Radio variable sources at 1400 MHz and their optical variability

Abstract

In the present study we have cross-correlated NVSS and FIRST radio catalogues having radio flux measurements at the same 1.4 GHz frequency. This way we benefit from repeated observations from both catalogues, as they give more accurate positions and fluxes and more important, reveal large differences between the two measured fluxes, thus allowing to establish radio variability. As a result 79,382 radio variables have been revealed, including 6301 with flux differences at 1.4 GHz larger than 15 mJy, 1917 with flux differences $>$ 45 mJy and 260 with flux differences $>$ 200 mJy. By using a special technique (Mickaelian & Sinamyan 2010, Mickaelian et al. 2011), 2425 optically variable objects out of 6301 radio sources have been revealed. 2425 radio sources with both high radio and optical variability into four categories have been divided. 1206 (19%) out of 6301 radio sources have activity types from available catalogues and 619 (25.5%) out of 2425 radio sources with at the same time radio and optical variability have activity types from available catalogues. In addition, 279 radio sources out of 2425 have high variability in optical range. We have established their activity types when available. The IR fluxes and colours for the 6301 variable radio sources have been studied. Colour–colour diagrams show that most of the “unknown” sources are galaxies. The activity types for 110 (42%) out of 260 extremely high variable radio sources also have been retrieved.

Introduction

Many active galaxies, especially active galactic nuclei (AGN), are strong in radio wavelengths (e.g. many objects in Véron-Cetty and Véron, 2010 have strong radio; the catalogue of Blazars by Massaro et al., 2015 is complied exclusively from objects having radio detection); hence studying radio emission from galaxies may be a key to identify the active ones among them. Radio galaxies, quasars, blazars, megamasers and other classes of objects are strong radio emitters. Radio galaxies and their relatives, radio-loud quasars and blazars, are types of AGN that are very luminous at radio wavelengths, with luminosities up to 10³⁹ W in the range of 10 MHz to 100 GHz. This radio emission is due to the synchrotron process; the observed radio structure is determined by the interaction between two opposite jets and the external medium, affected by relativistic beaming. The host galaxies are almost exclusively giant elliptical galaxies and radio galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work was done related to the effects of these objects on the intergalactic medium as well, particularly in galaxy groups and clusters.

For understanding some morphological (optical and radio) and physical properties of AGN and Starbursts (SB) investigations of active galaxies in radio wavelengths have been carried out. To have large sky coverage, we jointly use the two biggest radio catalogues, NVSS and FIRST, having measurements at the same radio frequency, 1.4 GHz and giving opportunity of repeated observations for achieving better positions and fluxes, as well as investigating radio variability.

The NRAO VLA Sky Survey (NVSS) is a radio continuum survey covering the sky north of −40° declination over 33,827 deg² (82% of the celestial sphere) at 1.4 GHz (centred on 21 cm HI line). VLA (Very Large Array) is a facility of the National Radio Astronomical Observatory (NRAO). The principal data products of the NVSS are a set of 2326 4 $\times$ 4° continuum “cubes” with three planes containing Stokes I, Q, and U images, plus a catalogue of almost 2 million discrete sources stronger than a flux density of about 2.5 mJy. The images all have 45${}^{″}$ FWHM angular resolution and nearly uniform sensitivity. Their RMS brightness fluctuations are approximately 0.45 mJy/beam $=$ 0.14 K (Stokes I) and 0.29 mJy/beam $=$ 0.09 K (Stokes Q and U). The RMS uncertainties in right ascension and declination vary from $\le \sim$1${}^{″}$ for the 400,000 sources stronger than 15 mJy to 7${}^{″}$ at the survey limit. The NVSS catalogue contains 1,773,484 sources with a source density at 52.4 sources/deg² (Condon et al., 1998).

The Faint Images of the Radio Sky at Twenty centimeters (FIRST) is based on observations made between 1993 and 2014. It also used the VLA at a frequency of 1.4 GHz (21 cm), and it covers 10,575 deg² of the sky (8444 deg² near the North Galactic Cap and 2131 deg² near the South Galactic Cap). FIRST goes down to a sensitivity of about 1 mJy with an angular resolution of about 5${}^{″}$. The images produced by an automated mapping pipeline have pixels of 1.8${}^{″}$, a typical RMS of 0.15 mJy, and a resolution of 5${}^{″}$. The source catalogue is derived from the images. Over most of the survey area, the detection limit is 1 mJy. A region along the equatorial strip (RA $=$ [21.3 ${}^h$,3.3 ${}^h$], Dec $=$ [-1°,1°]) has a deeper detection threshold because two epochs of observation were combined. The typical detection threshold in this region is 0.75 mJy. There are approximately 4500 sources below the 1 mJy threshold used for most previous versions of the catalogue. The FIRST includes 946,432 sources (Helfand et al., 2015).

Ofek and Frail (2011) investigated radio variability in NVSS and FIRST surveys. In this work authors investigated radio sources from FIRST and NVSS epoch. They have found 43 variable sources (0.1% of the sources) that vary by more than 4$\sigma$, and they constructed the mean structure function of these objects. This enabled them to explore radio variability on timescales between several months and about five years. This study concerns individual sources that may also be undergone follow-up studies after having our candidate radio variables. Variable radio sources are thought to be among the most interesting cosmic objects. A number of authors have studied the variability of radio sources, however mainly using observations of the given sources. We have attempted to derive variable radio sources at 1400 MHz by statistical method, investigate also their optical variability and understand if the radio variability has any correlation with the optical one. Our aim is to match NVSS and FIRST catalogues and to reveal common sources, thus having possibility to investigate the radio variability at 1.4 GHz and study their physical nature. After understanding physical nature of radio variability we plan to do multiwavelength investigation for these sources.

The current paper is organized in the following sections: cross-matching of NVSS and FIRST radio catalogues and construction of the sample of radio variable sources, optical variability of radio variable sources (we carry out a detailed optical photometry analysis and reveal many optically variable objects), the results of the variability analysis and extremely variable radio sources and summary and conclusions.