Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif

Abstract

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d ≲ 10 pc) anomalously dense (ne ∼ 102 cm−3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (η UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10 μas, yielding an apparent brightness temperature for an isotropic source of ≳1014 K.

Apertif: Phased array feeds for the Westerbork Synthesis Radio Telescope

“…Apart from the science planned for Apertif, its large FoV implies that the potential for detecting rare objects or making serendipitous discoveries is high. This is illustrated by the fact that in the survey data taken so far, 16 intra-hour variable radio sources were found serendipitously (Oosterloo et al 2020). Such variable sources are among the rarest objects on the sky.…”

Apertif, Phased Array Feeds for the Westerbork Synthesis Radio Telescope

“…Apart from the science planned for Apertif, its large field of view implies that the potential for detecting rare objects or making serendipitous discoveries is high. This is illustrated by the fact that in the survey data taken so far, 16 intra-hour variable radio sources were found serendipitously (Oosterloo et al 2020). Such variable sources are among the rarest objects on the sky.…”

ASKAP observations of multiple rapid scintillators reveal a degrees-long plasma filament

“…As the velocity of the Earth changes through the year, the rate of scintillation changes. This annual modulation, or annual cycle, of the scintillation rate was instrumental in establishing the scintillation nature of the IHV phenomenon (Jauncey et al 2000;Rickett et al 2001) and can be used to determine the velocity of the screen along with the characteristic scale, degree of anisotropy and orientation of the scintillation pattern (e.g., Jauncey & Macquart 2001;Dennett-Thorpe & de Bruyn 2003;Oosterloo et al 2020).…”

“…The method, described in detail in Bignall et al (2019), is designed to allow quantitative inference on the scintillation rate for epochs near standstills (when the Earth velocity is close to that of the screen) where traditional ACF HWHM estimates struggle due to very slow variations. It has been shown to produce results that closely follow the traditional ACF analysis on fast epochs, where comparison is possible (Oosterloo et al 2020). Unlike Bignall et al (2019), we used single light curves (per epoch, per source) due to observed relatively broadband nature of the scintillation relative to the ASKAP bandwidth.…”

“…Kedziora-Chudczer et al (1997) found the first extreme IHV, PKS 0405-385, in the southern IDV survey, among monitoring of 125 selected bright, flat-spectrum AGN with the Australia Telescope Compact Array (ATCA). Other extreme IHVs such as J1819+3845 (Dennett-Thorpe & de Bruyn 2000), PKS 1257−326 (Bignall et al 2003), and recently J1402+5347 (Oosterloo et al 2020) were discovered serendipitously. Several IDV/IHV surveys have been conducted (Rickett et al 1984;Kedziora-Chudczer et al 2001;Koay et al 2019) including the large-scale microarcsecond scintillation-induced variability survey (MASIV; Lovell et al 2003), but no other extreme variables have been identified.…”