Stacked X-ray Spectrum Research Articles

Statistical properties of cataclysmic variables in the local galactic disc: a joint analysis of Gaia and XMM–Newton data

ABSTRACT Cataclysmic variables (CVs) represent a key evolutionary phase of many low-mass stellar binaries and appear to collectively dominate the galactic X-ray background above ∼1 keV. Statistical properties of CVs, however, remain poorly understood, even in the solar neighbourhood. We aim to obtain an X-ray luminosity-complete sample of local CVs whose distances put them within three volumes. Here we present a pilot investigation on the X-ray and optical properties of local CVs or their candidates, mainly by cross-correlating the 3XMM DR8 and Gaia DR2 source catalogues. We first examine a nearly complete sample of CVs within 150 pc by characterizing their X-ray luminosity, X-ray to Gaia G-band flux ratio, and X-ray hardness ratio distributions, as well as their locations in the Gaia colour–magnitude diagram. We show that these distributions of this very local sample are consistent with those of a CV sample obtained at distances between 150 and 300 pc. We also present a catalogue of 15 new CV candidates within 500 pc based primarily on their X-ray to Gaia G-band flux ratios. The X-ray luminosities of these candidates are between 1029 to several 1031 erg s−1, and their stacked X-ray spectrum can be well described by a multitemperature optically thin thermal plasma model with a maximum temperature $T_{\rm max}=7.5^{+6.3}_{-2.4}$ keV. These properties are consistent with those of the very local CVs. In particular, one of the CV candidates is within 150 pc, indicating that a considerable number of local CVs are yet detected. These results provide a base for a more complete statistical understanding of CVs and their quantitative contribution to the galactic X-ray background.

The X-ray spectral and variability properties of typical radio-loud quasars

ABSTRACT We present X-ray spectral and long-term variability analyses of an unbiased sample of 361 optically selected radio-loud quasars (RLQs) utilizing sensitive serendipitous X-ray data from the Chandra and XMM-Newton archives. The spectral and temporal properties of RLQs are compared with those of radio-quiet quasars (RQQs) matched in L2500Å and z. The median power-law photon index (Γ) of RLQs is $1.84_{-0.01}^{+0.01}$, which is close to that of matched RQQs ($1.90_{-0.01}^{+0.02}$). No significant correlations between Γ and radio-loudness, Lx/Lx, rqq (the X-ray luminosity over that expected from the Lx–Luv relation for RQQs), redshift, or Eddington ratio are found for our RLQs. The stacked X-ray spectra of our RLQs show strong iron-line emission and a possible Compton-reflection hump. The intrinsic X-ray variability amplitude is ≈40 per cent for RLQs on time-scales of months-to-years in the rest frame, which is somewhat smaller than for the matched RQQs (≈60 per cent) on similar time-scales, perhaps due to the larger black hole masses and lower Eddington ratios in our RLQ sample. The X-ray spectral and variability results for our RLQs generally support the idea that the X-ray emission of typical RLQs is dominated by the disc/corona, as is also indicated by a recent luminosity correlation study.

Properties of AGN Multiband Optical Variability in the HSC SSP Transient Survey

Abstract We study variability of active galactic nuclei (AGNs) by using the deep optical multiband photometry data obtained from the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) survey in the COSMOS field. The images analyzed here were taken with 8, 10, 13, and 15 epochs over three years in the g, r, i, and z bands, respectively. We identified 491 robust variable AGN candidates, down to i = 25 mag and with redshift up to 4.26. Ninety percent of the variability-selected AGNs are individually identified with the X-ray sources detected in the Chandra COSMOS Legacy survey. We investigate their properties in variability by using structure function analysis and find that the structure function for low-luminosity AGNs (L bol ≲ 1045 erg s−1) shows a positive correlation with luminosity, which is the opposite trend for the luminous quasars. This trend is likely to be caused by a larger contribution of the host galaxy light for lower-luminosity AGNs. Using the model templates of galaxy spectra, we evaluate the amount of host galaxy contribution to the structure function analysis and find that dominance of the young stellar population is needed to explain the observed luminosity dependence. This suggests that low-luminosity AGNs at 0.8 ≲ z ≲ 1.8 are predominantly hosted in star-forming galaxies. The X-ray stacking analysis reveals the significant emission from the individually X-ray undetected AGNs in our variability-selected sample. The stacked samples show very large hardness ratios in their stacked X-ray spectrum, which suggests that these optically variable sources have large soft X-ray absorption by dust-free gas.

The ionic composition of the local absorber towards 3C 273

ABSTRACT Hot ionized gas is observed in the local vicinity of our galaxy through spectral absorption features. The most common hypothesis is that this gas forms a halo surrounding our Milky Way, in collisional ionization equilibrium. In this paper, we investigate the elemental abundance of this hot and ionized local gas. We use a 2.4 Ms stacked X-ray spectrum of the bright blazar 3C 273 and probe the local absorption features. Using ion-by-ion fitting of the X-ray absorption lines, we derive the column density of each ionization species. Based on the column densities, we reconstruct the absorption measure distribution, namely the hydrogenic column density as a function of temperature. We report the elemental abundances of C, N, Ne, and Fe relative to solar O. Previous measurements of local X-ray emission lines in conjunction with the present column densities indicate a scale height of 1–80 kpc and hydrogen number density of 10−4–10−3 cm−3 for the hot ionized gas. Additionally, we detect He-like O lines from the quasar broad line region with velocities of 6400 ± 1500 km s−1.

Constraints on the presence of a 3.5 keV dark matter emission line fromChandraobservations of the Galactic centre

Context. Recent findings of line emission at 3.5 keV in both individual and stacked X-ray spectra of galaxy clusters have been speculated to have dark matter origin. Aims. If the origin is indeed dark matter, the emission line is expected to be detectable from the Milky Way dark matter halo. Methods. We perform a line search in public Chandra X-ray observations of the region near Sgr A*. We derive upper limits on the line emission flux for the 2.0-9.0 keV energy interval and discuss their potential physical interpretations including various scenarios of decaying and annihilating dark matter. Results. While find no clear evidence for its presence, the upper flux limits are not inconsistent with the recent detections for conservative mass profiles of the Milky Way. Conclusions. The results depends mildly on the spectral modelling and strongly on the choice of dark matter profile.

Non-detection of X-ray emission from sterile neutrinos in stacked galaxy spectra

We conduct a comprehensive search for X-ray emission lines from sterile neutrino dark matter, motivated by recent claims of unidentified emission lines in the stacked X-ray spectra of galaxy clusters and the centers of the Milky Way and M31. Since the claimed emission lines lie around 3.5 keV, we focus on galaxies and galaxy groups (masking the central regions), since these objects emit very little radiation above $\sim 2$ keV and offer a clean background against which to detect emission lines. We develop a formalism for maximizing the signal-to-noise of decaying dark matter emission lines by weighing each X-ray event according to the expected dark matter profile. In total, we examine 81 and 89 galaxies with Chandra and XMM-Newton respectively, totaling 15.0 and 14.6 Ms of integration time. We find no significant evidence of any emission lines, placing strong constraints on the mixing angle of sterile neutrinos with masses between 4.8-12.4 keV. In particular, if the 3.57 keV feature from Bulbul et al. (2014) were due to 7.1 keV sterile neutrino emission, we would have detected it at $4.4\sigma$ and $11.8\sigma$ in our two samples. The most conservative estimates of the systematic uncertainties reduce these constraints to $4.4\sigma$ and 7.8$\sigma$, or letting the line energy vary between 3.50 and 3.60 keV reduces these constraints to $2.7\sigma$ and $11.0\sigma$ respectively. Unlike previous constraints, our measurements do not depend on the model of the X-ray background or on the assumed logarithmic slope of the center of the dark matter profile.

Relativistic Fe Kα line and ensemble spins of black holes in narrow-line Seyfert 1 galaxies

AbstractWhile a broad line of the Fe Kα emission is commonly found in the X-ray spectra of typical Seyfert galaxies, the situation is unclear in the case of Narrow Line Seyfert 1 galaxies (NLS1s)—an extreme subset which are generally thought to harbor less massive black holes with higher accretion rates. We report results of our study of the assemble property of the Fe K line in NLS1s by stacking the X-ray spectra of a large sample of 51 NLS1s observed with XMM-Newton. We find in the stacked X-ray spectra a prominent, broad emission feature over 4–7 keV, which is characteristic of the broad Fe Kα line. Our results suggest that a relativistic broad Fe line may in fact be common in NLS1s. The line profile is used to study the average spin of the black holes in the sample. We find, for the first time, that their black holes are constrained to be likely spinning at averagely low or moderate rates as a population. The implications of the results are discussed in the context of the black hole growth in NLS1 galaxies.

The Galactic plane at faint X-ray fluxes – II. Stacked X-ray spectra of a sample of serendipitous XMM–Newton sources

We have investigated the X-ray spectral properties of a sample of 138 X-ray sources detected serendipitously in $XMM-Newton$ observations of the Galactic plane, at an intermediate to faint flux level. We divide our sample into 5 subgroups according to the spectral hardness of the sources, and stack (i.e. co-add) the individual source spectra within each subgroup. As expected these stacked spectra show a softening trend from the hardest to the softest subgroups, which is reflected in the inferred line-of-sight column density. The spectra of the three hardest subgroups are characterized by a hard continuum plus superimpose Fe-line emission in the 6--7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe-K$\alpha$ line is 170$^{+35}_{-32}$ eV, whereas the 6.4-keV Fe-K fluorescence line from neutral iron and the 6.9-keV H-like Fe-Ly$\alpha$ line have EWs of 89$^{+26}_{-25}$ eV and 81$^{+30}_{-29}$ eV respectively, i.e. roughly half that of the 6.7-keV line. The remaining subgroups exhibit soft thermal spectra. Virtually all of the spectrally-soft X-ray sources can be associated with relatively nearby coronally-active late-type stars, which are evident as bright near-infrared (NIR) objects within the X-ray error circles. On a similar basis only a minority of the spectrally-hard X-ray sources have likely NIR identifications. The average continuum and Fe-line properties of the spectrally-hard sources are consistent with those of magnetic cataclysmic variables but the direct identification of large numbers of such systems in Galactic X-ray surveys, probing intermediate to faint flux levels, remains challenging.

THEXMM-NEWTONX-RAY SPECTRA OF THE MOST X-RAY LUMINOUS RADIO-QUIETROSATBRIGHT SURVEY-QSOs: A REFERENCE SAMPLE FOR THE INTERPRETATION OF HIGH-REDSHIFT QSO SPECTRA

We present the broadband X-ray properties of four of the most X-ray luminous (L{sub X} {>=} 10{sup 45} erg s{sup -1} in the 0.5-2 keV band) radio-quiet QSOs found in the ROSAT Bright Survey. This uniform sample class, which explores the extreme end of the QSO luminosity function, exhibits surprisingly homogenous X-ray spectral properties: a soft excess with an extremely smooth shape containing no obvious discrete features, a hard power law above 2 keV, and a weak narrow/barely resolved Fe K{alpha} fluorescence line for the three high signal-to-noise ratio (S/N) spectra. The soft excess can be well fitted with only a soft power law. No signatures of warm or cold intrinsic absorbers are found. The Fe K{alpha} centroids and the line widths indicate emission from neutral Fe (E = 6.4 keV) originating from cold material from distances of only a few light days or further out. The well-constrained equivalent widths (EW) of the neutral Fe lines are higher than expected from the X-ray Baldwin effect which has been only poorly constrained at very high luminosities. Taking into account our individual EW measurements, we show that the X-ray Baldwin effect flattens above L{sub X} {approx} 10{sup 44} erg s{sup -1} (2-10more » keV band) where an almost constant (EW) of {approx}100 eV is found. We confirm the assumption of having very similar X-ray active galactic nucleus properties when interpreting stacked X-ray spectra. Our stacked spectrum serves as a superb reference for the interpretation of low S/N spectra of radio-quiet QSOs with similar luminosities at higher redshifts routinely detected by XMM-Newton and Chandra surveys.« less

COSMIC RAY ACCELERATORS IN THE LARGE MAGELLANIC CLOUD

We present the broadband X-ray properties of four of the most X-ray luminous (L_X >= 10^{45} erg/s in the 0.5-2 keV band) radio-quiet QSOs found in the ROSAT Bright Survey. This uniform sample class, which explores the extreme end of the QSO luminosity function, exhibits surprisingly homogenous X-ray spectral properties: a soft excess with an extremely smooth shape containing no obvious discrete features, a hard power law above 2 keV, and a weak narrow/barely resolved Fe K-alpha fluorescence line for the three high signal-to-noise ratio (S/N) spectra. The soft excess can be well fitted with only a soft power law. No signatures of warm or cold intrinsic absorbers are found. The Fe K-alpha centroids and the line widths indicate emission from neutral Fe (E=6.4 keV) originating from cold material from distances of only a few light days or further out. The well-constrained equivalent widths (EW) of the neutral Fe lines are higher than expected from the X-ray Baldwin effect which has been only poorly constrained at very high luminosities. Taking into account our individual EW measurements, we show that the X-ray Baldwin effect flattens above L_X ~ 10^{44} erg/s (2-10 keV band) where an almost constant <EW> of ~100 eV is found. We confirm the assumption of having very similar X-ray AGN properties when interpreting stacked X-ray spectra. Our stacked spectrum serves as a superb reference for the interpretation of low S/N spectra of radio-quiet QSOs with similar luminosities at higher redshifts routinely detected by XMM-Newton and Chandra surveys.

The candidate filament close to the 3C 295 galaxy cluster: optical and X-ray spectroscopy

We present a detailed analysis of the overdensity of X-ray sources close to the 3C 295 galaxy cluster (z = 0.46) to assess whether it is associated with a filament of the large-scale structure of the Universe. We obtained optical spectra of the optical counterparts of eleven sources associated with the filament, finding that one is at z = 0.474. This is a type 1 AGN at 1.5 arcmin from the cluster center. We found three more sources with a redshift in the range 0.37 ÷ 0.53. We extracted the stacked X-ray spectrum of 47 X-ray sources belonging to the putative filament. We found a significant narrow (at the resolution of the Chandra ACIS-I) line at E ∼ 4. 4k eV, the energy of the iron Kα line at the redshift of the cluster. The detection of this line is confirmed at a confidence level of better than 3σ and its energy is constrained to be in the range 6.2−6.47 (at a 90% confidence level), excluding an identification with the 6.7 helium-like iron line from the hot cluster ICM at better than 4σ. We conclude that the detection of the redshifted line is a strong indication that at least several of the excess sources lie at z ∼ 0.46 and that AGNs are efficient tracers of the “filament” connected with the central cluster of galaxies.

X-ray absorption in distant type II QSOs

We present the results of the X-ray spectral analysis of an XMM-Newton-selected type II QSO sample with z>0.5 and 0.5-10 keV flux of 0.3-33 x 10^{-14} erg/s/cm^2. The distribution of absorbing column densities in type II QSOs is investigated and the dependence of absorption on X-ray luminosity and redshift is studied. We inspected 51 spectroscopically classified type II QSO candidates from the XMM-Newton Marano field survey, the XMM-Newton-2dF wide angle survey (XWAS), and the AXIS survey to set-up a well-defined sample with secure optical type II identifications. Fourteen type II QSOs were classified and an X-ray spectral analysis performed. Since most of our sources have only ~40 X-ray counts (PN-detector), we carefully studied the fit results of the simulated X-ray spectra as a function of fit statistic and binning method. We determined that fitting the spectra with the Cash-statistic and a binning of minimum one count per bin recovers the input values of the simulated X-ray spectra best. Above 100 PN counts, the free fits of the spectrum's slope and absorbing hydrogen column density are reliable. We find only moderate absorption (N_H=(2-10) x 10^22 cm^-2) and no obvious trends with redshift and intrinsic X-ray luminosity. In a few cases a Compton-thick absorber cannot be excluded. Two type II objects with no X-ray absorption were discovered. We find no evidence for an intrinsic separation between type II AGN and high X-ray luminosity type II QSO in terms of absorption. The stacked X-ray spectrum of our 14 type II QSOs shows no iron K-alpha line. In contrast, the stack of the 8 type II AGN reveals a very prominent iron K-alpha line at an energy of ~ 6.6 keV and an EW ~ 2 keV.

Multiwavelength Study of Massive Galaxies atz ∼ 2. II. Widespread Compton‐thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges

Approximately 20‐30% of 1.4 6.2 keV. The stacked X-ray spectrum rises steeply at > 10 keV, suggesting that these sources host Compton-thick Active Galactic Nuclei (AGNs) with column densities NH > ∼ 10 24 cm −2 and an average, unobscured X-ray luminosity L2−8keV ≈(1‐4) × 10 43 erg s −1 . Their sky density (∼ 3200 deg −2 ) and space density (∼ 2.6 × 10 −4 Mpc −3 ) are twice those of X-ray detected AGNs at z ≈ 2, and much larger than those of previously-known Compton thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought-after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching ∼ 50‐60% for M ∼ 10 11 M⊙, an effect likely connected with downsizing in galaxy formation. The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z = 2 is similar to the mass ratio of black holes to stars in local s pheroids, implying concurrent growth of both within the precursors of today’s massive galaxies. Subject headings:galaxies: evolution — galaxies: formation — galaxies: active — X-rays: galaxies

ChandraX‐Ray Sources in the LALA Cetus Field

The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep Chandra images on LALA fields. In this paper we present the Chandra X-ray sources detected in the Cetus field, along with an analysis of X-ray source counts, stacked X-ray spectrum, and optical identifications. A total of 188 X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0 keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived and compared with LALA field (172 ks exposure). Interestingly, we find consistent hard-band X-ray source density, but (36 ± 12)% higher soft-band X-ray source density in Cetus field. The weighted stacked spectrum of the detected X-ray sources can be fitted by a power law with photon index Γ = 1.55. Based on the weighted stacked spectrum, we find that the resolved fraction of the X-ray background drops from (72 ± 1)% at 0.5-1.0 keV to (63 ± 4)% at 6.0-8.0 keV. The unresolved spectrum can be fitted by a power law over the range 0.5-7 keV, with a photon index Γ = 1.22. We also present optical counterparts for 154 of the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a deep r'-band image obtained with the MMT.

Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey

Aims. Our goal is to probe the populations of and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample with wide multiwavelength coverage. Methods. We selected a sample of 136 X-ray sources detected at a significance of ≥3σ in the 2-10 keV band (F_(2-10) ≳ 10^(-14) erg cm^(-2) s^(-1)) in a ~1 deg^2 area in the XMM Medium Deep Survey (XMDS). The XMDS area is covered with photometry from the VVDS and CFHTLS surveys and infrared Spitzer data from the SWIRE survey. Based on the X-ray luminosity and X-ray to ratio, 132 sources are likely AGN, of which 122 have unambiguous - IR identification. The observed and IR spectral energy distributions of all identified sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. X-ray spectral analysis is performed individually for sources with a sufficient number of counts and using a stacking technique for subsamples of sources at different flux levels. Hardness ratios are used to estimate X-ray absorption in individual weak sources. Results. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template. We group them together in a single class of optically obscured AGN. These have red colors and in about 60% of cases show significant X-ray absorption (N_H > 10^(22) cm^(-2)). Sources with SEDs typical of type 1 AGN have blue colors and exhibit X-ray absorption in about 30% of cases. The stacked X-ray spectrum of AGN is flatter than that of type 1 AGN and has an average spectral slope of Γ = 1.6. The subsample of objects fitted by a star forming galaxy template has an even harder stacked spectrum, with Γ ~ 1.2-1.3. The fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than optical obscuration and its incidence is nearly constant with redshift and luminosity. This implies that at high luminosities X-ray absorption is not necessarily related to obscuration. The estimated surface densities of obscured, unobscured AGN and type 2 QSOs are respectively 138, 59 and 35 deg^(-2) at F > 10^(-14) erg cm^(-2) s^(-1).

Red active galactic nuclei in XMM-Newton/Sloan Digital Sky Survey fields

In this paper we combine archival and proprietary XMM-Newton observations (about 5 deg 2 ) that overlap with the Sloan Digital Sky Survey to explore the nature of the moderate-z X-ray population. We focus on X-ray sources with optically red colours (g - r > 0.4), which we argue are important for understanding the origin of the X-ray background (XRB). First, these systems constitute a significant fraction, about two-third, of the z? 1 X-ray population to the limit f(2-8 keV) ≈ 2 × 10 -14 erg s -1 cm -2 . Secondly, their luminosity function under evolution of the form α (1 + z) 3 suggests that they could be responsible for about 17 per cent of the diffuse XRB to z = 1. Thirdly, their stacked X-ray spectrum in the range 1-8 keV is consistent with a power-law distribution with r ≈ 1.4 (without fitting intrinsic absorption), that is, similar to the diffuse XRB. We find that the optically red X-ray population comprises a mixed bag of objects, both obscured [hydrogen column density (N H ) > 10 22 cm -2 ] and unobscured (N H 2 mag) AGNs identified in the Two-Micron All-Sky Survey (2MASS). The median N H of the red X-ray sources studied here is ≈10 21 cm -2 , lower than that found for the 2MASS AGNs, suggesting different populations.

ROSAT soft X-ray properties of the Large Bright Quasar Survey: modelling of stacked X-ray spectra

We develop and apply a novel method of analysis to study the X-ray spectral properties of 908 QSOs in the Large Bright Quasar Survey (LBQS) that were observed during the soft X-ray ROSAT All-Sky Survey (RASS). Due to the relatively short (<600sec) RASS exposure times, only 10% of the QSOs are detected in X-rays, so X-ray spectral model fits for individual QSOs are precluded by poor photon statistics. Spectral stacking provides effectively much more sensitive X-ray observations for an average QSO in bins of redshift, and for several classes of QSOs. We model the stacked X-ray spectra in a way that obviates both the distinction between detections and non-detections, and the need to stack together only those objects observed through similar Galactic column densities N_H. In application to the LBQS/RASS sample, we marginally confirm a flattening of the X-ray spectral index of QSOs toward higher redshifts. Radio-loud QSOs show flatter X-ray photon indices, in agreement with previous work. We show that the apparent flattening of the photon index with redshift is not due to an increasing fractional contribution from the radio-loud class of QSOs.