Non-detected Sources Research Articles

The eROSITA upper limits

Context. The soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory has successfully completed four of the eight planned all-sky surveys, detecting almost one million X-ray sources during the first survey (eRASS1). The catalog of this survey will be released as part of the first eROSITA data release (DR1). Aims. Based on X-ray aperture photometry, we provide flux upper limits for eRASS1 in several energy bands. We cover galactic longitudes between 180° ≲ l ≲ 360° (eROSITA-DE). These data are crucial for studying the X-ray properties of variable and transient objects, as well as non-detected sources in the eROSITA all-sky survey data. Methods. We performed aperture photometry on every pixel of the SRG/eROSITA standard pipeline data products for all available sky tiles in the single detection band (0.2–2.3 keV). Simultaneously, we performed the same analysis in the three-band detection at soft (0.2–0.6 keV), medium (0.6–2.3 keV), and hard (2.3–5.0 keV) energy bands. Based on the combination of products for the individual bands, we are also able to provide aperture photometry products and flux upper limits for the 0.2–5.0 keV energy band. The upper limits were calculated based on a Bayesian approach that utilizes detected counts and background within the circular aperture. Results. The final data products consist of tables with the aperture photometry products (detected counts, background counts, and exposure time), a close-neighbor flag, and the upper flux limit based on an absorbed power-law spectral model (Γ = 2.0, NH = 3 × 1020 cm−2). The upper limits are calculated using the one-sided 3σ confidence interval (CL) of a normal distribution, representing CL = 99.87%. The aperture photometry products allow for an easy computation of upper limits at any other confidence interval and spectral model. These data are stored in a database with hierarchical indexes in order to offer a fast query option. Conclusions. We provide a detailed description of the process of retrieving SRG/eROSITA upper limits for a large set of input positions, as well as of the eROSITA data, the X-ray aperture photometry, the upper limit calculation, and the final data products. The eROSITA upper flux limits represent either the maximum flux of potential non-detections or the 3σ upper flux uncertainty of detected sources. We emphasize the importance of choosing the right spectral model that ought to match the spectral shape of the source of interest: the wrong spectral model can produce discrepancies of up to 30% in the final flux upper limit value. We also describe the architecture of the database and the web tool, which are designed to handle large queries on input positions.

Exploring the connection between ultraviolet/optical variations and radio emission in radio-quiet quasars: clues about the origin of radio emission

ABSTRACT Radio emission in radio-quiet quasars (RQQs) has long been a mystery and its physical origin remains unclear. In previous work, we have found that quasars that are more variable in ultraviolet (UV)/optical have stronger X-ray emission, indicating a link between disc turbulence and X-ray corona heating. In this work, for the first time, we investigate the relation between UV/optical variability and the radio emission in RQQs selected from the Sloan Digital Sky Survey Stripe 82 survey and the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. We median stack the FIRST images and detect clear signals from RQQs in the co-added images of individually radio non-detected sources. Controlling the effects of other parameters, including redshift, black hole mass, bolometric luminosity and Eddington ratio, we find more variable RQQs, which are known to be relatively brighter in X-ray, and show tentatively weaker radio emission, contrary to the linear X-ray/radio correlation if the radio emission is from, or driven by, the corona. This discovery also suggests that if the radio emission in RQQs is driven by AGN activity (such as a weak jet), the underlying driving process is independent of the disc turbulence, which drives UV/optical variability and probably also corona heating. Alternatively, the radio emission could be a result of star formation in the host galaxies.

A Large Population of Luminous Active Galactic Nuclei Lacking X-Ray Detections: Evidence for Heavy Obscuration?

Abstract We present a large sample of infrared-luminous candidate active galactic nuclei (AGNs) that lack X-ray detections in Chandra, XMM-Newton, and NuSTAR fields. We selected all optically detected SDSS sources with redshift measurements, combined additional broadband photometry from WISE, UKIDSS, 2MASS, and GALEX, and modeled the spectral energy distributions (SEDs) of our sample sources. We parameterize nuclear obscuration in our SEDs with and uncover thousands of powerful obscured AGNs that lack X-ray counterparts, many of which are identified as AGN candidates based on straightforward WISE photometric criteria. Using the observed luminosity correlation between rest-frame 2–10 keV ( ) and rest-frame AGN ( ), we estimate the intrinsic X-ray luminosities of our sample sources and combine these data with flux limits from X-ray catalogs to determine lower limits on nuclear obscuration. Using the ratio of intrinsic-to-observed X-ray luminosity ( ), we find a significant fraction of sources with column densities approaching cm–2, suggesting that multiwavelength observations are necessary to account for the population of heavily obscured AGNs. We simulate the underlying distribution for the X-ray non-detected sources in our sample through survival analysis, and confirm the presence of AGN activity via X-ray stacking. Our results point to a considerable population of extremely obscured AGNs undetected by current X-ray observatories.

Water masers in Compton-thick AGN

Aims. Investigations of H2O maser galaxies at X-ray energies reveal that most active galactic nuclei (AGN) associated with water masers are characterized by high levels of absorption. With the aim of finding new maser sources for possible interferometric follow-ups, we have searched for water maser emission in a well-defined sample of heavily absorbed AGN (NH > 1023 cm−2), including Compton-thick (CT) sources. Methods. Previous surveys already searched for 22 GHz water maser emission in all the galaxies in this sample. With the goal of providing a detection or a stringent upper limit on the H2O luminosity, we reobserved some of the non-detected sources with the Green Bank Telescope. A new luminous H2O maser (LH2O ∼ 200 L⊙) was detected in the mid-IR-bright Seyfert 2 galaxy IRAS 15480−0344 and then followed-up with the Very Long Baseline Array. In order to shed light on the origin of the maser (jet, outflow, or disk), we recently observed the radio continuum emission in IRAS 15480-0344 with the European VLBI network (EVN) at 1.7 and 5.0 GHz. Results. With the newly discovered megamaser in IRAS 15480−0344 revealing a narrow (∼0.6 km s−1) and a broad (∼90 km s−1) component, the maser detection rate of the CT AGN sample is 50% (18/36), which is one of the highest ever found in maser surveys. The EVN maps show two bright sources (labeled SW and NE) in the nuclear region of IRAS 15480−0344, which we interpret as jet knots tracing regions where the radio plasma impacts dense molecular clouds. The narrow maser feature is approximately at the center of the imaginary line connecting the two continuum sources, likely pinpointing the core, and might be associated with the accretion disk or a nuclear outflow. The location of the broad maser feature, instead, coincides with source NE, suggesting that the maser emission might be produced by a jet-cloud interaction, as it was proposed for NGC 1068 and Mrk 348.

Bimodal radio variability in OVRO-40 m-monitored blazars

Blazars are known to show periods of quiescence followed by outbursts visible throughout the electromagnetic spectrum. We present a novel maximum likelihood approach to capture this bimodal behavior by examining blazar radio variability in the flux-density domain. We separate quiescent and flaring components of a source's light curve by modeling its flux-density distribution as a series of "off" and "on" states. Our modeling allows us to extract information regarding the flaring ratio, duty cycle, and the modulation index in the "off"-state, in the "on"-state, as well as throughout the monitoring period of each blazar. We apply our method to a flux-density-limited subsample from the Owens Valley Radio observatory's 15 GHz blazar monitoring program, and explore differences in the variability characteristics between BL Lacs and FSRQs as well as between $\gamma$-ray detected and non-detected sources. We find that: (1) BL Lacs are more variable and have relatively larger outbursts than the FSRQs, (2) unclassified blazar candidates in our sample show similar variability characteristics as the FSRQs, and (3) $\gamma$-ray detected differ from the $\gamma$-ray non-detected sources in all their variability properties, suggesting a link between the production of $\gamma$-rays and the mechanism responsible for the radio variability. Finally, we fit distributions for blazar flaring ratios, duty cycles, and on- and off- modulation indices that can be used in population studies of variability-dependent blazar properties.

KOSMA 12CO(2–1) and (3–2) observations toward Infrared Dark Clouds

Infrared Dark Clouds (IRDCs) are a valuable sample to study the initial conditions and the very early stages of the evolution of massive stars. Using the KOSMA telescope, we performed simultaneous single point 12CO(2–1) and (3–2) observations toward 117 IRDCs with a detection rate of 74 % for 12CO(2–1) and 58 % for 12CO(3–2). The non-detected sources are found not to represent real dark clouds. The distances of the IRDCs have been calculated using a Bayesian distance estimator indicating that the sources are mainly concentrated in the Outer, Perseus, Local, Sagittarius and Scutum arms. The excitation temperature and the filling factor have been solved simultaneously for the two emission lines. The average excitation temperature is about 10 K. The filling factors are used to correct the integrated intensity ratio of 12CO(3–2) to (2–1) resulting in a typical value of the ratio of $0.69\pm0.47$ for the majority of sources. A small number of sources exhibit enhanced values for the ratio because of the presence of a nearby HII region. The ratios do not show significant differences for sources with embedded far-infrared sources.

The X-ray properties of high-z Fanaroff-Riley type I candidates in the COSMOS field

We report the X-ray analysis of a sample of candidate high-redshift (1<z<2) FRI sources from the sample of Chiaberge et al. (2009), observed in the Chandra COSMOS field (C-COSMOS). Our main goals are to study their nuclear properties by means of unresolved X-ray emission, and to constrain the presence of clusters surrounding the FRI sources from the diffuse X-ray emission by the associated hot plasma. Among 19 FRI candidates, 6 have an X-ray unresolved counterpart in the C-COSMOS catalog. Two additional sources are not present in the C-COSMOS catalog but are clearly detected in the Chandra images. X-ray spectral analysis, when possible, or hardness ratio of the stacked emission from X-ray detected sources, suggest that some of them have significant intrinsic absorption (N_H~10^22 cm^-2), and high X-ray luminosities with respect to local FRIs. From the stacking analysis of the 11 non-detected sources, however, we find evidence for unresolved soft X-ray emission and no detected hard emission, suggesting an unabsorbed spectrum. Therefore, the X-ray properties vary significantly from source to source among these FRI candidates. From the analysis of the stacked images of all 19 FRI candidates we can rule out the presence of virialized haloes with temperatures larger than 2-3 keV; however, the upper limit on the average extended emission is still consistent with the presence of ~1-2 keV hot gas.

Multi-Wavelength Studies on H2O Maser Host Galaxies

H2O maser emissions have been found in external galaxies for more than 30 years. Main sciences associated with extragalactic H2O masers can be summarized roughly into three parts: maser emission itself, AGN sciences and cosmology exploration. Our work in this field focusses on two projects: X-ray data analysis of individual maser source using X-ray penetrability to explore maser host obscured AGN; multi-wavelength statistical properties of the whole published H2O maser sample. Here their nuclear radio properties were investigated in detail, based on their 6-cm and 20-cm radio observation data. Comparing the radio properties between maser-detected sources and non-detected sources at similar distance scale, we find that: (1) maser host galaxies tend to have higher nuclear radio luminosity; (2) the spectral index of both samples is comparable (∼0.6), within the error ranges. In addition, for AGN-maser sources, the isotropic maser luminosity tends to increase with rising radio luminosity. Thus we propose the nuclear radio luminosity as one good indicator for searching AGN-masers in the future.

THE RELATION BETWEEN RADIO POLARIZATION AND GAMMA-RAY EMISSION IN AGN JETS

We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.

Thermal Emission from Warm Dust in the Most Distant Quasars

We report new continuum observations of 14 -->z ~ 6 quasars at 250 GHz and 14 quasars at 1.4 GHz. We summarize all recent millimeter and radio observations of the sample of the 33 quasars known with -->5.71 ? z? 6.43 and present a study of the rest-frame far-infrared (FIR) properties of this sample. These quasars were observed with the Max Planck Millimeter Bolometer Array (MAMBO) at 250 GHz with mJy sensitivity, and 30% of them were detected. We also recover the average 250 GHz flux density of the MAMBO undetected sources at 4 ? by stacking the on-source measurements. The derived mean radio-to-UV spectral energy distributions (SEDs) of the full sample and the 250 GHz nondetections show no significant differences from lower redshift optical quasars. Obvious FIR excesses are seen in the individual SEDs of the strong 250 GHz detections, with FIR-to-radio emission ratios consistent with those of typical star-forming galaxies. Most 250 GHz-detected sources follow the LFIR-Lbol relationship derived from a sample of local IR-luminous quasars ( -->LIR > 1012 L?), while the average LFIR/Lbol ratio of the nondetections is consistent with that of the optically selected PG quasars. The MAMBO detections also tend to have weaker Ly? emission than the nondetected sources. We discuss possible FIR dust-heating sources and critically assess the possibility of active star formation in the host galaxies of the -->z ~ 6 quasars. The average star formation rate of the MAMBO nondetections is likely to be less than a few hundred M? yr?1, but in the strong detections, the host galaxy star formation is probably at a rate of 103 M? yr?1, which dominates the FIR dust heating.

An Infrared Photometric Study of Galaxies with Extragalactic H 2 O Maser Sources

All galaxies with extragalactic H2O maser sources observed so far are collected. With the 2MASS and the IRAS photometric data an infrared study is performed on those galaxies. By a comparison between the H2O maser detected sources and non-detected sources in the infrared it is indicated that infrared properties in the IRAS 12–25 μm and 60–100 μm are important for producing H2O masers in galaxies. It is also found that the H2O maser galaxies with different nuclear activity types have rather different infrared properties mainly in the IRAS 12–60 μm region.

Photometric Study of Galaxies with OH Megamasers in the Infrared

OH megamasers are the most luminous masers in the universe. The total of 109 OH megamaser (OHM) galaxies known so far have been collected, and cross-identifications of those OHM galaxies with Two Micron All Sky Survey (2MASS) sources are made in this paper. Using 2MASS and IRAS data, the infrared properties of OHM-detected and nondetected sources are compared. It is shown that the main difference between those two kinds of sources is the infrared color from 25 to 60 μm. Statistically, the OHM-detected sources have redder colors and steeper spectral indices in the 25-60 μm region than the nondetected sources. In addition, considering the spectral types, the properties of the OHM galaxies in the near-infrared and far-infrared are discussed. It is found that Seyfert 1 and Seyfert 2 sources with hidden broad-line regions (BLRs) have redder colors and steeper spectral indices than other sources, including LINER, composite spectrum nucleus, and H II sources in the near-infrared region, whereas the latter have redder colors and steeper spectral indices than the Seyfert 1 and Seyfert 2 sources with hidden BLRs in the far-infrared. It is also found that almost all sources either with or without an identified spectral type show a starburst-dominated feature in the far-infrared, while a considerable number of sources, in particular Seyfert 1 and Seyfert 2 sources with hidden BLRs, show an active galactic nucleus-dominated feature in the near-infrared. Furthermore, as far-infrared radiation at 35 and 53 μm is likely responsible for the OH maser pumping in OHM galaxies, the pumping efficiencies in the infrared are also analyzed in this paper. It is found that, statistically, the pumping efficiencies increase with far-infrared color. It is also found that, statistically, the OH peak fluxes are not proportional to the related infrared fluxes at either 35 or 53 μm. This result implies that the pump lines are optically thin and the pump sources are unsaturated.

The 172 ksChandraExposure of the LALA Bootes Field: X-Ray Source Catalog

We present an analysis of a deep, 172 ks Chandra observation of the Large Area Lyman Alpha survey (LALA) Bootes field, obtained with the Advanced CCD Imaging Spectrometer (ACIS-I) on board the Chandra X-Ray Observatory. This is one of the deepest Chandra images of the extragalactic sky; only the 2 Ms Chandra Deep Field North (CDF-N) and 1 Ms Chandra Deep Field South (CDF-S) are substantially deeper. In this paper we present the X-ray source catalog obtained from this image, along with an analysis of source counts and optical identifications. The X-ray image is composed of two individual observations obtained in 2002 and reaches 0.5–2.0 and 2.0–10.0 keV flux limits of 1.5 × 10-16 and 1.0 × 10-15 ergs cm-2 s-1, respectively, for point sources near the aim point. A total of 168 X-ray sources were detected: 160 in the 0.5–7.0 keV band, 132 in the 0.5–2.0 keV band, and 111 in the 2.0–7.0 keV band. The X-ray source counts were derived and compared with those from other Chandra deep surveys; the hard X-ray source density of the LALA Bootes field is 33% higher than that of CDF-S at the flux level of 2.0 × 10-15 ergs cm-2 s-1, confirming the field-to-field variances of the hard-band source counts reported by previous studies. The deep exposure resolves 72% of the 2.0–10.0 keV X-ray background. Our primary optical data are R-band imaging from the NOAO Deep Wide-Field Survey (NDWFS), with a limiting magnitude of R = 25.7 (Vega, 3 σ, and 4'' diameter aperture). We have found optical counterparts for 152 of the 168 Chandra sources (90%); 144 of these are detected in the R-band image, and eight have optical counterparts in other bands (either BW, V, I, or z'). Among the R-band nondetected sources, not more than 11 of them can possibly be at z > 5, based on the hardness ratios of their X-ray emission and nondetections in bluer bands (BW, V). The majority (~76%) of the X-ray sources are found to have log(fX/fR) within 0.0 ± 1; these are believed to be AGNs. Most of the X-ray–faint/optically bright sources [log(fX/fR) 1.0], which are harder in X-ray and are probably obscured AGNs.

Water Masers Associated with Circumstellar Shells

view Abstract Citations (46) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Water Masers Associated with Circumstellar Shells Benson, Priscilla J. ; Little-Marenin, Irene R. Abstract We report observational parameters for 62 H2O maser sources, 23 not previously reported, and upper limits of H2O maser emission for 223 nondetected sources. The detected long-period variables (LPVs) are at D < 1.8 kpc for a detection limit of about 1 Jy. The distance adjusted detection rate varies from 18% for Miras variables with P < 200 days to over 50% for stars with P> 300 days and seems to be determined primarily by the amount of material in their associated circumstellar shells, since detected stars have infrared to photospheric flux ratios [flux(12 μm flux(2.2 μm)] > 0.2, whereas nondetected stars have ratios less than 0.3. Stars in the overlap region, 0.2 < r <0.3, may have been observed at the wrong phase. The minimum mass-loss rate needed to sustain detectable H2O maser emission from stars in this program is 0.5 × 10-7 Msun yr-1. The mass-loss rate increases with period. Mira variables with H2O masers have more asymmetric light curves (f = 43) compared to nondetected Miras variables (f = 46). The average H2O maser flux (normalized to 1 kpc) for Mira variables in our observing program with P < 300 days is about 15 Jy km s-1, corresponding to L(H2O)/L(star) ≍ 1 × 10-10 or L(H20)/Lsun 3 x 10-7 and to a photon luminosity 1043-1044 s-1. The low detection rates for Mira variables with P < 200 days is primarily due to the smaller amount of material in their circumstellar shells as compared to the longer period Mira variables and does not appear to be a strong function of their lower metallicities. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192349 Bibcode: 1996ApJS..106..579B Keywords: STARS: CIRCUMSTELLAR MATTER; MASERS; STARS: MASS LOSS; STARS: VARIABLES: OTHER LONG-PERIOD VARIABLES full text sources ADS | data products SIMBAD (281)

The starburst-AGN connection - A sensitive VLBI survey of luminous IRAS galaxies

Results of an 18 cm VLBI survey of 31 luminous, radio-compact IR galaxies are presented. Over half the sample galaxies show high-brightness temperature emission, with T sub b greater than 10 exp 5 K and structure on scales of 5-150 mas. The limits for nondetected sources are similar, consistent with a picture in which most of these galaxies have compact cores at a level of a few percent of the total radio flux density. Characteristics of the extended radio structure, infrared properties, and optical excitation are not good indicators of the detectability of VLBI-scale emission. Structural information and energetic considerations rule out a single supernova interpretation of the compact emission in these galaxies, although we cannot exclude the possibility of several simultaneous extraordinarily luminous radio supernovae within the central few hundred cubic parsec. Our results instead favor the presence of an AGN obscured by starburst-related dust.