Steep-spectrum Radio Emission Research Articles

Serendipitous decametre detection of ultra steep spectrum radio emission in Abell 655

Some galaxy clusters contain non-thermal synchrotron emitting plasma that permeate the intracluster medium (ICM). The spectral properties of this radio emission are not well characterised at decametre wavelengths (ν < 30 MHz), primarily due to the severe corrupting effects of the ionosphere. Using a recently developed calibration strategy, we present LOFAR images below 30 MHz of the low-mass galaxy cluster Abell 655, which was serendipitously detected in an observation of the bright calibrator 3C 196. We combine this observation with LOFAR data at 144 MHz and new band 4 upgraded Giant Metrewave Radio Telescope observations centred at 650 MHz. In the 15–30 MHz LOFAR image, diffuse emission is seen with a physical extent of about 700 kpc. We argue that the diffuse emission detected in this galaxy cluster likely has multiple origins. At higher frequencies (650 MHz), the diffuse emission resembles a radio halo, while at lower frequencies the emission seems to consist of several components and bar-like structures. This detection of diffuse emission suggests that most low-frequency emission in this cluster comes from re-energised fossil plasma from old Active Galacitic Nucleus outbursts co-existing with the radio halo component. By counting the number of cluster radio detections in the decametre band, we estimate that around a quarter of the Planck clusters host re-energised fossil plasma that is detectable in the decametre band with LOFAR.

On the origin of diffuse radio emission in Abell 85 – insights from new GMRT observations

ABSTRACT Extended, steep, and ultra-steep spectrum radio emission in a galaxy cluster is usually associated with recent mergers. Simulations show that radio phoenixes are aged radio galaxy lobes whose emission reactivates when a low Mach shock compresses it. A85 hosts a textbook example of a radio phoenix at about 320 kpc southwest of the cluster centre. We present a new high-resolution 325 MHz GMRT radio map illustrating this radio phoenix’s complex and filamentary structure. The full extent of the radio structure is revealed for the first time from these radio images of A85. Using archival Chandra X-ray observations, we applied an automated 2D shock finder to the X-ray surface brightness and Adaptive Circular Binning temperature maps that confirmed a bow shock at the location of the radio phoenix. We also compared the Mach number from the X-ray data with the radio-derived Mach number in the same region using multifrequency radio observations and find that they are consistent within the 1σ error level.

A LOFAR study of non-merging massive galaxy clusters

Centrally located diffuse radio emission has been observed in both merging and non-merging galaxy clusters. Depending on their morphology and size, we distinguish between giant radio haloes, which occur predominantly in merging clusters, and mini haloes, which are found in non-merging, cool-core clusters. In recent years, cluster-scale radio emission has also been observed in clusters with no sign of major mergers, showing that our knowledge of the mechanisms that lead to particle acceleration in the intra-cluster medium (ICM) is still incomplete. Low-frequency sensitive observations are required to assess whether the emission discovered in these few cases is common in galaxy clusters or not. With this aim, we carried out a campaign of observations with the LOw Frequency ARay (LOFAR) in the frequency range 120–168 MHz of nine massive clusters selected from the Planck SZ catalogue, which had no sign of major mergers. In this paper, we discuss the results of the observations that have led to the largest cluster sample studied within the LOFAR Two-metre Sky Survey, and we present Chandra X-ray data used to investigate the dynamical state of the clusters, verifying that the clusters are currently not undergoing major mergers, and to search for traces of minor or off-axis mergers. We discover large-scale steep-spectrum emission around mini haloes in the cool-core clusters PSZ1G139.61+24 and RXJ1720.1+2638, which is not observed around the mini halo in the non-cool-core cluster A1413. We also discover a new 570 kpc-halo in the non-cool-core cluster RXCJ0142.0+2131. We derived new upper limits to the radio power for clusters in which no diffuse radio emission was found, and we discuss the implication of our results to constrain the cosmic-ray energy budget in the ICM. We conclude that radio emission in non-merging massive clusters is not common at the sensitivity level reached by our observations and that no clear connection with the cluster dynamical state is observed. Our results might indicate that the sloshing of a dense cool core could trigger particle acceleration on larger scales and generate steep-spectrum radio emission.

First evidence of diffuse ultra-steep-spectrum radio emission surrounding the cool core of a cluster

Diffuse synchrotron radio emission from cosmic-ray electrons is observed at the center of a number of galaxy clusters. These sources can be classified either as giant radio halos, which occur in merging clusters, or as mini halos, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterise the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters.

Multiband counterparts of two eclipsing ultraluminous X-ray sources in M 51

We present the discovery and interpretation of ionized nebulae around two ultraluminous X-ray sources in M 51; both sources share the rare property of showing X-ray eclipses by their companion stars and are therefore prime targets for follow-up studies. Using archival Hubble Space Telescope images, we found an elongated, 100-pc-long emission-line structure associated with one X-ray source (CXOM51 J132940.0+471237; ULX-1 for simplicity), and a more circular, ionized nebula at the location of the second source (CXOM51 J132939.5+471244; ULX-2 for simplicity). We observed both nebulae with the Large Binocular Telescope’s Multi-Object Double Spectrograph. From our analysis of the optical spectra, we argue that the gas in the ULX-1 bubble is shock-ionized, consistent with the effect of a jet with a kinetic power of ≈2 × 1039 erg s−1. Additional X-ray photoionization may also be present, to explain the strength of high-ionization lines such as He ii λ4686 and [Ne v] λ3426. On the other hand, the emission lines from the ULX-2 bubble are typical for photoionization by normal O stars suggesting that the nebula is actually an H ii region not physically related to the ULX but is simply a chance alignment. From archival Very Large Array data, we also detect spatially extended, steep-spectrum radio emission at the location of the ULX-1 bubble (consistent with its jet origin), but no radio counterpart for ULX-2 (consistent with the lack of shock-ionized gas around that source).

Abell 1033: birth of a radio phoenix

Extended steep-spectrum radio emission in a galaxy cluster is usually associated with a recent merger. However, given the complex scenario of galaxy cluster mergers, many of the discovered sources hardly fit into the strict boundaries of a precise taxonomy. This is especially true for radio phoenixes that do not have very well defined observational criteria. Radio phoenixes are aged radio galaxy lobes whose emission is reactivated by compression or other mechanisms. Here, we present the detection of a radio phoenix close to the moment of its formation. The source is located in Abell 1033, a peculiar galaxy cluster which underwent a recent merger. To support our claim, we present unpublished Westerbork Synthesis Radio Telescope and Chandra observations together with archival data from the Very Large Array and the Sloan Digital Sky Survey. We discover the presence of two sub-clusters displaced along the N-S direction. The two sub-clusters probably underwent a recent merger which is the cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum extended radio source very close to an AGN is proposed to be a newly born radio phoenix: the AGN lobes have been displaced/compressed by shocks formed during the merger event. This scenario explains the source location, morphology, spectral index, and brightness. Finally, we show evidence of a density discontinuity close to the radio phoenix and discuss the consequences of its presence.

A LINK BETWEEN X-RAY EMISSION LINES AND RADIO JETS IN 4U 1630-47?

Recently, Diaz Trigo et al. reported an XMM-Newton detection of relativistically Doppler-shifted emission lines associated with steep-spectrum radio emission in the stellar-mass black hole candidate 4U 1630-47 during its 2012 outburst. They interpreted these lines as indicative of a baryonic jet launched by the accretion disk. Here we present a search for the same lines earlier in the same outburst using high-resolution X-ray spectra from the Chandra High-Energy Transmission Grating Spectrometer. While our observations (eight months prior to the XMM-Newton campaign) also coincide with detections of steep spectrum radio emission by the Australia Telescope Compact Array, we find no evidence for any relativistic X-ray emission lines. Indeed, despite $\sim5\times$ brighter radio emission, our Chandra spectra allow us to place an upper limit on the flux in the blueshifted Fe XXVI line that is $\gtrsim20\times$ weaker than the line observed by Diaz Trigo et al. We explore several scenarios that could explain our differing results, including variations in the geometry of the jet or a mass-loading process or jet baryon content that evolves with the accretion state of the black hole. We also consider the possibility that the radio emission arises in an interaction between a jet and the nearby ISM, in which case the X-ray emission lines might be unrelated to the radio emission.

Into the central 10 pc of the most distant known radio quasar

Context: There are about 60 quasars known at redshifts z>5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs, and on the physical conditions in their environment. Aims: We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale, in order to compare the structural and spectral properties with those of other two z~6 radio-loud quasars, J0836+0054 (z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results: Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of <100 pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a "universal" feature of the most distant radio quasars, it is essential to study more, yet to be discovered radio-loud active galactic nuclei at z>6.

RADIO GALAXY NGC 1265 UNVEILS THE ACCRETION SHOCK ONTO THE PERSEUS GALAXY CLUSTER

We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that explains the complex radio morphology and spectrum by a past passage of the galaxy and radio bubble through a shock wave. Using analytical solutions to the full Riemann problem and hydrodynamical simulations, we study how this passage transformed the plasma bubble into a toroidal vortex ring. Adiabatic compression of the aged electron population causes it to be energized and to emit low-surface brightness and steep-spectrum radio emission. The large infall velocity of NGC 1265 and the low Faraday rotation measure values and variance of the jet strongly argue that this transformation was due to the accretion shock onto Perseus situated roughly at R_200. Estimating the volume change of the radio bubble enables inferring a shock Mach number of M = 4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of 6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for uncertainties in the equation of state of the radio plasma and volume of the torus. Extrapolating X-ray profiles, we obtain upper limits on the gas temperature and density in the infalling warm-hot intergalactic medium of kT < 0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio torus in combination with the direction of the galaxy's head and tail in the plane of the sky is impossible to reconcile with projection effects. Instead, this argues for post-shock shear flows that have been caused by curvature in the shock surface with a characteristic radius of 850 kpc. The energy density of the shear flow corresponds to a turbulent-to-thermal energy density of 14%. The shock-injected vorticity might be important in generating and amplifying magnetic fields in galaxy clusters. Future LOFAR observations of head-tail galaxies can be complementary probes of accretion shocks onto galaxy clusters.

Two types of shock in the hotspot of the giant quasar 4C74.26: a high-resolution comparison from Chandra, Gemini and MERLIN

New Chandra observations have resolved the structure of the X-ray luminous southern hotspot in the giant radio quasar 4C74.26 into two distinct features. The nearer one to the nucleus is an extremely luminous peak, extended some 5 kpc perpendicular to the orientation of the jet; 19 kpc projected further away from the central nucleus than this is an arc having similar symmetry. This X-ray arc is cospatial with near-infrared (near-IR) and optical emission imaged with Gemini and radio emission imaged with Multi-Element Radio-Linked Interferometer Network (MERLIN). We explore how this double feature corresponds to two shocks having very different characteristics in spectral energy distribution. We present the case that these observations are explained by the luminous X-ray peak being synchrotron emission with a flux density of ∼7 nanoJy. There is no steep spectrum radio, optical or near-IR emission directly associated with this shock. Beyond this point in the jet's flow, and following adiabatic losses, at least 19 kpc further downstream where the flow impinges on the intergalactic medium, the arc structure seen in sharp focus at radio wavelengths appears to be approximately mimicked at near-IR, optical and X-ray wavelengths. The radio emission is most naturally explained as synchrotron emission, but it would be unnatural to explain the X-rays as arising from the same emission mechanism since they show a non-monotonic spectrum. They are, however, explicable as inverse Compton scattering of photons in the cosmic microwave background, though this requires that the magnetic field strength in this slender arc/shock region responsible for the associated synchrotron radio emission is just a few per cent of the minimum energy value. The angular separation of the double shock structure (itself ≳19 kpc or 10 arcsec in size) from the active nucleus which fuels them of ∼550 kpc could present a challenge for connecting ‘unidentified’ hard X-ray or Fermi sources with their origins.

Deep 1.4 GHz Very Large Array Observations of the Radio Halo and Relic in Abell 2256

We present deep Very Large Array observations of the merging galaxy cluster A2256. This cluster is known to possess diffuse steep-spectrum radio relic emission in the peripheral regions. Our new observations provide the first detailed image of the central diffuse radio halo emission in this cluster. The radio halo extends over more than 800 kpc in the cluster core, while the relic emission covers a region of ~1125 kpc × 520 kpc. A spectral index map of the radio relic shows a spectral steepening from the northwest toward the southeast edge of the emission, with an average spectral index between 1369 and 1703 MHz of α = -1.2 across the relic. Polarization maps reveal high fractional polarization of up to 45% in the relic region with an average polarization of 20% across the relic region. The observed Faraday rotation measure is consistent with the Galactic estimate, and the dispersion in the rotation measure is small, suggesting that there is very little contribution to the rotation measure of the relic from the intracluster medium. We use these Faraday properties of the relic to argue that it is located on the front side of the cluster.

VLBA Identification of the Milliarcsecond Active Nucleus in the Seyfert Galaxy NGC 4151

The Seyfert galaxy NGC 4151 has been imaged at resolution better than 0.1 pc using a VLBI array consisting of the Very Long Baseline Array and three 100 m class telescopes. A flat-spectrum 3 mJy source with a monochromatic radio power of ~1037 ergs s-1 has been detected, apparently at the location of the active galactic nucleus (AGN) and its central black hole. The radio source has a minimum brightness temperature of 2.1 × 108 K and a size upper limit of 0.035 pc, about 10 times the diameter of the broad-line region and 15,000 times the diameter of the black hole's event horizon. An additional flat-spectrum component located within a parsec of the apparent nucleus is likely to be a knot in the inner radio jet. The presence of some steep-spectrum radio emission within 0.1 pc of the galaxy nucleus limits the emission measure of a possible ionized torus to a maximum value of 108 cm-6 pc. If the hard X-ray source in NGC 4151 is associated with the radio AGN, its radio–to–X-ray ratio is less than 10-5, putting NGC 4151 securely in the radio-quiet class of AGNs. The radio image reveals a 0.2 pc two-sided base to the well-known arcsecond radio jet. The apparent speeds of the jet components relative to the radio AGN are less than 0.050c and less than 0.028c at respective nuclear distances of 0.16 and 6.8 pc. These are the lowest speed limits yet found for a Seyfert galaxy and indicate nonrelativistic jet motions, possibly due to thermal plasma, on a scale only an order of magnitude larger than the broad-line region.

Radio Spectral Index Variations and Physical Conditions in Kepler’s Supernova Remnant

A new epoch of VLA measurements of Kepler's supernova remnant (SNR) was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Hα, infrared (IR), and X-ray data to better understand the three-dimensional structure and dynamics of Kepler's SNR and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from -0.85 to -0.6 are observed between 6 and 20 cm. The mean spectral index is -0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and IR emission in different locations around the remnant, although there is no single, global pattern. Spectral tomography shows that the flat- and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found at either a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward- and reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow shocked material (reshocked by the forward shock) from the progenitor's motion through the interstellar medium. The Hα and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Hα and IR emission in places and, in other places, traces the steep-spectrum radio emission. In the north, there is also an anticorrelation in the azimuthal profiles around the remnant of the flat-spectrum radio emission and the thermal X-ray, Hα, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfven wave damping in regions of high density.

The Nature and Origin ofza≈zeAbsorption Lines in the Redshift 0.20 Quasar, PKS 2135−147

We use new UV and optical spectra and an archival Hubble Space Telescope Wide Field Planetary Camera 2 (HST-WFPC2) image to study the za ? ze absorber in the ze ? 0.20 QSO PKS 2135-147. The UV spectra, obtained with the Faint Object Spectrograph on HST, show strong za ? ze absorption lines of C IV, N V, O VI, Ly?, and Ly?. The za ? ze line profiles are resolved, with deconvolved FWHM of 270-450 km s-1. The Lyman decrement and the O VI and N V doublet ratios indicate that there are also narrower, optically thick line components, and there is evidence in the C IV and Ly? profiles for two blended components. Lower limits on the total column densities are of order 1015 cm-2 for all ions. The ~2:1 ratio of the C IV doublets suggests that the total C IV column density is near the lower limit. If the absorber is photoionized by the QSO and the derived relative columns in C IV and H I are roughly correct, then the metallicity must be at least solar. The location of the za ? ze absorber remains uncertain. The line redshifts indicate that the clouds have little radial motion (less than ?200 km s-1) with respect to the QSO. This small velocity shift could mean that the absorber is outside of the deep gravitational potential of the QSO and the host-galaxy nucleus. Two ~L* galaxies in a small cluster centered on PKS 2135-147 lie within 36 h-1 kpc projected distance and have redshifts consistent with causing or contributing to the za ? ze lines. The extensive halo of the QSO's host galaxy could also contribute. Calculations show that the QSO is bright enough to photoionize gas up to O VI in the low-density halos of the host and nearby cluster galaxies. Nonetheless, there is indirect evidence for absorption much nearer the QSO, namely (1) the derived high (albeit uncertain) metallicity; (2) the relatively strong N V absorption lines, which might be caused by a higher nitrogen abundance in the metal-rich gas; and (3) strong, lobe-dominated, steep-spectrum radio emission, which is known to correlate with a much higher incidence of (probably intrinsic) za ? ze lines. We propose that the C IV/N V/O VI line ratios can be used as a general diagnostic of intrinsic versus intervening absorption, as long as the line saturation effects are understood.

The extraordinary radio source 3C 442

view Abstract Citations (15) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Extraordinary Radio Source 3C 442 Comins, Neil F. ; Owen, Frazer N. Abstract We have studied the distinctive system 3C 442 with VLA at 6, 20, and 90 cm. This source is associated with two galaxies, NGC 7236/7237. The image of the radio source shows a mass of filaments and diffuse emission surrounding the two galaxies. A weak resolved radio core lies at the position of NGC 7237, and two arcs of steep-spectrum radio emission cross the region containing the optical galaxies, but no opaque radio core or head-tail jet structures are found, as is usually the case for radio galaxies of similar radio luminosity. The spectral index over the entire system is roughly constant. Typically, α ~ 1.1 with some regions differing by +/- 0.2 from this value. The properties of this radio source are similar to a few other well- studied systems, all of which are associated with binary galaxies and some of which also show evidence for mergers. We propose that the unusual morphology of 3C 442 is due to a galactic merger in progress. The high ellipticity of the cluster of galaxies in which NGC 7236/7237 are imbedded suggests that they may be the central galaxies in a pair of merging clusters. We suggest that the collimated gas was produced by a previously existing radio galaxy whose radio source was disrupted by the gravitational influence of the merger on the gas surrounding the source. The large-scale structure of 3C 442 is the result of these electrons interacting with the hot gas of the disturbed intracluster medium, and thereby becoming trapped in the debris of the merger. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/170698 Bibcode: 1991ApJ...382..108C Keywords: Interacting Galaxies; Radio Galaxies; Galactic Clusters; Radio Emission; Very Large Array (Vla); Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 7236; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 7237; GALAXIES: INTERACTIONS; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (3) NED (1)

Further morphological studies of QSOs

Results are presented of optical mapping of 29 QSOs, together with results of VLA observations of 16 of the optically observed objects. Principal results are that QSOs to z = 0.45 have extended nebulosity, the average ratio of nuclear to fuzz luminosity, corrected for resolution, is about 0.5, some 8 times higher than Seyfert I values, the average fuzz luminosity is -21.5, fainter than giant ellipticals, and fuzz morphology does not resemble E or S galaxies, but is very similar to Seyfert I's to similar luminosity limits. In some cases, indications of spiral structure are seen, and a large fraction of asymmetrical morphology is noted. In the QSO 0241 + 622, steep spectrum radio emission is found displaced from the central source exactly along the minor optical axis.