THE SECOND<i>GALEX</i>ULTRAVIOLET VARIABILITY (GUVV-2) CATALOG

Wheatley, Jonathan; Welsh, Barry Y.; Browne, S. E.

doi:10.1088/0004-6256/136/1/259

Cited by 35 publications

(37 citation statements)

References 21 publications

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“…The largest group of known variable sources listed in the GALEX UV Variability-2 catalog are active galaxies (Wheatley et al 2008). This sample of sources was derived solely through selection due to their observed UV variability, without prior knowledge that such sources were in fact AGN.…”

Section: Introductionmentioning

confidence: 99%

GALEX observations of quasar variability in the ultraviolet

Welsh

Wheatley

Neil

2011

A&A

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Aims. Using archival observations recorded over a 5+ year timeframe with the NASA GALaxy Evolution eXplorer (GALEX) satellite, we present a study of the ultraviolet (UV) variability of 4360 quasars of redshifts up to z = 2.5 that have optical counterparts in the Sloan Digital Sky Survey DR5 spectroscopic catalog of Schneider et al. (2007, AJ, 134, 102). The observed changes in both the far UV (FUV: 1350−1785 Å) and near UV (NUV: 1770−2830 Å) AB magnitudes as a function of time may help differentiate between models of the emission mechanisms thought to operate in these active galaxies. Methods. A list of NUV and FUV variable quasars was derived from the UV light-curves of sources with 5 or more observational visits by GALEX that spanned a time-frame >3 months. By measuring the error in the derived mean UV magnitude from the series of GALEX observations for each source, quasars whose UV variability was greater than the 3-σ variance from the mean observed value were deemed to be (intrinsically) UV variable. This conservative selection criterion (which was applied to both FUV and NUV observations) resulted in identifying 550 NUV and 371 FUV quasars as being statistically significant UV variable objects. Results. Following the work of Vanden Berk et al. (2004, ApJ, 601, 692), we have performed a structure function (SF) analysis of these data to search for possible correlations between UV variability and parameters such as rest frame time-lag, redshift, luminosity and radio loudness. Firstly, we observe that the amplitudes of variability as a function of time-lag for both the NUV and FUV data are far larger than those observed at visible wavelengths. Secondly, the levels of FUV variability are greater than those observed in the NUV for a given value of time-lag. Also, the amplitudes of both the NUV and FUV variability of quasars increase as a function of rest frame time-lag, irrespective of the value of quasar redshift, for time-lags <200 days. For time-lags >300 days there is a pronounced rollover in the NUV SF for all redshift values, which is also observed (with a lower signiÞcance) in the FUV variability data. Although we find no strong relationship between UV variability and redshift, our data do show that higher redshift quasars appear to be more variable than their low redshift counterparts. Our data also show that, for all values of time-lag, the more luminous quasars tend to be slightly less UV variable, with perhaps the exception of FUV variable quasars for short time-lags. Although our data sample is small, we find that radio-loud quasars are marginally more variable than radio quiet ones by a factor ∼2 in the NUV and by a factor 1−3 in the FUV. Therefore, our present observations support the notion in which the radio properties of quasars have a limited influence on the observed UV variability of these objects. In summation, our present analysis favors a quasar model in which UV variability is mainly due to stochastic changes in the underlying continuum level, rather than models that favor gravitat...

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Section: Introductionmentioning

confidence: 99%

GALEX observations of quasar variability in the ultraviolet

Welsh

Wheatley

Neil

2011

A&A

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“…To determine the feasibility of using NUTS for temporal variability studies, we also looked at previous observations of transient events in archival data, particularly from the GALEX mission. We made use of the observations from the variability catalogs (Welsh et al, 2005;Wheatley et al, 2008) and time domain survey (Gezari et al, 2013), as well as additional catalogs of novae outbursts (Wils et al, 2010) and M dwarf flares (Welsh et al, 2007). In addition, there have been new archival studies of short timescale UV variability (de la Vega & Bianchi, 2018;Brasseur et al, 2019) using the gPhoton package (Million et al, 2016).…”

Section: Performance Estimatesmentioning

confidence: 99%

The Near Ultraviolet Transient Surveyor (NUTS): An ultraviolet telescope to observe variable sources

Ambily,

Sarpotdar,

Mathew

et al. 2022

Preprint

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Observing the ultraviolet (UV) sky for timevaryiable phenomena is one of the many exciting science goals that can be achieved by a relatively small aperture telescope in space. The Near Ultraviolet Transient Surveyor (NUTS) is a wide-field (3 • ) imager with a photoncounting detector in the near-UV (NUV, 200 -300 nm), to be flown on an upcoming small satellite mission. It has a Ritchey-Chrétien (RC) telescope design with correction optics to enable wide-field observations while

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“…The system was categorized as a high-amplitude variable in the second version of the GALEX Ultraviolet Variability (GUVV-2) Catalog (Wheatley et al 2008), where a maximum NUV flux difference of two magnitudes was identified within the available visits at that time. Welsh et al (2006) studied one of CR Draconis' flare events with high temporal sampling by extracting light curves from sky-projected, "extended" (-x) photon list files, produced as non-standard products of the GALEX mission pipeline.…”

Section: Example Science Application -Stellar Flares From Cr Draconismentioning

confidence: 99%

gPhoton: THE GALEX PHOTON DATA ARCHIVE

Million¹,

Fleming

Shiao

et al. 2016

ApJ

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ABSTRACTgPhoton is a new database product and software package that enables analysis of GALEX ultraviolet data at the photon level. The project's stand-alone, pure-Python calibration pipeline reproduces the functionality of the original mission pipeline to reduce raw spacecraft data to lists of time-tagged, skyprojected photons, which are then hosted in a publicly available database by the Mikulski Archive at Space Telescope (MAST). This database contains approximately 130 terabytes of data describing approximately 1.1 trillion sky-projected events with a timestamp resolution of five milliseconds. A handful of Python and command line modules serve as a front-end to interact with the database and to generate calibrated light curves and images from the photon-level data at user-defined temporal and spatial scales. The gPhoton software and source code are in active development and publicly available under a permissive license. We describe the motivation, design, and implementation of the calibration pipeline, database, and tools, with emphasis on divergence from prior work, as well as challenges created by the large data volume. We summarize the astrometric and photometric performance of gPhoton relative to the original mission pipeline. For a brief example of short time domain science capabilities enabled by gPhoton, we show new flares from the known M dwarf flare star CR Draconis. The gPhoton software has permanent object identifiers with the ASCL (ascl:1603.004) and DOI (doi:10.17909/T9CC7G). This paper describes the software as of version v1.27.2.

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THE SECONDGALEXULTRAVIOLET VARIABILITY (GUVV-2) CATALOG

Cited by 35 publications

References 21 publications

GALEX observations of quasar variability in the ultraviolet

GALEX observations of quasar variability in the ultraviolet

The Near Ultraviolet Transient Surveyor (NUTS): An ultraviolet telescope to observe variable sources

gPhoton: THE GALEX PHOTON DATA ARCHIVE

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