+267 is one of the most luminous known quasars at $z\sim2$ and is a strongly super-Eddington accreting SMBH candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad band covered by the new NuSTAR and the archival XMM-Newton data. Several measurements are in agreement with the super-Eddington scenario for PG1247+267: the soft power law ($Γ=2.3\pm0.1$); the weak ionized Fe emission line and a hint of the presence of outflowing ionized gas surrounding the SMBH. The presence of an extreme reflection component is instead at odds with the high accretion rate proposed for this quasar. This can be explained with three different scenarios; all of them are in good agreement with the existing data, but imply very different conclusions: i) a variable primary power law observed in a low state, superimposed on a reflection component echoing a past, higher flux state; ii) a power law continuum obscured by an ionized, Compton thick, partial covering absorber; and iii) a relativistic disk reflector in a lamp-post geometry, with low coronal height and high BH spin. The first model is able to explain the high reflection component in terms of variability. The second does not require any reflection to reproduce the hard emission, while a rather low high-energy cutoff of $\sim100$ keV is detected for the first time in such a high redshift source. The third model require a face-on geometry, which may affect the SMBH mass and Eddington ratio measurements. Deeper X-ray broad-band data are required in order to distinguish between these possibilities.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project. IV. Anomalous behavior of the broad ultraviolet emission lines in NGC 5548

During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert~1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far-UV continuum and broad emission-line variations decorrelated for ~60 to 70 days, starting ~75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterised by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission-line flux deficits occurred for the high-ionization collisionally excited lines, C IV and Si IV(+O IV]), and also He II(+O III]), while the anomaly in Ly-alpha was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with E_{\rm ph} > 54 eV, relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon BLR clouds by a moving veil of material lying between the inner accretion disk and inner BLR, perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies > 54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

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arXiv2016

SpIES: The Spitzer IRAC Equatorial Survey

We describe the first data release from the Spitzer-IRAC Equatorial Survey (SpIES); a large-area survey of 115 deg^2 in the Equatorial SDSS Stripe 82 field using Spitzer during its 'warm' mission phase. SpIES was designed to probe sufficient volume to perform measurements of quasar clustering and the luminosity function at z > 3 to test various models for "feedback" from active galactic nuclei (AGN). Additionally, the wide range of available multi-wavelength, multi-epoch ancillary data enables SpIES to identify both high-redshift (z > 5) quasars as well as obscured quasars missed by optical surveys. SpIES achieves 5σ depths of 6.13 μJy (21.93 AB magnitude) and 5.75 μJy (22.0 AB magnitude) at 3.6 and 4.5 microns, respectively - depths significantly fainter than WISE. We show that the SpIES survey recovers a much larger fraction of spectroscopically-confirmed quasars (98%) in Stripe 82 than are recovered by WISE (55%). This depth is especially powerful at high-redshift (z > 3.5), where SpIES recovers 94% of confirmed quasars, whereas WISE only recovers 25%. Here we define the SpIES survey parameters and describe the image processing, source extraction, and catalog production methods used to analyze the SpIES data. In addition to this survey paper, we release 234 images created by the SpIES team and three detection catalogs: a 3.6 μm-only detection catalog containing 6.1 million sources, a 4.5 μm-only detection catalog containing 6.5 million sources, and a dual-band detection catalog containing 5.4 million sources.

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arXiv2016

Discovery of extreme [OIII]5007A outflows in high-redshift red quasars

Black hole feedback is now a standard component of galaxy formation models. These models predict that the impact of black hole activity on its host galaxy likely peaked at z=2-3, the epoch of strongest star formation activity and black hole accretion activity in the Universe. We used XShooter on the Very Large Telescope to measure rest-frame optical spectra of four z~2.5 extremely red quasars with infrared luminosities ~10^47 erg/sec. We present the discovery of very broad (full width at half max= 2600-5000 km/sec), strongly blue-shifted (by up to 1500 km/sec) [OIII]5007A emission lines in these objects. In a large sample of obscured and red quasars, [OIII] kinematics are positively correlated with infrared luminosity, and the four objects in our sample are on the extreme end both in [OIII] kinematics and infrared luminosity. We estimate that ~3% of the bolometric luminosity in these objects is being converted into the kinetic power of the observed wind. Photo-ionization estimates suggest that the [OIII] emission might be extended on a few kpc scales, which would suggest that the extreme outflow is affecting the entire host galaxy of the quasar. These sources may be the signposts of the most extreme form of quasar feedback at the peak epoch of galaxy formation, and may represent an active "blow-out" phase of quasar evolution.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project. III. Optical Continuum Emission and Broad-Band Time Delays in NGC 5548

We present ground-based optical photometric monitoring data for NGC 5548, part of an extended multi-wavelength reverberation mapping campaign. The light curves have nearly daily cadence from 2014 January to July in nine filters (\emph{BVRI} and \emph{ugriz}). Combined with ultraviolet data from the \emph{Hubble Space Telescope} and \emph{Swift}, we confirm significant time delays between the continuum bands as a function of wavelength, extending the wavelength coverage from 1158\,Å to the $z$ band ($\sim\!9160$\,Å). We find that the lags at wavelengths longer than the {\it V} band are equal to or greater than the lags of high-ionization-state emission lines (such as He\,{\sc ii}\,$λ1640$ and $λ4686$), suggesting that the continuum-emitting source is of a physical size comparable to the inner broad-line region (BLR). The trend of lag with wavelength is broadly consistent with the prediction for continuum reprocessing by an accretion disk with $τ\propto λ^{4/3}$. However, the lags also imply a disk radius that is 3 times larger than the prediction from standard thin-disk theory, assuming that the bolometric luminosity is 10\% of the Eddington luminosity ($L = 0.1L_{\rm Edd}$). Using optical spectra from the Large Binocular Telescope, we estimate the bias of the interband continuum lags due to BLR emission observed in the filters. We find that the bias for filters with high levels of BLR contamination ($\sim\! 20\%$) can be important for the shortest continuum lags, and likely has a significant impact on the {\it u} and {\it U} bands owing to Balmer continuum emission.

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arXiv2016

The 2 Ms Chandra Deep Field-North Survey and the 250 ks Extended Chandra Deep Field-South Survey: Improved Point-Source Catalogs

We present improved point-source catalogs for the 2 Ms Chandra Deep Field-North (CDF-N) and the 250 ks Extended Chandra Deep Field-South (E-CDF-S), implementing a number of recent improvements in Chandra source-cataloging methodology. For the CDF-N/E-CDF-S, we provide a main catalog that contains 683/1003 X-ray sources detected with wavdetect at a false-positive probability threshold of $10^{-5}$ that also satisfy a binomial-probability source-selection criterion of $P<0.004$/$P<0.002$. Such an approach maximizes the number of reliable sources detected: a total of 196/275 main-catalog sources are new compared to the Alexander et al. (2003) CDF-N/Lehmer et al. (2005) E-CDF-S main catalogs. We also provide CDF-N/E-CDF-S supplementary catalogs that consist of 72/56 sources detected at the same wavdetect threshold and having $P$ of $0.004-0.1$/$0.002-0.1$ and $K_s\le22.9/K_s\le22.3$ mag counterparts. For all $\approx1800$ CDF-N and E-CDF-S sources, including the $\approx500$ newly detected ones (these being generally fainter and more obscured), we determine X-ray source positions utilizing centroid and matched-filter techniques; we also provide multiwavelength identifications, apparent magnitudes of counterparts, spectroscopic and/or photometric redshifts, basic source classifications, and estimates of observed AGN and galaxy source densities around respective field centers. Simulations show that both the CDF-N and E-CDF-S main catalogs are highly reliable and reasonably complete. Background and sensitivity analyses indicate that the on-axis mean flux limits reached represent a factor of $\approx1.5-2.0$ improvement over the previous CDF-N and E-CDF-S limits. We make our data products publicly available.

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arXiv2016

Peering through the Dust: NuSTAR Observations of Two FIRST-2MASS Red Quasars

Some reddened quasars appear to be transitional objects in the merger-induced black hole growth/galaxy evolution paradigm, where a heavily obscured nucleus starts to be unveiled by powerful quasar winds evacuating the surrounding cocoon of dust and gas. Hard X-ray observations are able to peer through this gas and dust, revealing the properties of circumnuclear obscuration. Here, we present NuSTAR and XMM-Newton/Chandra observations of FIRST-2MASS selected red quasars F2M 0830+3759 and F2M 1227+3214. We find that though F2M 0830+3759 is moderately obscured ($N_{\rm H,Z} = 2.1\pm0.2 \times10^{22}$ cm$^{-2}$) and F2M 1227+3214 is mildly absorbed ($N_{\rm H,Z} = 3.4^{+0.8}_{-0.7}\times10^{21}$ cm$^{-2}$) along the line-of-sight, heavier global obscuration may be present in both sources, with $N_{\rm H,S} = 3.7^{+4.1}_{-2.6} \times 10^{23}$ cm$^{-2}$ and $< 5.5\times10^{23}$ cm$^{-2}$, for F2M 0830+3759 and F2M 1227+3214, respectively. F2M 0830+3759 also has an excess of soft X-ray emission below 1 keV which is well accommodated by a model where 7% of the intrinsic AGN X-ray emission is scattered into the line-of-sight. While F2M 1227+3214 has a dust-to-gas ratio ($E(B-V)$/$N_{\rm H}$) consistent with the Galactic value, the $E(B-V)$/$N_{\rm H}$ value for F2M 0830+3759 is lower than the Galactic standard, consistent with the paradigm that the dust resides on galactic scales while the X-ray reprocessing gas originates within the dust-sublimation zone of the broad-line-region. The X-ray and 6.1$μ$m luminosities of these red quasars are consistent with the empirical relations derived for high-luminosity, unobscured quasars, extending the parameter space of obscured AGN previously observed by NuSTAR to higher luminosities.

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arXiv2016

NuSTAR observations of water megamaser AGN

Aims. Study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy properties of 14 nearby megamaser Active Galactic Nuclei observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus by combining NuSTAR spectral parameters with available physical quantities from VLBI mapping. Results. Most of the sources analyzed are heavily obscured, showing a column density in excess of $\sim 10^{23}$ cm$^{-2}$. In particular, $79\%$ are Compton-thick ($N_{\rm H} > 1.5 \times 10^{24}$ cm$^{-2}$). Using column densities measured by NuSTAR, with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, the torus dimensions can be predicted. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately connected to the inner part of the torus. It is probably made of a large number of molecular clouds connecting the torus and the outer part of the accretion disk, giving rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser emission and nuclear obscuration as a geometric effect.

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arXiv2016

IC 751: a new changing-look AGN discovered by NuSTAR

We present the results of five NuSTAR observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick ($N_{\rm\,H}\simeq 2\times 10^{24}\rm\,cm^{-2}$) to a Compton-thin ($N_{\rm\,H}\simeq 4\times 10^{23}\rm\,cm^{-2}$) state on timescales of $\lesssim 3$ months, which makes IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the line-of-sight column density at a $\sim2σ$ level are also found on a time-scale of $\sim 48$ hours ($ΔN_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}$). From the lack of spectral variability on timescales of $\sim 100$ ks we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region, and could therefore be related either to the external part of the broad-line region or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber ($N_{\rm\,H}\sim 3.8\times 10^{23}\rm\,cm^{-2}$) from that of the varying clouds [$N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}$], and to constrain that of the material responsible for the reprocessed X-ray radiation ($N_{\rm\,H} \sim 6 \times 10^{24}\rm\,cm^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2-10 and 10-50 keV band.

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arXiv2016

The Sloan Digital Sky Survey Reverberation Mapping Project: An Investigation of Biases in CIV Emission-Line Properties

We investigate the dependence on data quality of quasar properties measured from the CIV emission line region at high redshifts. Our measurements come from 32 epochs of Sloan Digital Sky Survey (SDSS) Reverberation Mapping Project spectroscopic observations of 482 z>1.46 quasars. We compare the differences between measurements made from the single-epoch and coadded spectra, focusing on the CIV emission line because of its importance for studies of high-redshift quasar demographics and physical properties, including black hole masses. In addition to statistical errors increasing (by factors of ~2-4), we find increasing systematic offsets with decreasing S/N. The systematic difference (measurement uncertainty) in our lowest S/N (<5) subsample between the single-epoch and coadded spectrum (i) CIV equivalent width is 17A (31A), (ii) centroid wavelength is <1A (2A), and fractional velocity widths, Delta V/V, characterized by (iii) the line dispersion, sigma_l, is 0.104 (0.12), and (iv) the mean absolute deviation (MAD) is 0.072 (0.11). These remain smaller than the 1-sigma measurement uncertainties for all subsamples considered. The MAD is found to be the most robust line-width characterization. Offsets in the CIV full-width at half maximum (FWHM) velocity width and the CIV profile characterized by FWHM/sigma_l are only smaller than the statistical uncertainties when S/N>10, although offsets in lower S/N spectra exceed the statistical uncertainties by only a factor of ~1.5. Characterizing the CIV line profile by the kurtosis is the least robust property investigated, as the median systematic coadded--single-epoch measurement differences are larger than the statistical uncertainties for all S/N subsamples.

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arXiv2016

The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Overview and Early Data

The Extended Baryon Oscillation Spectroscopic Survey (eBOSS) will conduct novel cosmological observations using the BOSS spectrograph at Apache Point Observatory. Observations will be simultaneous with the Time Domain Spectroscopic Survey (TDSS) designed for variability studies and the Spectroscopic Identification of eROSITA Sources (SPIDERS) program designed for studies of X-ray sources. eBOSS will use four different tracers to measure the distance-redshift relation with baryon acoustic oscillations (BAO). Using more than 250,000 new, spectroscopically confirmed luminous red galaxies at a median redshift z=0.72, we project that eBOSS will yield measurements of $d_A(z)$ to an accuracy of 1.2% and measurements of H(z) to 2.1% when combined with the z>0.6 sample of BOSS galaxies. With ~195,000 new emission line galaxy redshifts, we expect BAO measurements of $d_A(z)$ to an accuracy of 3.1% and H(z) to 4.7% at an effective redshift of z= 0.87. A sample of more than 500,000 spectroscopically-confirmed quasars will provide the first BAO distance measurements over the redshift range 0.9 2.1; these new data will enhance the precision of $d_A(z)$ and H(z) by a factor of 1.44 relative to BOSS. Furthermore, eBOSS will provide improved tests of General Relativity on cosmological scales through redshift-space distortion measurements, improved tests for non-Gaussianity in the primordial density field, and new constraints on the summed mass of all neutrino species. Here, we provide an overview of the cosmological goals, spectroscopic target sample, demonstration of spectral quality from early data, and projected cosmological constraints from eBOSS.

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arXiv2015

The Sloan Digital Sky Survey Reverberation Mapping Project: First Broad-line Hbeta and MgII Lags at z>~0.3 from six-Month Spectroscopy

Reverberation mapping (RM) measurements of broad-line region (BLR) lags in z>0.3 quasars are important for directly measuring black hole masses in these distant objects, but so far there have been limited attempts and success given the practical difficulties of RM in this regime. Here we report preliminary results of 15 BLR lag measurements from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project, a dedicated RM program with multi-object spectroscopy designed for RM over a wide redshift range. The lags are based on the 2014 spectroscopic light curves alone (32 epochs over 6 months) and focus on the Hbeta and MgII broad lines in the 100 lowest-redshift (z<0.8) quasars included in SDSS-RM; they represent a small subset of the lags that SDSS-RM (including 849 quasars to z~4.5) is expected to deliver. The reported preliminary lag measurements are for intermediate-luminosity quasars at 0.3<~z<0.8, including 9 Hbeta lags and 6 MgII lags, for the first time extending RM results to this redshift-luminosity regime and providing direct quasar black hole mass estimates over ~ half of cosmic time. The MgII lags also increase the number of known MgII lags by several-fold, and start to explore the utility of MgII for RM at high redshift. The location of these new lags at higher redshifts on the observed BLR size-luminosity relationship is statistically consistent with previous Hbeta results at z<0.3. However, an independent constraint on the relationship slope at z>0.3 is not yet possible due to the limitations in our current sample. Our results demonstrate the general feasibility and potential of multi-object RM for z>0.3 quasars.

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arXiv2015

Mid-infrared luminous quasars in the GOODS-Herschel fields: a large population of heavily-obscured, Compton-thick quasars at z~2

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars ($ν$L(6 micron)>6x10$^{44}$ erg/s) at redshift z~1-3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS-Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, N$_H$>1.5x10$^{24}$ cm$^{-2}$) quasars at the peak era of nuclear and star-formation activities. Despite being very bright in the mid-IR band, ~30% of these quasars are not detected in the extremely deep 2 Ms and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (~67%) are obscured by column densities N$_H$>10$^{22}$ cm$^{-2}$; this fraction reaches ~80% when including the X-ray undetected sources (9 out of 33), which are likely to be the most heavily-obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be ~24-48%, and their space density to be $Φ$=(6.7$\pm$2.2)x10$^{-6}$ Mpc$^{-3}$. From the investigation of the quasar host galaxies in terms of star-formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily-obscured quasars have, on average, more disturbed morphologies than the unobscured/moderately-obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is ~40%, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z~2.

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arXiv2015

The NuSTAR Extragalactic Surveys: Initial Results and Catalog from the Extended Chandra Deep Field South

We present initial results and the source catalog from the NuSTAR survey of the Extended Chandra Deep Field South (hereafter, ECDFS) - currently the deepest contiguous component of the NuSTAR extragalactic survey program. The survey covers the full ~30 arcmin x 30 arcmin area of this field to a maximum depth of ~360 ks (~220 ks when corrected for vignetting at 3-24 keV), reaching sensitivity limits of ~1.3 x 10^-14 erg/cm2/s (3-8 keV), ~3.4 x 10^-14 erg/cm2/s (8-24 keV) and ~3.0 x 10^-14 erg/cm2/s (3-24 keV). Fifty four (54) sources are detected over the full field, although five of these are found to lie below our significance threshold once contaminating flux from neighboring (i.e., blended) sources is taken into account. Of the remaining 49 that are significant, 19 are detected in the 8-24 keV band. The 8-24 keV to 3-8 keV band ratios of the twelve sources that are detected in both bands span the range 0.39-1.7, corresponding to a photon index range of Gamma ~ 0.5-2.3, with a median photon index of 1.70 +/- 0.52. The redshifts of the 49 sources in our main sample span the range z = 0.21-2.7, and their rest-frame 10-40 keV luminosities (derived from the observed 8-24 keV fluxes) span the range L(10-40 keV) ~ (0.7-300) x 10^43 erg/s, sampling below the "knee" of the X-ray luminosity function out to z ~ 0.8-1. Finally, we identify one NuSTAR source that has neither a Chandra nor an XMM-Newton counterpart, but that shows evidence of nuclear activity at infrared wavelengths, and thus may represent a genuine, new X-ray source detected by NuSTAR in the ECDFS.

astro-ph.HE↗

arXiv2015

The NuSTAR Extragalactic Surveys: Overview and Catalog from the COSMOS Field

To provide the census of the sources contributing to the X-ray background peak above 10 keV, NuSTAR is performing extragalactic surveys using a three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity and medium area tier, covering 1.7 deg2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ~3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9 and 6.4 x 10^-14 erg/cm2/s in the 3-24 keV, 3-8 keV and 8-24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 10^42 -10^45.5 erg/s luminosities and redshift z=0.04-2.5. Thirty two sources are detected in the 8-24 keV band with fluxes ~100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but four are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick AGN measured from the hardness ratio is between 13-20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z=0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 10^24 /cm2, implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.

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