C. H. Ishwara-Chandra

Giant radio sources

We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437-244 and 1025-229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (\ RM \ < 20 rad m(-2)). The giant sources are defined to be those with overall projected size greater than or equal to 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity-size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton lass owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.

SPECTRAL AND TIMING EVOLUTION OF THE BLACK HOLE X-RAY NOVA 4U 1543-47 DURING ITS 2002 OUTBURST

We present an X-ray spectral and timing analysis of 4U 1543-47 during its 2002 outburst based on 49 pointed observations obtained using the Rossi X-ray Timing Explorer (RXTE). The outburst reached a peak intensity of 4.2 Crab in the 2-12 keV band and declined by a factor of 32 throughout the month-long observation. A 21.9 +- 0.6 mJy radio flare was detected at 1026.75 MHz two days before the X-ray maximum; the radio source was also detected late in the outburst, after the X-ray source entered the low hard state. The X-ray light curve exhibits the classic shape of a rapid rise and an exponential decay. The spectrum is soft and dominated by emission from the accretion disk. The continuum is fit with a multicolor disk blackbody (kT_{max} = 1.04 keV) and a power-law (Gamma ~ 2.7). Midway through the decay phase, a strong low-frequency QPO (nu = 7.3-8.1 Hz) was present for several days. The spectra feature a broad Fe K alpha line that is asymmetric, suggesting that the line is due to relativistic broadening rather than Comptonization. Relativistic Laor models provide much better fits to the line than non-relativistic Gaussian models, particularly near the beginning and end of our observations. The line fits yield estimates for the inner disk radius that are within 6 R_g; this result and additional evidence indicates that this black hole may have a non-zero angular momentum.We present an X-ray spectral and timing analysis of 4U 1543-47 during its 2002 outburst based on 49 pointed observations obtained using the Rossi X-ray Timing Explorer (RXTE). The outburst reached a peak intensity of 4.2 Crab in the 2-12 keV band and declined by a factor of 32 throughout the month-long observation. A 21.9 +- 0.6 mJy radio flare was detected at 1026.75 MHz two days before the X-ray maximum; the radio source was also detected late in the outburst, after the X-ray source entered the low hard state. The X-ray light curve exhibits the classic shape of a rapid rise and an exponential decay. The spectrum is soft and dominated by emission from the accretion disk. The continuum is fit with a multicolor disk blackbody (kT_{max} = 1.04 keV) and a power-law (Gamma ~ 2.7). Midway through the decay phase, a strong low-frequency QPO (nu = 7.3-8.1 Hz) was present for several days. The spectra feature a broad Fe K alpha line that is asymmetric, suggesting that the line is due to relativistic broadening rather than Comptonization. Relativistic Laor models provide much better fits to the line than non-relativistic Gaussian models, particularly near the beginning and end of our observations. The line fits yield estimates for the inner disk radius that are within 6 R_g; this result and additional evidence indicates that this black hole may have a non-zero angular momentum.

ON THE ORIGIN OF THE VARIOUS TYPES OF RADIO EMISSION IN GRS 1915+105

We investigate the association between the radio plateau states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. To investigate the properties of the source during a superluminal flare, we present Giant Metrewave Radio Telescope observations during a radio flare that turned out to be a preplateau flare, as shown by the contemporaneous Ryle Telescope observations. A major superluminal ejection was observed at the end of this plateau state (as described by V. Dhawan et al.), associated with highly variable X-ray emission showing X-ray soft dips. This episode thus has all three types of radio emission: a preplateau flare, a plateau state, and superluminal jets. We analyze all the available Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array data during this episode and show that (1) the preflare plateau state consists of a three-component X-ray spectrum that includes a multicolor disk-blackbody, a Comptonized component, and a power law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source, identify several candidate superluminal flare events, and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the plateau state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares, with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are the interaction of the matter blob ejected during the X-ray soft dips with the steady jet already established during the plateau state. This picture can explain all the types of radio emission observed from this source in terms of its X-ray-emission characteristics.

A COMBINED LOW-RADIO FREQUENCY/X-RAY STUDY OF GALAXY GROUPS. I. GIANT METREWAVE RADIO TELESCOPE OBSERVATIONS AT 235 MHz AND 610 MHz

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 (AGNs) 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 co-exist with current activity is found in six groups of the sample.

Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase

Context. We explore the physics behind one of the brightest radio afterglows ever, GRB 030329, at late times when the jet is nonrelativistic. Aims. We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements. Methods. We observed the GRB 030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268−1128 days after the burst. We modeled all the available radio data and derived the physical parameters. Results. The index of the electron energy distribution is p = 2.1, the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at tNR ∼ 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous findings. Conclusions. Our findings indicate that the blast wave is roughly spherical at tNR, and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB 030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further.

LOOKING FOR A PULSE: A SEARCH FOR ROTATIONALLY MODULATED RADIO EMISSION FROM THE HOT JUPITER, τ BOÖTIS b

Hot Jupiters have been proposed as a likely population of low-frequency radio sources due to electron cyclotron maser emission of similar nature to that detected from the auroral regions of magnetized solar system planets. Such emission will likely be confined to specific ranges of orbital/rotational phase due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio observations of the hot Jupiter τ Bootis b, consisting of 40 hr carefully scheduled to maximize coverage of the planets 79.5 hr orbital/rotational period in an effort to detect such rotationally modulated emission. The resulting image is the deepest yet published at these frequencies and leads to a 3σ upper limit on the flux density from the planet of 1.2 mJy, two orders of magnitude lower than predictions derived from scaling laws based on solar system planetary radio emission. This represents the most stringent upper limits for both quiescent and rotationally modulated radio emission from a hot Jupiter yet achieved and suggests that either (1) the magnetic dipole moment of τ Bootis b is insufficient to generate the surface field strengths of >50 G required for detection at 150 MHz or (2) Earth lies outside the beaming pattern of the radio emission from the planet.

Giant Metrewave Radio Telescope Discovery of a Millisecond Pulsar in a Very Eccentric Binary System

We report the discovery of the binary millisecond pulsar J0514-4002A, which is the first known pulsar in the globular cluster NGC 1851 and the first pulsar discovered using the Giant Metrewave Radio Telescope (GMRT). The pulsar has a rotational period of 4.99 ms, an orbital period of 18.8 days, and the most eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum mass of 0.9 M_sun and its nature is presently unclear. After accreting matter from a low-mass companion star which spun it up to a (few) millisecond spin period, the pulsar eventually exchanged the low-mass star for its more massive present companion. This is exactly the same process that could form a system containing a millisecond pulsar and a black hole; the discovery of NGC 1851A demonstrates that such systems might exist in the Universe, provided that stellar mass black holes exist in globular clusters.

Deep GMRT 150-MHz observations of the LBDS-Lynx region: ultrasteep spectrum radio sources

It has been known for nearly three decades that high-redshift radio galaxies exhibit steep radio spectra, and hence ultrasteep spectrum radio sources provide candidates for high-redshift radio galaxies. Nearly all radio galaxies with z > 3 have been found using this redshift–spectral index correlation. We have started a programme with the Giant Metrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels about 10 to 100 times deeper than the known high-redshift radio galaxies which were identified primarily using the already available radio catalogues. In our programme, we have obtained deep, high-resolution radio observations at 150 MHz with GMRT for several ‘deep’ fields which are well studied at higher radio frequencies and in other bands of the electromagnetic spectrum, with an aim to detect candidate high-redshift radio galaxies. In this paper we present results from the deep 150-MHz observations of the LBDS-Lynx field, which has been already imaged at 327, 610 and 1412 MHz with the Westerbork Synthesis Radio Telescope and at 1400 and 4860 MHz with the Very Large Array. The 150-MHz image made with GMRT has an rms noise of ∼0.7 mJy beam−1 and a resolution of ∼19 × 15 arcsec2. It is the deepest low-frequency image of the LBDS-Lynx field. The source catalogue of this field at 150 MHz has about 765 sources down to ∼20 per cent of the primary beam response, covering an area of about 15 deg2. Spectral index was estimated by cross-correlating each source detected at 150 MHz with the available observations at 327, 610, 1400 and 4860 MHz and also using available radio surveys such as the Westerbork Northern Sky Survey at 327 MHz and the NRAO VLA Sky Survey and Faint Images of the Radio Sky at Twenty-cm at 1400 MHz. We find about 150 radio sources with spectra steeper than 1. About two-third of these are not detected in the Sloan Digital Sky Survey, hence are strong candidate high-redshift radio galaxies, which need to be further explored with deep infrared imaging and spectroscopy to estimate the redshift.

Radio observations of a few giant sources

We present multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) of a sample of 17 largely giant radio sources (GRSs). These observations have either helped clarify the radio structures or provided new information at a different frequency. The broad line radio galaxy, J0313+413, has an asymmetric, curved radio jet and a variable radio core, consistent with a moderate angle of inclination to the line of sight. We attempt to identify steep spectrum radio cores (SSCs), which may be a sign of recurrent activity, and find four candidates. If confirmed, this would indicate a trend for SSCs to occur preferentially in GRSs. From the structure and integrated spectra of the sources, we suggest that the lobes of emission in J0139+399 and J0200+408 may be owing to an earlier cycle of nuclear activity. We find that inverse-Compton losses with the cosmic microwave background radiation dominate over synchrotron radiative losses in the lobes of all the sources, consistent with earlier studies. We also show that the prominence of the bridge emission decreases with increasing redshift, possibly owing to inverse-Compton losses. This could affect the appearance and identification of GRSs at large redshifts.

Low-frequency radio monitoring of microquasars

Microquasars are radio-emitting X-ray binaries (REXBs) with a radio morphology like quasars and high X-ray luminosity. Sixteen known microquasar candidates were extensively monitored for the first time at low radio frequencies using the Giant Meter-wave Radio Telescope (GMRT) between 6-June 2003 and 22-Jan. 2005 at 0.235/0.61 (simultaneous) and 1.28 GHz. Nine out of sixteen sources were detected positively by the GMRT including all six high-mass X-ray binaries (HMXBs) and three low-mass X-ray binaries (LMXBs). Among the nine sources emitting at low frequencies, six are persistent in radio and three are transient at radio wavelengths. In the case of four persistent radio sources (Scorpius X-1, Cyg X-1, Cyg X-3, and LSI+61303) the contemporaneous data suggests a spectral turnover (S ν = ν α ,α > 0) and agrees with the synchrotron self absorption (SSA) effect expected at lower frequencies. The radio spectra of SS433 and LS5039 show a power law decay (S ν = ν α ,α < 0) with no signature of SSA even at the very low frequency of 0.235 GHz. This unique result suggests either that these sources are scatter-broadened at lower frequencies or that the low-frequency radio emission from these sources are superimposed by the emission from an extended region located near these sources. Five sources, GRO J1655-40, XTE J1118+480, 1E1740.7-2942, XTE J1748-288, and GRS 1758-258 were never detected during our observations, thus suggesting that they show the SSA effect at lower frequencies or that they are too faint to be detected at GMRT frequencies. Because interstellar scintillation becomes dominant at low frequencies and may lead to flux-density fluctuations, the scintillation time scale for each microquasar was calculated and compared to the variability time scale in the data. We confirm from these studies that Cyg X-1 and SS433 are most likely affected by scintillation and that LSI+61303, LS 5039, Sco X-1, and XTE J1118+480 may possibly be affected by scintillation. A comparative study of the radio luminosity from centimeter-(GHz) to meter-wavelength (MHz) suggests a decrease by a few orders of magnitude as one goes lower in frequency. We have also plotted the RXTE/ASM X-ray light curve for all the sixteen known microquasars. Based on the ASM data, the X-ray light curve can be classified as: (a) persistent, (b) quasi-persistent or (c) transient. From the analysis of these types and the information about their companion star, the persistent or transient nature of the radio jet can be confirmed. This paper provides a general review of the main observational results obtained up to now, as well as different models for the production of low-frequency radio emissions from these sources.