Radio observations of a few giant sources (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2007
We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of a sample of giant radio sources (GRSs), and high-frequency observations of three of these sources with the Very Large Array (VLA). From multifrequency observations of the lobes we estimate the magnetic field strengths using three different approaches, and show that these differ at most by a factor of ∼3. For these large radio sources the inverse-Compton losses usually dominate over synchrotron losses when estimates of the classical minimum energy magnetic field are used, consistent with earlier studies. However, this is often not true if the magnetic fields are close to the values estimated using the formalism of Beck & Krause. We also examine the spectral indices of the cores and any evidence of recurrent activity in these sources. We probe the environment using the symmetry parameters of these sources and suggest that their environments are often asymmetric on scales of ∼1 Mpc, consistent with earlier studies.
Discovery of a new, 2.2-Mpc giant radio galaxy at a redshift of 0.57
Monthly Notices of the Royal Astronomical Society
We report the discovery of one of the largest and most distant Giant Radio Galaxy (GRG) in the Lynx field which was discovered using deep Giant Metre-wave Radio Telescope (GMRT) 150 MHz observations. The core is detected at 150 MHz and also in the VLA FIRST survey. Spectroscopic observations carried out using the IUCAA Giravali Observatory(IGO) provided a redshift value of 0.57. This redshift was later confirmed with data from the Sloan Digital Sky Survey (Data Release 12). The angular size of the GRG is 5.5 arcmin and at the redshift of 0.57, its linear size is 2.2 Mpc. At this high redshift, only a few radio sources are known to have such large linear size. In order to estimate the spectral index of the bridge emission as well as the spectral age of the source, we observed this source at L-band, 610 MHz and 325 MHz bands with the GMRT. We present the spectral ageing analysis of the source which puts an upper limit of 20 Myr on the spectral age. The better resolution maps presented here as opposed to the original 150 MHz map shows evidence for a second episode of emission. We also find that the core is detected at all four frequencies with a spectral index of 0.85, which is steeper than normal, hence we speculate that the core may be a compact steep spectrum source (CSS), which makes this giant radio galaxy a candidate triple-double radio galaxy.
Monthly Notices of The Royal Astronomical Society, 1999
Giant radio sources (GRSs), defined to be those with a projected linear size greater than approximately 1 Mpc, are the largest single objects in the Universe, and are useful for probing a number of astrophysical questions. In this paper we present the results of a multifrequency study of a sample of moderate-redshift giants with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). From spectral index studies, we have estimated the ages and velocities of advancement for a few sources, and identified possible sites of reacceleration. In the GRSs, inverse-Compton losses with the cosmic microwave background radiation dominate over synchrotron radiative losses which could affect the identification of giants at large redshifts. We also show that the prominence of the bridge emission decreases with increasing redshift, possibly due to inverse-Compton losses, and estimate the expected X-ray flux from some of the objects in our sample.
A multifrequency study of possible relic lobes in giant radio sources
Monthly Notices of the …, 2009
We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of three giant radio sources (GRSs, J0139+3957, J0200+4049 and J0807+7400) with relaxed diffuse lobes which show no hotspots and no evidence of jets. The largest of these three, J0200+4049, exhibits a depression in the centre of the western lobe, while J0139+3957 and J0807+7400 have been suggested earlier by Klein et al. and Lara et al. respectively to be relic radio sources. We estimate the ages of the lobes. We also present Very Large Array (VLA) observations of the core of J0807+7400, and determine the core radio spectra for all three sources. Although the radio cores suggest that the sources are currently active, we explore the possibility that the lobes in these sources are due to an earlier cycle of activity.
The dynamics of the giant radio galaxy 3C���457
Monthly Notices of the Royal Astronomical Society, 2009
We present multifrequency radio observations with the Giant Metrewave Radio Telescope and Very Large Array, and X-ray observations with the X-ray Multi-Mirror Mission (XMM-Newton) telescope of the giant radio source (GRS) 3C 457. We have detected the core, lobes and the environment of the GRS in X-ray. We examine the relationships between the radio and X-ray emission, determine the radio spectrum over a large frequency range and attribute the X-ray emission from the lobes to the inverse Compton scattering of cosmic microwave background photons. The magnetic field strength of the lobes is very close to the equipartition value. Both the lobes are in pressure balance near the hotspots and apparently underpressured towards the core. The X-ray spectrum of the core of the GRS consists of an unabsorbed soft power-law component and a heavily absorbed hard power-law component. The soft unabsorbed component is likely to be related to the radio jets. There is no strong evidence of Fe Kα emission line in our data.
Monthly Notices of the Royal Astronomical Society, 2008
Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) are used to determine the spectral breaks in consecutive strips along the lobes of a sample of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sources range from ∼6 to 36 Myr with a median value of ∼20 Myr using the classical equipartition fields. Using the magnetic field estimates from the Beck & Krause formalism the spectral ages range from ∼5 to 38 Myr with a median value of ∼22 Myr. These ages are significantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, confirming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots, as is the case in earlier studies of smaller sources. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from ∼0.55 to 0.88 with a median value of ∼0.6. We discuss these values in the light of theoretical expectations, and show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.
The Astrophysical Journal, 2011
We present new Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz of 18 X-ray bright galaxy groups. These observations are part of an extended project, presented here and in future papers, which combines low-frequency radio and X-ray data to investigate the interaction between central active galactic nuclei (AGN) and the intra-group medium (IGM). The radio images show a very diverse population of group-central radio sources, varying widely in size, power, morphology and spectral index. Comparison of the radio images with Chandra and XMM-Newton X-ray images shows that groups with significant substructure in the X-ray band and marginal radio emission at > ∼ 1 GHz host low-frequency radio structures that correlate with substructures in IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction in our sample, are found in half of the systems, and are typically associated with small, low-or mid-power double radio sources. Two systems, NGC5044 and NGC4636, possess multiple cavities, which are isotropically distributed around the group center, possibly due to group weather. In other systems the radio/X-ray correlations are less evident. However, the AGN/IGM interaction can manifest itself through the effects of the high-pressure medium on the morphology, spectral properties and evolution of the radio-emitting plasma. In particular, the IGM can confine fading radio lobes in old/dying radio galaxies and prevent them from dissipating quickly. Evidence for radio emission produced by former outbursts that coexist with current activity is found in six groups of the sample.
Dynamical age vs spectral age of the lobes of selected giant radio sources (GRSs)
arXiv: Cosmology and Nongalactic Astrophysics, 2009
Dynamical ages of the opposite lobes determined {\sl independently} of each other suggest that their ratios are between sim\simsim1.1 to sim\simsim1.4. Demanding similar values of the jet power and the radio core density for the same GRS, we look for a {\sl self-consistent} solution for the opposite lobes, which results in different density profiles along them found by the fit. A comparison of the dynamical and spectral ages shows that their ratio is between sim\simsim1 and sim\simsim5, i.e. is similar to that found for smaller radio galaxies. Two causes of this effect are pointed out.
IGR J17488–2338: a newly discovered giant radio galaxy
Astronomy & Astrophysics, 2014
We present the discovery of a large scale radio structure associated with IGR J17488-2338, a source recently discovered by INTEGRAL and optically identified as a broad line AGN at redshift 0.24. At low frequencies, the source properties are those of an intermediate-power FR II radio galaxy with a linear size of 1.4 Mpc. This new active galaxy is therefore a member of a class of objects called Giant Radio Galaxies (GRGs), a rare type of radio galaxies with physical sizes larger than 0.7 Mpc; they represent the largest and most energetic single entities in the Universe and are useful laboratories for many astrophysical studies. Their large scale structures could be due either to special external conditions or to uncommon internal properties of the source central engine The AGN at the centre of IGR J17488-2338 has a black hole of 1.3×10 9 solar masses, a bolometric luminosity of 7×10 46 erg s −1 and an Eddington ratio of 0.3, suggesting that it is powerful enough to produce the large structure observed in radio. The source is remarkable also for other properties, among which its X-ray absorption, at odds with its type 1 classification, and the presence of a strong iron line which is a feature not often observed in radio galaxies.
A Case for Renewed Activity in the Giant Radio Galaxy J0116473
The Astrophysical …, 2002
We present ATCA radio observations of the giant radio galaxy J0116−473 at 12 and 22 cm wavelengths in total intensity and polarization. The images clearly reveal a bright inner-double structure within more extended edge-brightened lobe emission. The lack of hotspots at the ends of the outer lobes, the strong core and the inner-double structure with its edge-brightened morphology lead us to suggest that this giant radio galaxy is undergoing a renewed nuclear activity: J0116−473 appears to be a striking example of a radio galaxy where a young double source is evolving within older lobe material. We also report the detection of a Mpc-long linear feature which is oriented perpendicular to the radio axis and has a high fractional polarization.