Probing the origin of low-frequency radio emission in PG quasars with the uGMRT – I

ABSTRACT We present the results from 685 MHz observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) of 22 quasars belonging to the Palomar-Green (PG) quasar sample. Only four sources reveal extended radio structures on ∼10–30 kpc scales, while the rest are largely a combination of a radio core unresolved at the uGMRT resolution of ∼3–5 arcsec, surrounded by diffuse emission on few kpc to ∼10 kpc scales. A few sources reveal signatures of barely resolved jets and lobes in their spectral index images that are created using the 685 MHz uGMRT data and similar resolution GHz-frequency data from the Very Large Array. On the basis of their position on the radio–IR correlation as well as the spectral index images, we find that the radio emission in the two radio-loud (RL) quasars and nearly one-third of the radio-quiet (RQ) quasars is active galactic nucleus (AGN) dominated whereas the remaining sources appear to have significant contributions from stellar-related processes along with the AGN. While the two RL sources exhibit inverted spectral index in their cores, the RQ sources exhibit a range of spectral indices varying from flat to steep (−0.1 $\gtrsim $ αR $\gtrsim $ −1.1) indicating the presence of unresolved jets/lobes or winds. Except for a significant correlation between the 685 MHz radio luminosity and the Eddington ratio, we do not find strong correlations between other 685 MHz radio properties and black hole (BH) properties in the RQ PG sources. This lack of correlations could be explained by the contribution of stellar-related emission, or radio emission from previous AGN activity episodes that may not be related to the current BH activity state.

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Characterizing the radio emission from the binary galaxy cluster merger Abell 2146

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834025 ◽

2019 ◽

Vol 622 ◽

pp. A21 ◽

Cited By ~ 4

Author(s):

D. N. Hoang ◽

T. W. Shimwell ◽

R. J. van Weeren ◽

H. J. A. Röttgering ◽

A. Botteon ◽

...

Keyword(s):

Particle Acceleration ◽

Radio Emission ◽

Low Frequency ◽

Galaxy Cluster ◽

Relativistic Electrons ◽

Extended Source ◽

Very Large Array ◽

Radio Halo ◽

Extended Radio ◽

Relativistic Particles

Context. Collisions of galaxy clusters generate shocks and turbulence in the intra-cluster medium (ICM). The presence of relativistic particles and magnetic fields is inferred through the detection of extended synchrotron radio sources such as haloes and relics and implies that merger shocks and turbulence are capable of (re-)accelerating particles to relativistic energies. However, the precise relationship between merger shocks, turbulence, and extended radio emission is still unclear. Studies of the most simple binary cluster mergers are important to help understand the particle acceleration in the ICM. Aims. Our main aim is to study the properties of the extended radio emission and particle acceleration mechanism(s) associated with the generation of relativistic particles in the ICM. Methods. We measure the low-frequency radio emission from the merging galaxy cluster Abell 2146 with LOFAR at 144 MHz. We characterize the spectral properties of the radio emission by combining these data with data from archival Giant Metrewave Radio Telescope (GMRT) at 238 MHz and 612 MHz and Very Large Array (VLA) at 1.5 GHz. Results. We observe extended radio emission at 144 MHz behind the NW and SE shocks. Across the NW extended source, the spectral index steepens from −1.06 ± 0.06 to −1.29 ± 0.09 in the direction of the cluster centre. This spectral behaviour suggests that a relic is associated with the NW upstream shock. The precise nature of the SE extended emission is unclear. It may be a radio halo bounded by a shock or a superposition of a relic and halo. At 144 MHz, we detect a faint emission that was not seen with high-frequency observations, implying a steep (α < −1.3) spectrum nature of the bridge emission. Conclusions. Our results imply that the extended radio emission in Abell 2146 is probably associated with shocks and turbulence during cluster merger. The relativistic electrons in the NW and SE may originate from fossil plasma and thermal electrons, respectively.

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Low frequency radio counterparts of HESS J1731−347 a.k.a SNR G353.6−0.7

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317004689 ◽

2017 ◽

Vol 12 (S331) ◽

pp. 201-205

Author(s):

A. J. Nayana ◽

Poonam Chandra

Keyword(s):

Radio Emission ◽

Radio Telescope ◽

Supernova Remnants ◽

Low Frequency ◽

High Energy ◽

Spectral Indices ◽

Thermal Radio Emission ◽

Very High Energy ◽

Γ Rays ◽

Very High

AbstractHESS J1731−347 a.k.a. SNR G353.6−0.7 is one of the five known very high energy (VHE, Energy > 0.1 TeV) shell-type supernova remnants. We carried out Giant Metrewave Radio Telescope (GMRT) observations of this TeV SNR in 1390, 610 and 325 MHz bands. We detected the 325 and 610 MHz radio counterparts of the SNR G353.6−0.7 (Nayana et al. 2017). We also determined the spectral indices of individual filaments and our values are consistent with the non-thermal radio emission. We compared the radio morphology with that of VHE emission. The peak in radio emission corresponds to the faintest feature in the VHE emission. We explain this anti-correlated emission in a possible leptonic origin of the VHE γ-rays.

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Thermal radio absorption as a tracer of the interaction of SNRs with their environments

Astronomy and Astrophysics ◽

10.1051/0004-6361/202141635 ◽

2021 ◽

Vol 653 ◽

pp. A62

Author(s):

G. Castelletti ◽

L. Supan ◽

W. M. Peters ◽

N. E. Kassim

Keyword(s):

Spectral Index ◽

Supernova Remnants ◽

Surrounding Medium ◽

Low Frequency ◽

Power Laws ◽

Line Of Sight ◽

Pulsar Wind Nebulae ◽

Very Large Array ◽

Murchison Widefield Array ◽

Thermal Absorption

We present new images and continuum spectral analysis for 14 resolved Galactic supernova remnants (SNRs) selected from the 74 MHz Very Large Array Low-Frequency Sky Survey Redux (VLSSr). We combine new integrated measurements from the VLSSr with, when available, flux densities extracted from the Galactic and Extragalactic All-Sky Murchison Widefield Array Survey and measurements from the literature to generate improved integrated continuum spectra sampled from ~15 MHz to ~217 GHz. We present the VLSSr images. When possible we combine them with publicly available images at 1.4 GHz, to analyse the resolved morphology and spectral index distribution across each SNR. We interpret the results and look for evidence of thermal absorption caused by ionised gas either proximate to the SNR itself, or along its line of sight. Three of the SNRs, G4.5+6.8 (Kepler), G28.6−0.1, and G120.1+1.4 (Tycho), have integrated spectra which can be adequately fit with simple power laws. The resolved spectral index map for Tycho confirms internal absorption which was previously detected by the Low Frequency Array, but it is insufficient to affect the fit to the integrated spectrum. Two of the SNRs are pulsar wind nebulae, G21.5−0.9 and G130.7+3.1 (3C 58). For those we identify high-frequency spectral breaks at 38 and 12 GHz, respectively. For the integrated spectra of the remaining nine SNRs, a low frequency spectral turnover is necessary to adequately fit the data. In all cases we are able to explain the turnover by extrinsic thermal absorption. For G18.8+0.3 (Kes 67), G21.8−0.6 (Kes 69), G29.7−0.3 (Kes 75), and G41.1−0.3 (3C 397), we attribute the absorption to ionised gas along the line of sight, possibly from extended H II region envelopes. For G23.3−0.3 (W41) the absorption can be attributed to H II regions located in its immediate proximity. Thermal absorption from interactions at the ionised interface between SNR forward shocks and the surrounding medium were previously identified as responsible for the low frequency turnover in SNR G31.9+0.0 (3C 391); our integrated spectrum is consistent with the previous results. We present evidence for the same phenomenon in three additional SNRs G27.4+0.0 (Kes 73), G39.2–0.3 (3C 396), and G43.3–0.2 (W49B), and derive constraints on the physical properties of the interaction. This result indicates that interactions between SNRs and their environs should be readily detectable through thermal absorption by future low frequency observations of SNRs with improved sensitivity and resolution.

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Radio morphology of southern narrow-line Seyfert 1 galaxies with Very Large Array observations*

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2373 ◽

2020 ◽

Vol 498 (1) ◽

pp. 1278-1297

Author(s):

S Chen ◽

E Järvelä ◽

L Crepaldi ◽

M Zhou ◽

S Ciroi ◽

...

Keyword(s):

Star Formation ◽

Radio Emission ◽

Large Mass ◽

Large Array ◽

Narrow Line ◽

Diffuse Emission ◽

Jets And Outflows ◽

Very Large Array ◽

Radio Jets ◽

New Radio

ABSTRACT We present the results of new radio observations carried out with the Karl G. Jansky Very Large Array C-configuration at 5.5 GHz for a sample of southern narrow-line Seyfert 1 galaxies (NLS1s). This work increases the number of known radio-detected NLS1s in the Southern hemisphere, and confirms that the radio emission of NLS1s is mainly concentrated in a central region at kpc-scale and only a few sources show diffuse emission. In radio-quiet NLS1s, the radio luminosity tends to be higher in steep-spectrum sources and be lower in flat-spectrum sources, which is opposite to radio-loud NLS1s. This may be because the radio emission of steep NLS1s is dominated by misaligned jets, active galactic nucleus driven outflows, or star formation superposing on a compact core. Instead the radio emission of flat NLS1s may be produced by a central core that has not yet developed radio jets and outflows. We discover new NLS1s harbouring kpc-scale radio jets and confirm that a powerful jet does not require a large-mass black hole to be generated. We also find sources dominated by star formation. These NLS1s could be new candidates in investigating the radio emission of different mechanisms.

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Spectral Index Versus Frequency and Models for the Continua of AGN and QSOs

Symposium - International Astronomical Union ◽

10.1017/s0074180900140835 ◽

1989 ◽

Vol 134 ◽

pp. 201-202

Author(s):

Wayne A. Stein

Keyword(s):

Accretion Disk ◽

Spectral Index ◽

Power Law ◽

Spectral Distribution ◽

Secondary Electron ◽

Low Frequency ◽

Spectral Indices ◽

Ultraviolet Continuum ◽

Electron Synchrotron ◽

X Ray

The observed spectral index as a function of frequency of QSO continua must be explained in models. It is generally increasing (F(ν) ∝ ν−α, α increasing) with higher frequency in the infrared (downward curvature). The visual to ultraviolet continuum has been shown to be a broken power law with F(ν) ∝ ν−0.5 at low frequency and a break to larger α at νo ∼ 3×1015 Hz. X-ray observations frequently exhibit a flat continuum with α < 1. One prominent example is 3C273 for which α1–3μm → 2, αvis ∼ 0.5 and αx ∼ 0.5. These spectral indices arise naturally in Secondary Electron Synchrotron Self-Compton (SESSC) models. Some accretion disk models approach these spectral indices for the visual-ultraviolet portion of the spectral distribution.

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Radio observations of the merging galaxy cluster Abell 520

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833900 ◽

2019 ◽

Vol 622 ◽

pp. A20 ◽

Cited By ~ 10

Author(s):

D. N. Hoang ◽

T. W. Shimwell ◽

R. J. van Weeren ◽

G. Brunetti ◽

H. J. A. Röttgering ◽

...

Keyword(s):

Mach Number ◽

Radio Emission ◽

Low Frequency ◽

Galaxy Cluster ◽

Shock Acceleration ◽

Relativistic Electrons ◽

Diffusive Shock Acceleration ◽

Cluster Emission ◽

X Ray ◽

Extended Radio

Context. Extended synchrotron radio sources are often observed in merging galaxy clusters. Studies of the extended emission help us to understand the mechanisms in which the radio emitting particles gain their relativistic energies. Aims. We examine the possible acceleration mechanisms of the relativistic particles that are responsible for the extended radio emission in the merging galaxy cluster Abell 520. Methods. We performed new 145 MHz observations with the LOw Frequency ARay (LOFAR) and combined these with archival Giant Metrewave Radio Telescope (GMRT) 323 MHz and Very Large Array (VLA) 1.5 GHz data to study the morphological and spectral properties of extended cluster emission. The observational properties are discussed in the framework of particle acceleration models associated with cluster merger turbulence and shocks. Results. In Abell 520, we confirm the presence of extended (760 × 950 kpc2) synchrotron radio emission that has been classified as a radio halo. The comparison between the radio and X-ray brightness suggests that the halo might originate in a cocoon rather than from the central X-ray bright regions of the cluster. The halo spectrum is roughly uniform on the scale of 66 kpc. There is a hint of spectral steepening from the SW edge towards the cluster centre. Assuming diffusive shock acceleration (DSA), the radio data are suggestive of a shock Mach number of ℳSW = 2.6−0.2+0.3 that is consistent with the X-ray derived estimates. This is in agreement with the scenario in which relativistic electrons in the SW radio edge gain their energies at the shock front via acceleration of either thermal or fossil electrons. We do not detect extended radio emission ahead of the SW shock that is predicted if the emission is the result of adiabatic compression. An X-ray surface brightness discontinuity is detected towards the NE region that may be a counter shock of Mach number ℳNEX = 1.52±0.05. This is lower than the value predicted from the radio emission which, assuming DSA, is consistent with ℳNE = 2.1 ± 0.2. Conclusions. Our observations indicate that the radio emission in the SW of Abell 520 is likely effected by the prominent X-ray detected shock in which radio emitting particles are (re-)accelerated through the Fermi-I mechanism. The NE X-ray discontinuity that is approximately collocated with an edge in the radio emission hints at the presence of a counter shock.

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G51.04+0.07 and its environment: Identification of a new Galactic supernova remnant at low radio frequencies

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832995 ◽

2018 ◽

Vol 616 ◽

pp. A98 ◽

Cited By ~ 2

Author(s):

L. Supan ◽

G. Castelletti ◽

W. M. Peters ◽

N. E. Kassim

Keyword(s):

Radio Emission ◽

Flux Density ◽

Supernova Remnant ◽

Low Frequency ◽

Neutral Hydrogen ◽

Radio Continuum ◽

Very Large Array ◽

Continuum Spectrum ◽

Galactic Emission ◽

Nonthermal Radio Emission

We have identified a new supernova remnant (SNR), G51.04+0.07, using observations at 74 MHz from the Very Large Array Low-Frequency Sky Survey Redux (VLSSr). Earlier, higher frequency radio continuum, recombination line, and infrared data had correctly inferred the presence of nonthermal radio emission within a larger, complex environment including ionised nebulae and active star formation. However, our observations have allowed us to redefine at least one SNR as a relatively small source (7.′5 × 3′in size) located at the southern periphery of the originally defined SNR candidate G51.21+0.11. The integrated flux density of G51.04+0.07 at 74 MHz is 6.1 ± 0.8 Jy, while its radio continuum spectrum has a slope α = −0.52 ± 0.05 (S v ∝ vα), typical of a shell-type remnant. We also measured spatial variations in the spectral index between 74 and 1400 MHz across the source, ranging from a steeper spectrum (α = −0.50 ± 0.04) coincident with the brightest emission to a flatter component (α = −0.30 ± 0.07) in the surrounding fainter region. To probe the interstellar medium into which the redefined SNR is likely evolving, we have analysed the surrounding atomic and molecular gas using the 21 cm neutral hydrogen (HI) and 13CO(J = 1 − 0) emissions. We found that G51.04+0.07 is confined within an elongated HI cavity and that its radio emission is consistent with the remains of a stellar explosion that occurred ~6300 yr ago at a distance of 7.7 ± 2.3 kpc. Kinematic data suggest that the newly discovered SNR lies in front of HII regions in the complex, consistent with the lack of a turnover in the low frequency continuum spectrum. The CO observations revealed molecular material that traces the central and northern parts of G51.04+0.07. The interaction between the cloud and the radio source is not conclusive and motivates further study. The relatively low flux density (~1.5 Jy at 1400 MHz) of G51.04+0.07 is consistent with this and many similar SNRs lying hidden along complex lines of sight towards inner Galactic emission complexes. It would also not be surprising if the larger complex studied here hosted additional SNRs.

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Spectral Index - Redshift Relation for Radio Galaxies and Quasars

Symposium - International Astronomical Union ◽

10.1017/s0074180900081882 ◽

1996 ◽

Vol 175 ◽

pp. 569-570

Author(s):

R.D. Dagkesamanskii

Keyword(s):

Flux Density ◽

Spectral Index ◽

Low Frequency ◽

Radio Galaxies ◽

Cosmological Evolution ◽

Radio Sources ◽

Spectral Indices ◽

Extragalactic Radio Sources

Cosmological evolution of synchrotron spectra of the powerful extragalactic radio sources was studied by many authors. Some indications of such an evolution had been found firstly by analysis of ‘spectral index - flux density’ (α – S) relation for the sample of relatively strong radio sources. Later Gopal-Krishna and Steppe extended the analysis to weaker sources and found that the slope of αmed(S) curve changes dramatically at intermediate flux densities. Gopal-Krishna and Steppe pointed out that the maxima of the αmed(S) curve and of differential source counts are at almost the same flux density ranges (see, Fig. 2). It has to be noticed that the all mentioned results were obtained using the low-frequency spectral indices and on the basis of low frequency samples.

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Implications of the Low-frequency Turnover in the Spectrum of Radio Knot C in DG Tau

The Astrophysical Journal ◽

10.3847/1538-4357/ac2b26 ◽

2021 ◽

Vol 923 (1) ◽

pp. 61

Author(s):

C.-I. Björnsson

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Radio Emission ◽

Magnetic Field Strength ◽

Spectral Index ◽

Low Frequency ◽

Relativistic Electrons ◽

Fermi Acceleration ◽

Emission Volume ◽

The Magnetic Field

Abstract The synchrotron spectrum of radio knot C in the protostellar object DG Tau has a low-frequency turnover. This is used to show that its magnetic field strength is likely to be at least 10 mG, which is roughly two orders of magnitude larger than previously estimated. The earlier, lower value is due to an overestimate of the emission volume together with an omission of the dependence of the minimum magnetic field on the synchrotron spectral index. Since the source is partially resolved, this implies a low volume-filling factor for the synchrotron emission. It is argued that the high pressure needed to account for the observations is due to shocks. In addition, cooling of the thermal gas is probably necessary in order to further enhance the magnetic field strength as well as the density of relativistic electrons. It is suggested that the observed spectral index implies that the energy of the radio-emitting electrons is below that needed to take part in first-order Fermi acceleration. Hence, the radio emission gives insights to the properties of its pre-acceleration phase. Attention is also drawn to the similarities between the properties of radio knot C and the shock-induced radio emission in supernovae.

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Unveiling the nature of the unidentified gamma-ray sources: blazar counterparts at low radio frequencies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314003433 ◽

2013 ◽

Vol 9 (S304) ◽

pp. 95-95

Author(s):

Francesco Massaro ◽

R. D'Abrusco ◽

M. Giroletti ◽

A. Paggi ◽

N. Masetti ◽

...

Keyword(s):

Radio Emission ◽

Gamma Ray ◽

Low Frequency ◽

Radio Galaxies ◽

Large Array ◽

New Approach ◽

Very Large Array ◽

Very Low Frequency ◽

Sky Survey ◽

Order Of Magnitude

AbstractAbout one third of the gamma-ray sources detected by Fermi have still no firmly established counterpart at lower energies. Here we propose a new approach to find candidate counterparts for the unidentified gamma-ray sources (UGSs) based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the information derived from the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at least one counterpart in the WENSS. We also performed an extensive research in literature to look for infrared and optical counterparts of the gamma-ray blazar candidates selected with the low-frequency radio observations to confirm their nature. On the basis of our multifrequency research we identify 23 new gamma-ray blazar candidates out of 32 UGSs investigated. I will also present the first analysis of very low frequency radio emission of blazars based on the recent Very Large Array Low-Frequency Sky Survey (VLSS) at 74 MHz. I show that blazars present radio flat spectra when evaluated at 74 MHz, about an order of magnitude in frequency lower than previous analyses. The implications of these findings in the contest of the blazars – radio galaxies connection will be discussed.

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