J/ApJ/769/125 1.4GHz radio variability in FIRST & SDSS Stripe 82 (Hodge+, 2013)
Millijansky radio variability in SDSS stripe 82.
Hodge J.A., Becker R.H., White R.L., Richards G.T.
<Astrophys. J., 769, 125 (2013)>
=2013ApJ...769..125H 2013ApJ...769..125H
ADC_Keywords: Galaxies, radio ; QSOs ; Surveys
Keywords: quasars: general; radio continuum: galaxies; surveys
Abstract:
We report on a blind survey for extragalactic radio variability that
was carried out by comparing two epochs of data from the Faint Images
of the Radio Sky at Twenty centimeters survey with a third epoch from
a new 1.4GHz survey of SDSS Stripe 82. The three epochs are spaced
seven years apart and have an overlapping area of 60deg2. We uncover
89 variable sources down to the millijansky level, 75 of which are
newly identified, and we find no evidence for transient phenomena.
This new sample of variable sources allows us to infer an upper limit
to the mean characteristic timescale of active galactic nucleus radio
variability of 14yr. We find that only 1% of extragalactic sources
have fractional variability fvar>3, while 44% of Galactic sources
vary by this much. The variable sample contains a larger fraction of
quasars than a comparable non-variable control sample, though the
majority of the variable sources appear to be extended galaxies in the
optical. This implies that either quasars are not the dominant
contributor to the variability of the sample, or that the deep optical
data allow us to detect the host galaxies of some low-z quasars. We
use the new, higher resolution data to report on the morphology of the
variable sources. Finally, we show that the fraction of sources that
are variable remains constant or increases at low flux densities. This
may imply that next generation radio surveys with telescopes like
Australian Square Kilometer Array Pathfinder and MeerKAT will see a
constant or even increasing fraction of variable sources down into the
sub-millijansky regime.
Description:
We used three different epochs of radio data to conduct our study of
radio variability.
The first two epochs (1995 and 2002) come from the FIRST survey
(Becker et al. 1995, see VIII/90). The FIRST survey was conducted with
the VLA at 1.4GHz and covered >9000deg2 in the north and south
Galactic caps. The survey utilized the VLA's B-configuration, giving
it a resolution of 5".
The third epoch of radio data comes from a recent high-resolution VLA
survey of SDSS Stripe 82 (Hodge et al. 2011AJ....142....3H 2011AJ....142....3H). This
1.4GHz survey covered 92deg2 to a typical rms of 52uJy. The
resolution achieved was 1.8", or roughly three times better than
FIRST. These data constitute Epoch III and may sometimes be referred
to as the "Stripe 82" radio data.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 103 89 Epoch III (2007-2009) variable sources
table2.dat 104 58 Epoch I (1995) and II (2002) variable sources
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See also:
VIII/90 : The FIRST Survey Catalog, Version 12Feb16 (Becker+ 2012)
VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010)
VIII/87 : The Allen Telescope Array 20cm Survey (ATATS). I. (Croft+ 2010)
J/ApJ/742/49 : Variable radio sources in FIRST (Thyagarajan+, 2011)
J/ApJ/740/65 : VLA search for 5GHz radio transients (Ofek+, 2011)
J/ApJ/739/76 : Pi GHz Sky Survey (PiGSS). II. (Bower+, 2011)
J/ApJ/737/45 : Variable 1.4GHz radio sources from NVSS & FIRST (Ofek+, 2011)
J/ApJ/714/1305 : The Deep SWIRE Field (DSF) IV. (Strazzullo+, 2010)
J/ApJS/180/67 : Phot. selection of quasars from SDSS. II. (Richards+, 2009)
J/MNRAS/397/281 : Radio imaging in Lockman Hole (Ibar+, 2009)
J/ApJ/689/108 : MASIV survey. II. First four epochs (Lovell+, 2008)
J/ApJ/648/629 : Radio obs. of late M, L, and T dwarfs (Berger+, 2006)
J/ApJS/155/257 : NBC Quasar Candidate Catalog (Richards+, 2004)
J/AJ/127/2565 : Faint FIRST variable radio sources (de Vries+, 2004)
http://sundog.stsci.edu/ : VLA FIRST survey home page
Byte-by-byte Description of file: table[12].dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Hour of right ascension (J2000)
4- 5 I2 min RAm Minute of right ascension (J2000)
7- 12 F6.3 s RAs Second of right ascension (J2000)
14 A1 --- DE- Sign of declination (J2000)
15- 16 I2 deg DEd Degree of declination (J2000)
18- 19 I2 arcmin DEm Arcminute of declination (J2000)
21- 25 F5.2 arcsec DEs Arcsecond of declination (J2000)
27- 33 F7.2 mJy Spk1 [1/1482]? 1.4GHz peak flux density of Epoch I
35- 41 F7.2 mJy Sint1 [0.6/1525] Integrated flux density of Epoch I
43- 45 I3 uJy rms1 [131/778] rms of Epoch I
47- 53 F7.2 mJy Spk2 [1.2/1350]? 1.4GHz peak flux density of Epoch II
55- 61 F7.2 mJy Sint2 [0.5/1361] Integrated flux density of Epoch II
63- 65 I3 uJy rms2 [130/250] rms of Epoch II
67- 73 F7.2 mJy Spk3 [1/1129]? 1.4GHz peak flux density of Epoch III
75- 81 F7.2 mJy Sint3 [0.2/1242] Integrated flux density of Epoch III
83- 87 I5 uJy rms3 [48/11014]? rms of Epoch III
89- 92 F4.2 arcsec Maj [0.09/4] Deconvolved major axis value measured
from the Epoch III data
94- 97 F4.2 --- fvar [1/3.2]? variability amplitude fvar (1)
99-100 A2 --- MF Morphological classification in FIRST (2)
102-104 A3 --- MS Morphological classification in Stripe 82 (2)
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Note (1): Fractional variability is defined as fvar=Smax/Smin
(Equation (2)) where S=Spk for the FIRST epochs I/II and
S=Sint for Stripe 82 Epoch III. See section 4.2.
Note (2): We follow de Vries et al. (2004, J/AJ/127/2565) and define the
following six morphological classes (more details in section 4.5):
PS = Isolated point source;
CJ = "core-jet" morphology, either as two separate components or as a single
component with a clear elongation;
CL = "core-lobe" morphology, where the core is surrounded by two distinct
lobe components, which are not variable;
CH = "core-halo" morphology, consisting of a point source core surrounded by
a diffuse radio halo;
CX = complex source, consisting of multiple components;
CX? = Same as CX but less certain
HS = hot-spot variability.
See section 4.5 for further explanations.
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History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 17-Dec-2014