Giant Radio Research Articles

Foregrounds for redshifted 21-cm studies of reionization: Giant Meter Wave Radio Telescope 153-MHz observations

Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ~ 1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum Cl. We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V 2 (U, 0). From our observations, we find that V 2 (U, 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ(U, Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ(U, Δν), in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures.

A Giant Radio Jet Ejected by an Ultramassive Black Hole in a Single-lobed Radio Galaxy

We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black hole (>10^9 solar mass) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II megajet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.

Production of TeV Gamma Radiation in the Vicinity of the Supermassive Black Hole in the Giant Radio Galaxy M87

Although the giant radio galaxy M87 harbors many distinct regions of broadband nonthermal emission, the recently reported fast variability of TeV γ-rays from M87, on a timescale of days, strongly constrains the range of speculations concerning the possible sites and scenarios of particle acceleration responsible for the observed TeV emission. A natural production site of this radiation is the immediate vicinity of the central supermassive black hole (BH). Because of its low bolometric luminosity, the nucleus of M87 can be effectively transparent for γ-rays up to an energy of 10 TeV, which makes this source an ideal laboratory for the study of particle acceleration processes close to the BH event horizon. We critically analyze different possible radiation mechanisms in this region and argue that the observed very high energy γ-ray emission can be explained as the inverse Compton emission of ultrarelativistic electron-positron pairs produced through the development of an electromagnetic cascade in the BH magnetosphere. We demonstrate, through detailed numerical calculations of acceleration and radiation of electrons in the magnetospheric vacuum gap, that this pulsar magnetosphere-like scenario can satisfactorily explain the main properties of the TeV γ-ray emission from M87.

Deep radio images of the HEGRA and Whipple TeV sources in the Cygnus OB2 region

Context. The modern generation of Cherenkov telescopes has revealed a new population of gamma-ray sources in the Galaxy. Some of them have been identified with previously known X-ray binary systems while other remain without clear counterparts a lower energies. Our initial goal here was reporting on extensive radio observations of the first extended and yet unidentified source, namely TeV J2032+4130. This object was originally detected by the HEGRA telescope in the direction of the Cygnus OB2 region and its nature has been a matter of debate during the latest years. The situation has become more complex with the Whipple and MILAGRO telescopes new TeV detections in the same field which could be consistent with the historic HEGRA source, although a different origin cannot be ruled out. Aims. We aim to pursue our radio exploration of the TeV J2032+4130 position that we initiated in a previous paper but taking now into account the latest results from new Whipple and MILAGRO TeV telescopes. The data presented here are an extended follow up of our previous work. Methods. Our investigation is mostly based on interferometric radio observations with the Giant Metre Wave Radio Telescope (GMRT) close to Pune (India) and the Very Large Array (VLA) in New Mexico (USA). We also conducted near infrared observations with the 3.5 m telescope and the OMEGA2000 camera at the Centro Astronomico Hispano Aleman (CAHA) in Almeria (Spain). Results. We present deep radio maps centered on the TeV J2032+4130 position at different wavelengths. In particular, our 49 and 20 cm maps cover a field of view larger than half a degree that fully includes the Whipple position and the peak of MILAGRO emission. Our most important result here is a catalogue of 153 radio sources detected at 49 cm within the GMRT antennae primary beam with a full width half maximum (FWHM) of 43 arc-minute. Among them, peculiar sources inside the Whipple error ellipse are discussed in detail, including a likely double-double radio galaxy and a one-sided jet source of possible blazar nature. This last object adds another alternative counterpart possibility to be considered for both the HEGRA, Whipple and MILAGRO emission. Moreover, our multi-configuration VLA images reveal the non-thermal extended emission previously reported by us with improved angular resolution. Its non-thermal spectral index is also confirmed thanks to matching beam observations at the 20 and 6 cm wavelengths.

Simultaneous VLBI/GMRT/RXTE observation of SS433

Aims. Our aim in this paper is to discover the spectral characteristics of the enigmatic compact object SS433 during a very rare occasion when the blue jet was pointing towards us (the precessional phase $\psi=0$) and when the orbital phase ϕ of its companion was nearly zero (inferior conjunction) and compare this with the result obtained when $\phi\sim 0.5$ (superior conjunction). Methods. We employed the Giant Meter Radio Telescope (GMRT) for the photometric observation at 1280 MHz, European VLBI Network (EVN) for photometry and imaging at 4990 MHz, and the Rossi X-ray Timing Explorer (RXTE) for photometry and spectrum at $3{-}25$ keV. Results. At this opportune moment, the companion totally blocked the disk and the base of the jet, the X-ray intensity was reduced by $20\%$ with respect to the previous day, and the intensity went up by about $65\%$ on the next day. The X-ray intensity is about one-third compared to RXTE results obtained during the superior conjunction on 13 March 2004. By taking the difference in the spectra, we obtained the spectrum of the system core including the contribution from any accretion shock that may have occurred during interaction between the accreting stream and the disk. The EVN observation clearly showed the separation of a radio blob, so using an adiabatic expansion model we estimate when the blob might have been separated. Conclusions. We see a clear dependence of X-ray intensity emitted from the base of the jet on the photon energy. We find that the core is much brighter in hard X-rays. The conjunction seems to have had no effect on the radio data.

A GHz Flare in a Quiescent Black Hole and A Determination of the Mass Accretion Rate

If the total energy of a radio flare is known, one can estimate the mass accretion rate of the disk by assuming equipartition of magnetic energy and the gravitational potential energy of accreting matter. We present here an example of how such an estimate could be done. Our recent radio observation using the Giant Meter Radio Telescope (GMRT) of the galactic black hole transient A0620-00 at 1.280 GHz revealed a micro-flare of a few milli-Jansky. Assuming a black hole mass of $10 M_\odot$ for the compact object, we find the accretion rate to be at the most ${\dot M} = (8.5 \pm 1.4) \times 10^{-11} (\frac{x}{3})^{5/2} M_\odot$ yr$^{-1}$, where, $x$ is the distance from the hole in units of Schwarzschild radius. This is consistent with the earlier estimate of the accretion rate based on optical and X-ray observations. We claim that this procedure is general enough to be used for any black hole candidate.

SS 433: results of a recent multiwavelength campaign

We conducted a multiwavelength campaign in 2002 September-October, to observe SS 433. We used the Giant Meter Radio Telescope for radio observations, the Physical Research Laboratory Infrared Telescope at Mt Abu for infrared (IR), the ARIES telescope at Nainital for optical photometry, the telescope at the Vainu Bappu observatory for spectral measurements and the Rossi X-ray Timing Explorer for X-ray observations. We find sharp variations in intensity on time-scales of a few minutes in the X-ray, IR and radio wavelengths. Differential photometry in the IR observations clearly indicates significant intrinsic variations on short time-scales of minutes throughout the campaign. Combining the results for these wavelengths, we find a signature of delay of about two days between the IR and radio signals. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broad-band spectrum averaged over the campaign duration.

A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR 1806-20

The brightest giant flare from the soft γ-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least 1 order of magnitude more energetic than those of the two other SGR giant flares. Starting from about 1 week after the burst, a very bright (~80 mJy), fading radio afterglow was detected. Follow-up observations revealed the multifrequency light curves of the afterglow and the temporal evolution of the source size. Here we show that these observations can be understood in a two-component explosion model. In this model, one component is a relativistic collimated outflow responsible for the initial giant flare and the early afterglow, and the other component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss the triggering mechanisms of these two components within the framework of the magnetar model.

A Wide‐Field, Low‐Frequency Radio Image of the Field of M31. II. Source Classification and Discussion

We have previously presented the results of a 325 MHz radio survey of M31, conducted with the A-configuration of the Very Large Array. In this survey, a total of 405 radio sources between <6 and 170 in extent were mapped with a resolution of 6 and a sensitivity of ~0.6 mJy/beam. Here, we compare the resultant source list and image with other radio, IR, optical, X-ray observations and catalogs of the region. Through this, we were able to identify five supernova remnant (SNR) candidates and three pulsar wind nebula (PWN) candidates in M31, as well as three Milky Way radio stars, a possible Milky Way Planetary Nebula, and a bevy of interesting extragalactic objects: a BL Lac, a Giant Radio Galaxy, a galaxy merger, and several high-z radio galaxy candidates. In addition, a large number of compact (<6) extremely steep spectrum sources ($\alpha \leq -1.6; S_{\nu} \propto \nu^{\alpha}$) were detected whose nature is unknown.

Multi-frequency study of the B3 VLA sample

We present 74-MHz data of 365 B3 VLA sources, which were obtained as a by-product of the observations of Tschager et al. (2003, A&A, 402, 171) who observed a large area of sky with the VLA in A-array. Apart from the lowest- frequency observations of B3 VLA sources performed so far contributing to our on-going multi-frequency study of this survey, these A-array data also provide the first morphological information at this low frequency. The most intriguing result is the discovery of two new Giant Radio Galaxy candidates, B3 1232+397B at a redshift of 3.22 and B3 1419+419 at a redshift of 0.367. This means that B3 1232+397B would be the most distant GRG known to date. At the same time, these two objects are two new examples of sources with recurrent activity. Our results give a foretaste of the impact which extremely low-frequency radio continuum observations will have for the study of source evolution.

Mass accretion rate of the galactic black hole A0620-00 in its quiescent state

Our recent radio observation using the Giant Meter Radio Telescope (GMRT) of the galactic black hole transient A0620-00 at 1.280GHz revealed a micro-flare of a few milli-Ja nsky. Assuming an equipartition of magnetic energy and the gravitational potential energy of accreting matter, it is p ossible to estimate the upper limit of the accretion rate. As suming a black hole mass of 10M⊙ residing at the center, we find this to be at the most ˙ M = (8.5± 1.4)× 10 −11 M⊙ yr −1 . This is consistent with earlier estimates of accretion rates based on optical a nd X-ray observations.

The Large Adaptive Reflector: A Giant Radio Telescope with an Aero Twist

A multi-national grouping of radio astronomers has identified the need for a major new instrument, called the Square Kilometer Array (SKA), to be constructed in the coming decade. This giant radio telescope will be 100 times more powerful than any in existence today. The innovative Canadian concept for the underlying design of the telescope, the Large Adaptive Reflector (LAR), is among the best candidates and is well positioned to be adopted by the international community. The LAR is a concept for a low-cost, large-aperture, wideband, radio telescope, designed to operate over the wavelength range from 2mt o 1.4 cm. Theproposed design for the LAR includes two central components. The first is a long focal length, large-diameter parabolic reflector, composed of actuated hexagonal panels, mounted on the ground. The second component is a focal package supported at a height of 500 m by a large helium balloon (aerostat) and a system of three or more taut tethers. The telescope is steered by simultaneously changing the lengths of the tethers with winches (thus changing the position of the feed) and by modifying the shape of the reflector. Simulations have shown that in operating wind conditions, the position of the feed platform can be stabilized to within a few centimetres. This paper gives an overview of the present state of the LAR design, with an emphasis on the airborne subsystem. Construction of a 1/3-scale model of the tethered aerostat subsystem, with a footprint of only 0.5 km 2 , is presently underway in Penticton, B.C. It will allow a validation of the underlying design and the study of some fascinating issues in the design and control of this system. Continued on page 240

A Case for Renewed Activity in the Giant Radio Galaxy J0116−473

We present Australia Telescope Compact Array (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 hot spots 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 renewed nuclear activity: J0116-473 appears to be a striking example of a radio galaxy in which a young double source is evolving within older lobe material. We also report the detection of a megaparsec-long linear feature that is oriented perpendicular to the radio axis and has a high fractional polarization.

Solar Radio Astronomy at Low Frequencies

This review is concerned to study of sun at frequencies lower than 1.4 GHz. Emphasis is made on results which illustrate the topics in which GMRT could play a major role. Coordinated studies including spectral and imaging radio observations are important for research in solar physics. Joint observations between the Giant Meter Radio Telescope (GMRT) with radio instruments located in the same longitude range are encouraged. This review inludes three distinct topics: Electron beams and radio observations- Radio signatures of Coronal Mass Ejections- Radio signatures of coronal and interplanetary shocks.

Energy Density and Radiation Losses in Giant Radio Galaxies

AbstractThe volumes, equipartition energy density, and radiation losses due to the synchrotron and inverse Compton interactions as a function of redshift and projected linear size of Giant radio galaxies are discussed. The new results are based on data from three samples: 1) Ishwara-Chandra &amp; Saikia (1999), 2) Schoenmakers (Ph.D. Thesis, 1999), and 3) Machalski, Jamrozy &amp; Zola (2001).

Giant Radio Galaxies and the Inter Galactic Medium

Using the WENSS survey, we have selected a complete sample of 26 Giant Radio Galaxies. We have investigated the properties of their radio structures using multifrequency radio observations. A spectral ageing analisys reveals that, among other things, that these sources are typically 50 — 100 Myr old and that their lobes have higher advance velocities than smaller sources of similar radio power. Further, we find pressure gradients in their radio lobes, suggesting that the lobes are still overpressured with respect to the environment. We find no evidence for a cosmological evolution of the radio lobe pressures with increasing redshift, at least up to z ∼ 0.3, other than that caused by our selection technique. A faint, higher redshift sample of giant sources is needed to constrain the pressure in their environments, the IGM. We are well underway with selecting such a sample, and the GMRT can play an important role in studying the radio properties of these sources.

Real-time signal processor for pulsar studies

This paper describes the design, tests and preliminary results of a real-time parallel signal processor built to aid a wide variety of pulsar observations. The signal processor reduces the distortions caused by the effects of dispersion, Faraday rotation, doppler acceleration and parallactic angle variations, at a sustained data rate of 32 Msamples/sec. It also folds the pulses coherently over the period and integrates adjacent samples in time and frequency to enhance the signal-to-noise ratio. The resulting data are recorded for further off-line analysis of the characteristics of pulsars and the intervening medium. The signal processing for analysis of pulsar signals is quite complex, imposing the need for a high computational throughput, typically of the order of a Giga operations per second (GOPS). Conventionally, the high computational demand restricts the flexibility to handle only a few types of pulsar observations. This instrument is designed to handle a wide variety of Pulsar observations with the Giant Metre Wave Radio Telescope (GMRT), and is flexible enough to be used in many other high-speed, signal processing applications. The technology used includes field-programmable-gate-array(FPGA) based data/code routing interfaces, PC-AT based control, diagnostics and data acquisition, digital signal processor (DSP) chip based parallel processing nodes and C language based control software and DSP-assembly programs for signal processing. The architecture and the software implementation of the parallel processor are fine-tuned to realize about 60 MOPS per DSP node and a multiple-instruction-multiple-data (MIMD) capability.

Trying to Enlarge the Sky Coverage of the FAST

AbstractThe proposed FAST project is a novel giant spherical radio telescope, with active elements which form a temporary paraboloid to track radio objects in the sky. Efforts have been made to extend the limited sky coverage that is a characteristic disadvantage of Arecibo-style radio telescopes. Three measures under investigation are introduced in this paper. The expected performance of the telescope is described, and a brief comparison is made with some of the largest existing radio telescopes.

Trying to Enlarge the Sky Coverage of the Fast

The proposed FAST project is a novel giant spherical radio telescope, with active elements which form a temporary paraboloid to track radio objects in the sky. Efforts have been made to extend the limited sky coverage that is a characteristic disadvantage of Arecibo-style radio telescopes. Three measures under investigation are introduced in this paper. The expected performance of the telescope is described, and a brief comparison is made with some of the largest existing radio telescopes.

Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies

Sensitive images of low-level, Mpc-sized radio cocoons offer new opportunities to probe large scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 (Mack et al. 1997,1998) reveal significant asymmetries and particularities in the morphology, radio spectrum and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315 vicinity confirms its location to be near, or at a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus Supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a `weather station' (Burns 1998) probing the flow of the elusive and previously invisible IGM gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma-rays and ultra high energy cosmic rays is briefly discussed.