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 deg 2 . Correcting for various selection effects and for the extragalactic variable population of active galactic nuclei, we conclude there are ∼ 1.6 Galactic sources deg −2 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.

A search for variable and transient radio sources in the extended Chandra Deep Field South at 5.5 GHz

“…On the timescales of days to 15 years, Becker et al (2010) examined the variability of sources towards the Galactic plane in the flux density range 1−100 mJy. A surface density of 1.6 deg −2 variables was reported (8% of the sample).…”

“…At ∼5 GHz, the deepest variability survey to date is that of Becker et al (2010), which probed a flux density greater than one mJy, an order of magnitude higher in flux than that of the Carilli et al (2003) survey. The Becker et al (2010) survey showed that the surface density of variable sources towards the Galactic Plane above one mJy is ∼1.6 deg −2 on timescales of days to years.…”

“…The Becker et al (2010) survey showed that the surface density of variable sources towards the Galactic Plane above one mJy is ∼1.6 deg −2 on timescales of days to years. In comparison Gregory & Taylor (1986) showed that a much stricter limit of ∼ 10 −3 deg −2 is found above 70 mJy on timescales of days and years.…”

VAST: An ASKAP Survey for Variables and Slow Transients

“…They found that 2% of the sources within their sample were highly variable (defined as S ≥ 50%) and hence derived a rate of ρ = 18 deg −2 . Also in search of highly variable radio sources, Becker et al (2010) surveyed the Galactic plane and found ρ = 1.6 deg −2 above a sensitivity of 1 mJy at 4.8 GHz. At low Galactic latitude and sampling a multitude of timescales (days to years), Ofek et al (2011) used the VLA to study transient and variables in a field covering 2.66 deg 2 to a sensitivity limit of ∼100 μJy at 5 GHz.…”

Density of warm ionized gas near the Galactic center: low radio frequency observations

“…While the possibility of some of these to be members of new types of Galactic sources cannot be ruled out, but Becker et al (2010) finds Galactic variable sources to have either a flat or inverted spectrum. Except one source, there is no other source whose spectral index is flatter than −0.3, and hence they are unlikely to be variable Galactic sources.…”