ALBERT

Description: We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at redshift z  ~ 1.3 that exhibits large-scale extended, nearly co-spatial, radio and X-ray emission from radio lobes, but no detection of Active Galactic Nuclei core, jets and hotspots. The total angular extent of the GRG at the observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is found to be ~2.4 arcmin, that corresponds to a total projected linear size of ~1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz shows high spectral curvature ( ${\alpha }_{\rm 0.610 \,GHz}^{\rm 1.4 \,GHz}$ – ${\alpha }_{\rm 0.240 \,GHz}^{\rm 0.325 \,GHz}$  〉 1.19) with sharp steepening above 0.325 GHz, consistent with relic radio emission that is ~8  x  10 6 yr old. The radio spectral index map between observed frame 0.325 and 1.4 GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from outer-end to inner-end, indicating backflow of plasma in the lobes. The extended X-ray emission characterized by an absorbed power law with photon index ~1.86 favours inverse-Compton scattering of the Cosmic Microwave Background (ICCMB) photons as the plausible origin. Using both X-ray and radio fluxes under the assumption of ICCMB we estimate the magnetic field in the lobes to be 3.3 μG. The magnetic field estimate based on energy equipartition is ~3.5 μG. Our work presents a case study of a rare example of a GRG caught in dying phase in the distant Universe.

Description: We present the Giant Metrewave Radio Telescope 240 MHz observations of the nearby luminous Fanaroff–Riley type I radio source 3C 270, in the group-central elliptical NGC 4261. Combining these data with reprocessed Very Large Array 1.55 and 4.8 GHz observations, we produce spectral-index maps that reveal a constant spectral index along the jets and a gradual steepening from the ends of the jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted to the integrated spectrum of the source gives an asymptotic low-frequency index of $\alpha _{\rm {inj}}=0.53_{-0.02}^{+0.01}$ , while JP models fitted to the observed spectral index trend along the lobes allow us to estimate radiative ages of ~29 and ~37 Myr for the west and east lobes, respectively. Our age estimates are a factor of 2 lower than the 75-Myr upper limit derived from X-ray data. We find unlikely the scenario of an early supersonic phase in which the lobe expanded into the ISM at approximately Mach 6 (3500 km s –1 ), and suggest that either the source underwent multiple active galactic nuclei outbursts with possible large changes in jet power, or possibly that the source age that we find is due to a backflow that transports young electrons from the jet tips through the lobes towards the nucleus relatively quickly. We calculate that in the lobes the energy ratio of non-radiating to radiating particles is ~4–24 indicating significant gas entrainment. If the lobes are in pressure balance with their surroundings, the total energy required to heat the entrained material is 10 58  erg, ~40 per cent of the total enthalpy of the lobes.

Description: LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbital cycle of 26.5 d and a superorbital modulation with a period of 4.6 yr. We have performed a detailed study of the low-frequency radio emission of LS I +61 303 by analysing all the archival Giant Metrewave Radio Telescope data at 150, 235 and 610 MHz, and conducting regular LOw Frequency ARray observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency. We have obtained the light curves of the source at 150, 235 and 610 MHz, all of them showing orbital modulation. The light curves at 235 and 610 MHz also show the existence of superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkable differences with these light curves. At 15 GHz we see clear outbursts, whereas at low frequencies we see variability with wide maxima. The light curve at 235 MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of ~0.5 orbital phases in the maxima. We model the shifts between the maxima at different frequencies as due to the expansion of a one-zone emitting region assuming either free–free absorption or synchrotron self-absorption with two different magnetic field dependences. We always obtain a subrelativistic expansion velocity, in some cases being close to the stellar wind one.

Description: We have studied in detail the 0.15–15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical properties of its emission. We have analysed two archival Very Large Array monitorings, all the available archival Giant Metrewave Radio Telescope (GMRT) data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with Giant Metrewave Radio Telescope and Westerbork Synthesis Radio Telescope. The data show that the radio emission of LS 5039 is persistent on day, week and year time-scales, with a variability 25 per cent at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at ~0.5GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for the size of the emitting region of ~0.85 mas, setting a magnetic field of B  ~ 20 mG, an electron density of n e  ~ 4  x 10 5 cm –3 and a mass-loss rate of $\dot{M}\sim 5\times 10^{-8} \,\mathrm{M_{\odot }\, yr^{-1}}$ . These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for the first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.

Description: We have studied in detail the 0.15–15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical properties of its emission. We have analysed two archival Very Large Array monitorings, all the available archival Giant Metrewave Radio Telescope (GMRT) data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with Giant Metrewave Radio Telescope and Westerbork Synthesis Radio Telescope. The data show that the radio emission of LS 5039 is persistent on day, week and year time-scales, with a variability 25 per cent at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at ~0.5GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for the size of the emitting region of ~0.85 mas, setting a magnetic field of B  ~ 20 mG, an electron density of n e  ~ 4  x 10 5 cm –3 and a mass-loss rate of $\dot{M}\sim 5\times 10^{-8} \,\mathrm{M_{\odot }\, yr^{-1}}$ . These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for the first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.

Description: Double-lobe radio galaxies in the local Universe have traditionally been found to be hosted in elliptical or lenticular galaxies. We report the discovery of four spiral-host double-lobe radio galaxies (J0836+0532, J1159+5820, J1352+3126, and J1649+2635) that are discovered by cross-matching a large sample of 187 005 spiral galaxies from SDSS DR7 (Sloan Digital Sky Survey Data Release 7) to the full catalogues of FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAO VLA Sky Survey). J0836+0532 is reported for the first time. The host galaxies are forming stars at an average rate of 1.7–10 M  yr –1 and possess supermassive black holes (SMBHs) with masses of a few times 10 8  M . Their radio morphologies are similar to Fanaroff–Riley type II radio galaxies with total projected linear sizes ranging from 86 to 420 kpc, but their total 1.4-GHz radio luminosities are only in the range 10 24 –10 25  W Hz –1 . We propose that the formation of spiral-host double-lobe radio galaxies can be attributed to more than one factor, such as the occurrence of strong interactions, mergers, and the presence of unusually massive SMBHs, such that the spiral structures are not destroyed. Only one of our sources (J1649+2635) is found in a cluster environment, indicating that processes other than accretion through cooling flows e.g. galaxy–galaxy mergers or interactions could be plausible scenarios for triggering radio-loud active galactic nuclei activity in spiral galaxies.

Description: We present observations at 610 and 235 MHz using the Giant Metrewave Radio Telescope (GMRT) of the recurrent nova V745 Scorpii which recorded its last outburst on 2014 February 6. This is the second symbiotic recurrent nova whose light curve at low frequencies has been followed in detail, the first being RS Ophiuchi in 2006. We fitted the 610 MHz light curve by a model of synchrotron emission from an expanding shell being modified by radiative transfer effects due to local absorbing gas consisting of a uniformly distributed and a clumpy component. Using our model parameters, we find that the emission at 235 MHz peaked around day 35 which is consistent with our GMRT observations. The two main results of our study are (1) The radio emission at a given frequency is visible sooner after the outburst in successive outbursts of both V745 Scorpii and RS Ophiuchi. The earlier detection of radio emission is interpreted to be caused by decreasing foreground densities. (2) The clumpy material, if exists, is close to the white dwarf and can be interpreted as being due to the material from the hot accretion disc. The uniform density gas is widespread and attributed to the winds blown by the white dwarf. We present implications of these results on the evolution of both novae. Such studies along with theoretical understanding have the potential of resolving several outstanding issues such as why all recurrent novae are not detectable in synchrotron radio and whether recurrent novae are progenitor systems of Type 1a supernova.

Description: Seyfert and LINER galaxies are known to exhibit compact radio emission on ~10–100 pc scales, but larger Kiloparsec-Scale Radio structures (KSRs) often remain undetected in sub-arcsec high-resolution observations. We investigate the prevalence and nature of KSRs in Seyfert and LINER galaxies using the 1.4 GHz VLA FIRST and NVSS observations. Our sample consists of 2651 sources detected in FIRST and of these 1737 sources also have NVSS counterparts. Considering the ratio of total to peak flux density ( = ( S int / S peak ) 1/2 ) as a parameter to infer the presence of extended radio emission we show that ≥30 per cent of FIRST-detected sources possess extended radio structures on scales larger than 1.0 kpc. The use of low-resolution NVSS observations help us to recover faint extended KSRs that are resolved out in FIRST observations and results in ≥42.5 per cent KSR sources in FIRST–NVSS sub-sample. This fraction is only a lower limit owing to the combination of projection, resolution and sensitivity effects. Our study demonstrates that KSRs may be more common than previously thought and are found across all redshifts, luminosities and radio loudness. The extranuclear radio luminosity of KSR sources is found to be positively correlated with the core radio luminosity as well as the [O iii ] 5007 Å line luminosity and this can be interpreted as KSRs being powered by AGN rather than star formation. The distributions of the FIR-to-radio ratios and mid-IR colours of KSR sources are also consistent with their AGN origin. However, contribution from star formation cannot be ruled out particularly in sources with low radio luminosities.

Description: We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index α ≥ +2.0, a previously unexplored spectral domain. The search is expected to yield strong candidates for α ≥ +2.5, for which the standard synchrotron self-absorption (characterized by a single power-law energy distribution of relativistic electron population) would not be a plausible explanation, even in an ideal case of a perfectly homogeneous source of incoherent synchrotron radiation. Such sharply inverted spectra, if found, would require alternative explanations, e.g. free–free absorption, or non-standard energy distribution of relativistic electrons which differs from a power law (e.g. Maxwellian). The search was carried out by comparing two sensitive low-frequency radio surveys made with sub-arcminute resolution, namely, the Westerbork In the Southern Hemisphere (WISH) survey at 352 MHz and TGSS/DR5 at 150 MHz. The overlap region between these two surveys contains 7056 WISH sources classified as ‘single’ and brighter than 100 mJy at 352 MHz. We focus here on the seven of these sources for which we find α 〉 +2.0. Two of these are undetected at 150 MHz and are particularly good candidates for α 〉 +2.5. Five of the seven sources exhibit a ‘Gigahertz-Peaked-Spectrum’.

Description: Low-frequency observations at 325 and 610 MHz have been carried out for two ‘radio-loud’ Seyfert galaxies, NGC 4235 and NGC 4594 (Sombrero galaxy), using the Giant Meterwave Radio Telescope (GMRT). The 610 MHz total intensity and 325–610 MHz spectral index images of NGC 4235 tentatively suggest the presence of a ‘relic’ radio lobe, most likely from a previous episode of active galactic nucleus (AGN) activity. This makes NGC 4235 only the second known Seyfert galaxy after Mrk 6 to show signatures of episodic activity. Spitzer and Herschel infrared spectral energy distribution (SED) modelling using the clumpydream code predicts star formation rates (SFRs) that are an order of magnitude lower than those required to power the radio lobes in these Seyferts (~0.13–0.23 M  yr –1 compared to the required SFR of ~2.0–2.7 M  yr –1 in NGC 4594 and NGC 4235, respectively). This finding along with the detection of parsec and sub-kpc radio jets in both Seyfert galaxies, that are roughly along the same position angles as the radio lobes, strongly support the suggestion that Seyfert lobes are AGN powered. SED modelling supports the ‘true’ type 2 classification of NGC 4594: this galaxy lacks significant dust obscuration as well as a prominent broad-line region. Between the two Seyfert galaxies, there is an inverse relation between their radio-loudness and Eddington ratio and a direct relation between their Eddington-scaled jet power and bolometric power.