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ABSTRACT: A comprehensive search for variable and transient radio sources has been
conducted using ~55,000 snapshot images of the FIRST survey. We present an
analysis leading to the discovery of 1,627 variable and transient objects down
to mJy levels over a wide range of timescales (few minutes to years).
Variations observed range from 20% to a factor of 25. Multi-wavelength matching
for counterparts reveals the diverse classes of objects exhibiting variability,
ranging from nearby stars and pulsars to galaxies and distant quasars.
Interestingly, more than half of the objects in the sample have either no
classified counterparts or no corresponding sources at any other wavelength and
require multi-wavelength follow-up observations. We discuss these classes of
variables and speculate on the identity of objects that lack multi-wavelength
counterparts.
07/2011;
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ABSTRACT: Using three epochs of VLA observations of the Galactic Plane in the first quadrant taken ~15 years apart, we have conducted a search for a population of variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm. We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting for various selection effects and for the extragalactic variable population of active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg which vary by more than 50% on a time scale of years (or shorter). We show that these sources are much more highly variable than extragalactic objects; more than 50% show variability by a factor >2 compared to <10% for extragalactic objects in the same flux density range. We also show that the fraction of variable sources increases toward the Galactic center (another indication that this is a Galactic population), and that the spectral indices of many of these sources are flat or inverted. A small number of the variables are coincident with mid-IR sources and two are coincident with X-ray emitters, but most have no known counterparts at other wavelengths. Intriguingly, one lies at the center of a supernova remnant, while another appears to be a very compact planetary nebula; several are likely to represent activity associated with star formation regions. We discuss the possible source classes which could contribute to the variable cohort and followup observations which could clarify the nature of these sources. Comment: 11 pages, 7 figures; to be published in the Astronomical Journal; data available on MAGPIS website at http://third.ucllnl.org/gps/
05/2010;
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Knox S. Long,
William P. Blair,
P. Frank Winkler,
Robert H. Becker,
Terrance J. Gaetz,
Parviz Ghavamian, David J. Helfand,
John P. Hughes,
Robert P. Kirshner,
Kip D. Kuntz,
Emily K. McNeil,
Thomas G. Pannuti,
Paul P. Plucinsky,
Destry Saul,
Ralph Tüllmann,
and Benjamin Williams
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ABSTRACT: M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S II]:Hα ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of ~2 × 1034 erg s-1, we have detected 82 of 137 SNR candidates, yielding confirmation of (or at least strongly support for) their SNR identifications. This provides the largest sample of remnants detected at optical and X-ray wavelengths in any galaxy, including the Milky Way. A spectral analysis of the seven X-ray brightest SNRs reveals that two, G98-31 and G98-35, have spectra that appear to indicate enrichment by ejecta from core-collapse supernova explosions. In general, the X-ray-detected SNRs have soft X-ray spectra compared to the vast majority of sources detected along the line of sight to M33. It is unlikely that there are any other undiscovered thermally dominated X-ray SNRs with luminosities in excess of ~4 × 1035 erg s-1 in the portions of M33 covered by the ChASeM33 survey. We have used a combination of new and archival optical and radio observations to attempt to better understand why some objects are detected as X-ray sources and others are not. We have also developed a morphological classification scheme for the optically identified SNRs and discussed the efficacy of this scheme as a predictor of X-ray detectability. Finally, we have compared the SNRs found in M33 to those that have been observed in the Galaxy and the Magellanic Clouds. There are no close analogs of Cas A, Kepler's SNR, Tycho's SNR, or the Crab Nebula in the regions of M33 surveyed, but we have found an X-ray source with a power-law spectrum coincident with a small-diameter radio source that may be the first pulsar-wind nebula recognized in M33.
The Astrophysical Journal Supplement Series 03/2010; 187(2):495. · 13.46 Impact Factor
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ABSTRACT: We compare the relative merits of AGN selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated optical follow-up are drawn from the SEXSI program. Mid-infrared data in these fields are derived from Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the IRAC color-color AGN `wedge' selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGN by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGN in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGN are detected at X-ray energies in the moderately deep (~100 ks) Chandra data. We hypothesize that the IRAC AGN that lack X-ray detections are predominantly high-luminosity AGN that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2-8 keV band three times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR. Comment: Accepted to ApJ; 20 pages, 6 figures, 6 tables
11/2009;
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ABSTRACT: We have conducted a pilot survey for z>3.5 quasars by combining the FIRST radio survey with the SDSS. While SDSS already targets FIRST sources for spectroscopy as quasar candidates, our survey includes fainter quasars and greatly improves the discovery rate by using strict astrometric criteria for matching the radio and optical positions. Our method allows for selection of high-redshift quasars with less color bias than with optical selection, as using radio selection essentially eliminates stellar contamination. We report the results of spectroscopy for 45 candidates, including 29 quasars in the range 0.37 < z < 5.2, with 7 having redshifts z>3.5. We compare quasars selected using radio and optical criteria, and find that radio-selected quasars have a much higher fraction of moderately-reddened objects. We derive a radio-loud quasar luminosity function at 3.5<z<4.0, and find that it is in good agreement with expectations from prior SDSS results. Comment: 15 pages, 11 figures, accepted for publication in AJ
09/2009;
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ABSTRACT: The FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey now covers 1550 deg2 of sky, where 07h16 α 17h40 and 28°.3 δ 42°. This yields a catalog of 138,665 sources above the survey threshold of 1 mJy, about one-third of which are in double-lobed and multicomponent sources. We have used these data to obtain the first high-significance measurement of the two-point angular correlation for a deep radio sample. We find that the correlation function between 002 and 2° is well fitted by a power law of the form Aθγ, where A 3 × 10–3 and γ -1.1. On small scales (θ < 02), double and multicomponent sources are shown to have a larger clustering amplitude than that of the whole sample. Sources with flux densities below 2 mJy are found to have a shallower slope than that obtained for the whole sample, consistent with there being a significant contribution from starbursting galaxies at these faint fluxes. The cross-correlation of radio sources and Abell clusters is determined. A preliminary approach to inferring spatial information is outlined.
The Astrophysical Journal 01/2009; 473(1):7. · 6.02 Impact Factor
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ABSTRACT: From examination of only 4 deg2 of sky in the NOAO Deep Wide-Field Survey (NDWFS) region, we have identified the first radio-loud quasar at a redshift z > 6. The object, FIRST J1427385+331241, was discovered by matching the FLAMINGOS Extragalactic Survey (FLAMEX) IR survey to Faint Images of the Radio Sky at Twenty cm (FIRST) survey radio sources with NDWFS counterparts. One candidate z > 6 quasar was found, and spectroscopy with the Keck II telescope confirmed its identification yielding a redshift z = 6.12. The object is a broad absorption line (BAL) quasar with an optical luminosity of MB ~ -26.9 and a radio-to-optical flux ratio ~60. Two Mg II absorptions systems are present at redshifts of z = 2.18 and z = 2.20. We briefly discuss the implications of this discovery for the high-redshift quasar population.
The Astrophysical Journal 12/2008; 652(1):157. · 6.02 Impact Factor
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ABSTRACT: Combining radio observations with optical and infrared color selection, demonstrated in our pilot study to be an efficient selection algorithm for finding red quasars, we have obtained optical and infrared spectroscopy for 120 objects in a complete sample of 156 candidates from a sky area of 2716 deg2. Consistent with our initial results, we find that our selection criteria (J - K > 1.7, R - K > 4.0) yield a ~50% success rate for discovering quasars substantially redder than those found in optical surveys. Comparison with UVX- and optical color-selected samples shows that 10% of the quasars are missed in a magnitude-limited survey. Simultaneous two-frequency radio observations for part of the sample indicate that a synchrotron continuum component is ruled out as a significant contributor to reddening the quasars' spectra. We go on to estimate extinctions for our objects assuming that their red colors are caused by dust. Continuum fits and Balmer decrements suggest E(B - V) values ranging from near zero to 2.5 mag. Correcting the K-band magnitudes for these extinctions, we find that for K ≤ 14.0, red quasars make up between 25% and 60% of the underlying quasar population; owing to the incompleteness of 2MASS at fainter K-band magnitudes, we can only set a lower limit to the radio-detected red quasar population of >20%-30%.
The Astrophysical Journal 12/2008; 667(2):673. · 6.02 Impact Factor
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ABSTRACT: We report the discovery of a Lyα-emitting galaxy at redshift z = 6.545 serendipitously identified in the course of spectroscopic follow-up of hard X-ray sources on behalf of the Serendipitous Extragalactic X-Ray Source Identification (SEXSI) survey. The line flux of the galaxy, 2.1 × 10-17 ergs cm-2 s-1, is similar to line fluxes probed by narrowband imaging surveys; the 5.2 arcmin2 surveyed implies a surface density of z ≈ 6.5 Lyα emitters somewhat higher than that inferred from narrowband surveys. This source marks the sixth Lyα-emitting galaxy identified at z ≈ 6.5, a redshift putatively beyond the epoch of reionization when the damping wings of the neutral hydrogen of the intergalactic medium is capable of severely attenuating Lyα emission. By comparing the Lyα luminosity functions at z ≈ 5.7 and z ≈ 6.5, we infer that the intergalactic medium may remain largely reionized from the local universe out to z ≈ 6.5.
The Astrophysical Journal 12/2008; 619(1):12. · 6.02 Impact Factor
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Robert H. Becker,
Richard L. White,
Michael D. Gregg,
Sally A. Laurent-Muehleisen,
Michael S. Brotherton,
Chris D. Impey,
Frederic H. Chaffee,
Gordon T. Richards, David J. Helfand,
Mark Lacy,
Frederic Courbin,
and Deanne D. Proctor
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ABSTRACT: We present the results of an extension of the FIRST Bright Quasar Survey (FBQS) to the South Galactic cap, and to a fainter optical magnitude limit. Radio source counterparts with SERC R magnitudes brighter than 18.9 which meet the other FBQS criteria are included. We supplement this list with a modest number of additional objects to test our completeness for quasars with extended radio morphologies. The survey covers 589 deg2 in two equatorial strips in the southern cap. We have obtained spectra for 86% of the 522 candidates and find 321 radio-selected quasars of which 264 are reported here for the first time. A comparison of this fainter sample with the FBQS sample shows the two to be generally similar. Fourteen new broad absorption line (BAL) quasars are included in this sample. When combined with the previously identified BAL quasars in our earlier papers, we can discern a break in the frequency of BAL quasars with radio loudness, namely that the relative number of high-ionization BAL quasars drops by a factor of 4 for quasars with a radio-loudness parameter R* > 100.
The Astrophysical Journal Supplement Series 12/2008; 135(2):227. · 13.46 Impact Factor
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ABSTRACT: We have obtained single-epoch optical photometry for 202 quasars, taken from the FIRST Bright Quasar Survey, which span a wide range in radio loudness. Comparison with the magnitudes of these objects on the POSS-I plates provides by far the largest sample of long-term variability amplitudes for radio-selected quasars yet produced. We find the quasars to be more variable in the blue than in the red band, consistent with work on optically selected samples. The previously noted trend of decreasing variability with increasing optical luminosity applies only to radio-quiet objects. Furthermore, we do not confirm a rise in variability amplitude with redshift, nor do we see any dependence on radio flux or luminosity. The variability over a radio-optical flux ratio range spanning a factor of 60,000 from radio-quiet to extreme radio-loud objects is largely constant, although there is a suggestion of greater variability in the extreme radio-loud objects. We demonstrate the importance of Malmquist bias in variability studies and develop a procedure to correct for the bias in order to reveal the underlying variability properties of the sample.
The Astronomical Journal 12/2007; 121(4):1872. · 4.03 Impact Factor
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ABSTRACT: Comparison of the VLA FIRST survey with various catalogs of bright stars allows an examination of the prevalence of stellar radio emission independent of optical selection criteria. This FIRST unbiased survey for radio stars covers nearly 5000 deg2 of the northern sky to a flux density limit of 0.7 mJy at 20 cm. Using astrometric catalogs that include proper-motion information, we have detected 26 stellar radio sources, doubling the number of such objects previously known in this region of high-latitude sky. We also show that, in the absence of good proper motions, even the 1'' precision of the FIRST positions is insufficient to avoid crippling chance coincidence rates. We calculate the fraction of radio detections as a function of stellar magnitude and show that, when proper motions from the Guide Star Catalog II become available, the number of stellar radio source detections should increase fourfold.
The Astronomical Journal 12/2007; 117(3):1568. · 4.03 Impact Factor
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ABSTRACT: We present a technique to explore the radio sky into the nanoJansky regime by employing image stacking using the FIRST survey. We first discuss the non-intuitive relationship between the mean and median values of a distribution that is dominated by noise, followed by an analysis of the systematic effects present in FIRST's 20cm VLA snapshot images. Image stacking allows us to recover the properties of source populations with fluxes a factor of 30 or more below the rms noise level. Mean estimates of radio flux density, luminosity, etc., are derivable for any source class having arcsecond positional accuracy. We use this technique to compute the mean radio properties for 41,295 quasars from the SDSS DR3 catalog. There is a tight correlation between optical and radio luminosity, with the radio luminosity increasing as the 0.85 power of optical luminosity. This implies declining radio-loudness with optical luminosity: the most luminous objects (M=-28.5) have average radio-to-optical ratios 3 times lower than the least luminous objects (M=-20). There is also a striking correlation between optical color and radio loudness: quasars that are either redder or bluer than the norm are brighter radio sources, with objects 0.8 magnitudes redder than the SDSS composite spectrum having radio-loudness ratios that are higher by a factor of 10. We explore the longstanding question of whether a radio-loud/radio-quiet dichotomy exists in quasars, finding that optical selection effects probably dominate the distribution function of radio loudness, which has at most a modest (~20%) inflection between the radio-loud and radio-quiet ends of the distribution. We also find, surprisingly, that broad absorption line quasars have higher mean radio flux densities, with the greatest disparity arising in the rare low-ionization BAL subclass.
08/2006;
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ABSTRACT: From examination of only 4 deg^2 of sky in the NOAO Deep Wide-Field Survey
(NDWFS) region, we have identified the first radio-loud quasar at a redshift
z>6. The object, FIRST J1427385+331241, was discovered by matching the FLAMEX
IR survey to FIRST survey radio sources with NDWFS counterparts. One candidate
z>6 quasar was found, and spectroscopy with the Keck II telescope confirmed its
identification, yielding a redshift z=6.12. The object is a Broad Absorption
Line (BAL) quasar with an optical luminosity of M_B ~ -26.9 and a
radio-to-optical flux ratio ~ 60. Two MgII absorptions systems are present at
redshifts of z=2.18 and z=2.20. We briefly discuss the implications of this
discovery for the high-redshift quasar population.
07/2006;
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ABSTRACT: We present the catalog of 477 spectra from the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program, a survey designed to probe the dominant contributors to the 2-10 keV cosmic X-ray background. Our survey covers 1 deg^2 of sky to 2-10 keV fluxes of 10^-14 erg cm^-2 s^-1, and 2 deg^2 for fluxes of 3 x 10^-14 erg cm^-2 s^-1. Our spectra reach to R <24 and have produced redshifts for 438 hard X-ray sources. The vast majority of the 2-10 keV-selected sample are AGN with redshifts between 0.1 and 3. We find that few sources at z<1 have high X-ray luminosities, reflecting a dearth of high-mass, high-accretion-rate sources at low redshift, a result consistent with other recent wide-area surveys. Half of our sources show significant obscuration, with N_H>10^22 cm^-2, independent of unobscured luminosity. We classify 168 sources as emission-line galaxies; all are X-ray luminous objects with optical spectra lacking both high-ionization lines and evidence of a non-stellar continuum. The redshift distribution of these emission-line galaxies peaks at a significantly lower redshift than does that of the sources we spectroscopically identify as AGN. We conclude that few of these sources can be powered by starburst activity. Stacking spectra for a subset of these sources, we detect [Ne V] emission, a clear signature of AGN activity, confirming that the majority of these objects are Seyfert 2s in which the high-ionization lines are diluted by stellar emission. We find 33 objects lacking broad lines in their optical spectra which have quasar X-ray luminosities (Lx>10^44 erg s^-1), the largest sample of such objects identified to date. In addition, we explore 17 AGN associated with galaxy clusters and find that the cluster-member AGN sample has a lower fraction of broad-line AGN than does the background sample. Comment: Accepted to ApJ; 57 pages, 25 figures, 5 tables
03/2006;
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ABSTRACT: We present a near-infrared quasar composite spectrum spanning the wavelength range 0.58 - 3.5 um. The spectrum has been constructed from observations of 27 quasars obtained at the NASA IRTF telescope and satisfying the criteria Ks < 14.5 and Mi < -23; the redshift range is 0.118 < z < 0.418. The signal-to-noise is moderate, reaching a maximum of 150 between 1.6 and 1.9 um. While a power-law fit to the continuum of the composite spectrum requires two breaks, a single power-law slope of alpha=-0.92 plus a 1260 K blackbody provides an excellent description of the spectrum from H-alpha to 3.5 um, strongly suggesting the presence of significant quantities of hot dust in this blue-selected quasar sample. We measure intensities and line widths for ten lines, finding that the Paschen line ratios rule out Case B recombination. We compute K-corrections for the J, H, K, and Spitzer 3.6 um bands which will be useful in analyzing observations of quasars up to z=10. Comment: 39 pages, 11 figures, accepted for publication in ApJ Spectra will be made available in the online version of the journal, once published. Until then, if you wish to obtain the composite spectrum send an email request to eilatg[at]astro.columbia.edu
11/2005;
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ABSTRACT: We present the Multi-Array Galactic Plane Imaging Survey (MAGPIS), which maps portions of the first Galactic quadrant with an angular resolution, sensitivity and dynamic range that surpasses existing radio images of the Milky Way by more than an order of magnitude. The source detection threshold at 20 cm is in the range 1--2 mJy over the 85% of the survey region (5 deg < l < 32 deg, |b| < 0.8 deg) not covered by bright extended emission. We catalog over 3000 discrete sources (diameters mostly <30 arcsec) and present an atlas of ~400 diffuse emission regions. New and archival data at 90 cm for the whole survey area are also presented. Comparison of our catalogs and images with the MSX mid-infrared data allow us to provide preliminary discrimination between thermal and non-thermal sources. We identify 49 high-probability supernova remnant candidates, increasing by a factor of seven the number of known remnants with diameters smaller than 5 arcmin in the survey region; several are pulsar wind nebula candidates and/or very small diameter remnants (D<45 arcsec). We report the tentative identification of several hundred H II regions based on a comparison with the mid-IR data; they range in size from unresolved ultra-compact sources to large complexes of diffuse emission on scales of half a degree. In several of the latter regions, cospatial nonthermal emission illustrates the interplay between stellar death and birth. We comment briefly on plans for followup observations and our extension of the survey; when complemented by data from ongoing X-ray and mid-IR observations, we expect MAGPIS to provide the most complete census yet obtained of the birth and death of massive stars in the Milky Way. Catalogs and images are available on the MAGPIS web site (http://third.ucllnl.org/gps). Comment: 21 pages, 10 figures, 4 tables; submitted to AJ
10/2005;
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ABSTRACT: We discovered independently the shell-type supernova remnant G12.82-0.02, recently reported by Brogan et al. (2005), which is coincident with the unidentified TeV gamma-ray source revealed in the HESS survey of the Galactic plane. Estimating the ambient starlight at the location of this source from the integrated Ly alpha luminosity of the nearby H II region W33, we conclude that inverse Compton emission is a viable explanation for the observed TeV emission. Examining remnants in the survey of Aharonian et al. (2005a) including those detected above 200 Gev and those not detected, we find a strikingly large range of more than three orders of magnitude in the radio to TeV flux ratios. We briefly explore the possible explanations of this range and the implications for the TeV emission mechanism.
06/2005;
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ABSTRACT: Archival data have been combined with recent observations of the Galactic plane using the Very Large Array to create new catalogs of compact centimetric radio sources. The 20 cm source catalog covers a longitude range of -20 deg < l < 120 deg; the latitude coverage varies from +- 0.8 deg to +- 2.7 deg. The total survey area is ~331 sq deg; coverage is 90% complete at a flux density threshold of ~14 mJy, and over 5000 sources are recorded. The 6 cm catalog covers 43 sq deg in the region -10 deg < l < 42 deg, |b| < 0.4 deg to a 90% completeness threshold of 2.9 mJy; over 2700 sources are found. Both surveys have an angular resolution of ~6 arcsec. These catalogs provide a 30% (at 20 cm) to 50% (at 6 cm) increase in the number of high-reliability compact sources in the Galactic plane, as well as providing greatly improved astrometry, uniformity, and reliability; they should prove useful for comparison with new mid- and far-infrared surveys of the Milky Way.
02/2005;
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ABSTRACT: We utilize new VLA Galactic plane catalogs at 5 and 1.4 GHz covering the first Galactic quadrant (350<l<42, |b|<0.4) in conjunction with the MSX6C Galactic plane catalog to construct a large sample of ultra-compact HII regions. A radio catalog of this region was first published by Becker et al. (1994), but we have added new observations and re-reduced the data with significantly improved calibration and mosaicing procedures, resulting in a tripling of the number of 5 GHz sources detected. Comparison of the new 5 GHz catalog and the MSX6C Galactic plane catalog resulted in a sample of 687 matches, out of which we estimate only 15 to be chance coincidences. Most of the matches show red MSX colors and a thermal radio spectrum. The scale height of their Galactic latitude distribution is very small (FWHM of 16' or ~40 pc). These properties suggest that the sample is dominated by young ultra-compact HII regions, most of which are previously uncataloged. High reliability and low reliability source lists are available for download with the paper. Comment: High reliability and low reliability source lists are now available for download with the paper
10/2004;
