J/ApJS/238/25 UV variability with GALEX gPhoton archive. I. (de la Vega+, 2018)
Searching for short-timescale variability in the ultraviolet with the
GALEX gPhoton archive.
I. Artifacts and spurious periodicities.
de la Vega A., Bianchi L.
<Astrophys. J. Suppl. Ser., 238, 25 (2018)>
=2018ApJS..238...25D 2018ApJS..238...25D
ADC_Keywords: Photometry, ultraviolet; Cross identifications; Stars, variable
Keywords: novae, cataclysmic variables ;
stars: oscillations (including pulsations) ;
stars: variables: general ; ultraviolet: stars ; white dwarfs
Abstract:
In order to develop and test a methodology to search for UV
variability over the entire Galaxy Evolution Explorer (GALEX) database
down to the shortest timescales, we analyzed time-domain photometry of
∼5000 light curves of ∼300 bright (mFUV, mNUV≤14) and blue
(mFUV-mNUV<0) GALEX sources. Using the gPhoton database tool, we
discovered and characterized instrumentally induced variabilities in
time-resolved GALEX photometry that may severely impact automated
searches for short-period variations. The most notable artifact is a
quasi-sinusoidal variation mimicking light curves typical of
pulsators, seen occasionally in either one or both detectors, with
amplitudes of up to 0.3mag and periods corresponding to the
periodicity of the spiral dithering pattern used during the
observation (P∼120s). Therefore, the artifact may arise from
small-scale response variations. Other artifacts include visit-long
"sagging" or "hump" in flux, occurring when the dithering pattern is
not a spiral, or a one-time change in flux level during the exposure.
These instrumentally caused variations were not reported before, and
are not due to known (and flagged) artifacts such as hotspots, which
can be easily eliminated. To characterize the frequency and causality
of such artifacts, we apply Fourier transform analysis to both light
curves and dithering patterns, and examine whether artificial
brightness variations correlate with visit or instrumental parameters.
Artifacts do not correlate with source position on the detector. We
suggest methods to identify artifact variations and to correct them
when possible.
Description:
The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer orbiting
observatory, surveyed the sky in the ultraviolet (UV) from 2003 to
2013. Two microchannel plate (MCP) photon-counting detectors, one in
the far-UV (FUV, range 1350-1750Å) and one in the near-UV (NUV,
range 1750-2750Å), each with a 1.25 degree field-of-view (FOV),
recorded cascades of electrical signals (known as "events") from
photons landing on the MCPs with a time resolution of 5 milliseconds.
Recently, Million+ (2016ApJ...833..292M 2016ApJ...833..292M) released the first
database tool enabling time-resolved GALEX photometry, gPhoton. This
paper presents the first comprehensive analysis of the short-term
variability detection capabilities using gPhoton.
See section 1 for further explanations.
Our sample of bright stars is extracted from the General Release 6 and
7 (GR6+7) GALEX merged catalog (MCAT). See section 2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 137 304 The sources of the GALEX (GR6+7) merged catalog
(MCAT) culled sample
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See also:
II/312 : GALEX-DR5 (GR5) sources from AIS and MIS (Bianchi+ 2011)
II/335 : Revised catalog of GALEX UV sources (GUVcat_AIS GR6+7) (Bianchi+ 2017)
J/ApJS/173/673 : M dwarf UV flares in GALEX (Welsh+, 2007)
J/AJ/136/259 : Second GALEX UV variability catalog (GUVV-2) (Wheatley+, 2008)
J/ApJ/766/60 : GALEX Time Domain Survey I. UV variable sources (Gezari+, 2013)
J/ApJ/845/171 : Rapidly pulsating sdB stars with GALEX (Boudreaux+, 2017)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- objid GALEX objid identifier
21- 30 F10.6 deg RAdeg Right Ascension in decimal degrees (J2000)
32- 41 F10.6 deg DEdeg Declination in decimal degrees (J2000)
43- 48 F6.3 mag FUVmag [10.6/14] Apparent FUV band AB magnitude
50- 54 F5.3 mag e_FUVmag [0/0.004] Uncertainty in FUVmag
56- 61 F6.3 mag NUVmag [10.8/14] Apparent NUV band AB magnitude
63- 67 F5.3 mag e_NUVmag [0/0.003] Uncertainty in NUVmag
69- 77 F9.3 s FUVexp [502/67899] Total FUV band exposure time
79- 87 F9.3 s NUVexp [502/77766] Total NUV band exposure time
89- 91 I3 --- nFUV [1/132] Number of FUV band visits
93- 95 I3 --- nNUV [1/170] Number of NUV band visits
97-121 A25 --- ID SIMBAD identifier
123-129 F7.3 mag delFUV [0/9.5]?=-99 Total FUV band magnitude change (1)
131-137 F7.3 mag delNUV [0/2.1]?=-99 Total NUV band magnitude change (1)
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Note (1): Maximum variation in brightness, Δmag, defined in Section 4.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 30-Nov-2018