Variable Radio Sources in the Galactic Plane

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.

11 pages, 7 figures; to be published in the Astronomical Journal; data available on MAGPIS website at http://third.ucllnl.org/gps/

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• A radio spectral index map and catalogue at 147-1400 MHz covering 80 per cent of the sky
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• A VLA search for 5 GHz radio transients and variables at low Galactic latitudes
• A 22-year Southern Sky Survey for Transient and Variable Radio Sources using the Molonglo Observatory Synthesis Telescope
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• AMI Galactic Plane Survey at 16 GHz: I -- Observing, mapping and source extraction
• GLOSTAR -- Radio Source Catalog I: $28^{\circ }<\text{l}<36^{\circ }$ and $|b|<1^{\circ }$
• The structure function of variable 1.4 GHz radio sources based on NVSS and FIRST observations
• The Allen Telescope Array Fly's Eye Survey for Fast Radio Transients
• An automated archival VLA transients survey
• A Search for Radio Transients in VLA Archival Images of the 3C 286 Field
• The Allen Telescope Array Pi GHz Sky Survey I. Survey Description and Static Catalog Results for the Bootes Field
• A search for variable and transient radio sources in the extended Chandra Deep Field South at 5.5 GHz
• The Allen Telescope Array Pi GHz Sky Survey II. Daily and Monthly Monitoring for Transients and Variability in the Bootes Field
• ASGARD: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results
• The Allen Telescope Array Pi GHz Sky Survey - III: The ELAIS-N1, Coma, and Lockman Hole Fields
• An insight into the extragalactic transient and variable microJy radio sky across multiple decades
• Millijansky Radio Variability in SDSS Stripe 82
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• A multiple-beam CLEAN for imaging intra-day variable radio sources
• Observations of Galactic star-forming regions with the Cosmic Background Imager at 31 GHz
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• Structural and spectral properties of Galactic plane variable radio sources
• Features and prospects for kilonova remnant detection with current and future surveys
• Astrometry of variable compact radio sources: A search for Galactic black hole X-ray binaries