arXiv subjects

N. G. Kantharia

Publications and source records attributed to N. G. Kantharia.

32 records · Page 2Linked to original sources

arXiv2009

Double radio peak and non-thermal collimated ejecta in RS Ophiuchi following the 2006 outburst

We report MERLIN, VLA, OCRA-p, VLBA, Effelsberg and GMRT observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial "free expansion" phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4+/-2x10^{-7} M_\odot is consistent with a WD mass of 1.4 M_\odot. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data.

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arXiv2008

Chandra Observations of Nuclear X-ray Emission from Low Surface Brightness Galaxies

We present Chandra detections of x-ray emission from the AGN in two giant Low Surface Brightness (LSB) galaxies, UGC 2936 and UGC 1455. Their x-ray luminosities are 1.8\times10^{42} ergs/s and 1.1\times10^{40} ergs/s respectively. Of the two galaxies, UGC 2936 is radio loud. Together with another LSB galaxy UGC 6614 (XMM archival data) both appear to lie above the X-ray-Radio fundamental plane and their AGN have black hole masses that are low compared to similar galaxies lying on the correlation. However, the bulges in these galaxies are well developed and we detect diffuse x-ray emission from four of the eight galaxies in our sample. Our results suggest that the bulges of giant LSB galaxies evolve independently of their halo dominated disks which are low in star formation and disk dynamics. The centers follow an evolutionary path similar to that of bulge dominated normal galaxies on the Hubble Sequence but the LSB disks remain unevolved. Thus the bulge and disk evolution are decoupled and so whatever star formation processes produced the bulges did not affect the disks.

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arXiv2008

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

We present deep multifrequency observations using the Giant Metrewave Radio Telescope at 153, 244, 610 and 1260 MHz of a field centered on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as halos and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey, NRAO VLA Sky Survey and the Faint Images of the Radio Sky at Twenty-centimeters survey. Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index, $α\sim$0.8 (S$(ν)\proptoν^{-α}$), which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with $α\geq$1.3. We examine their optical fields and discuss the nature of some of these sources.

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arXiv2007

Low radio frequency signatures of ram pressure stripping in Virgo spiral NGC 4254

We report the detection of extended low radio frequency continuum emission beyond the optical disk of the spiral galaxy NGC 4254 using the Giant Metrewave Radio Telescope. NGC 4254, which has an almost face-on orientation, is located in the outskirts of the Virgo cluster. Since such extended emission is uncommon in low inclination galaxies, we believe it is a signature of magnetised plasma pushed out of the disk by ram pressure of the intracluster medium as NGC 4254 falls into the Virgo cluster. The detailed spectral index distribution across NGC 4254 shows that the steepest spectrum alpha < -1 (S propto nu^{alpha}) arises in the gas beyond the optical disk. This lends support to the ram pressure scenario by indicating that the extended emission is not from the disk gas but from matter which has been stripped by ram pressure. The steeper spectrum of the extended emission is reminiscent of haloes in edge-on galaxies. The sharp fall in intensity and enhanced polarization in the south of the galaxy, in addition to enhanced star formation reported by others provide evidence towards the efficacy of ram pressure on this galaxy. HI 21cm observations show that the gas in the north lags in rotation and hence is likely the atomic gas which is carried along with the wind. NGC 4254 is a particularly strong radio emitter with a power of 7x10^{22} Watts/Hz at 240 MHz. We find that the integrated spectrum of the galaxy flattens at lower frequencies and is well explained by an injection spectrum with alpha_0=-0.45+-0.12. We end by comparing published simulation results with our data and conclude that ram pressure stripping is likely to be a significant contributor to evolution of galaxies residing in X-ray poor groups and cluster outskirts.

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arXiv2007

GMRT Observations of the 2006 outburst of the Nova RS Ophiuchi: First detection of emission at radio frequencies < 1.4 GHz

The first low radio frequency (<1.4 GHz) detection of the outburst of the recurrent nova RS Ophiuchi is presented in this letter. Radio emission was detected at 0.61 GHz on day 20 with a flux density of ~48 mJy and at 0.325 GHz on day 38 with a flux density of ~ 44 mJy. This is in contrast with the 1985 outburst when it was not detected at 0.327 GHz even on day 66. The emission at low radio frequencies is clearly non-thermal and is well-explained by a synchrotron spectrum of index alpha ~ -0.8 (S propto nu^alpha) suffering foreground absorption due to the pre-existing, ionized, warm, clumpy red giant wind. The absence of low frequency radio emission in 1985 and the earlier turn-on of the radio flux in the current outburst are interpreted as being due to higher foreground absorption in 1985 compared to that in 2006, suggesting that the overlying wind densities in 2006 are only ~30% of those in 1985.

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arXiv2007

GMRT and VLA observations at 49cm and 20cm of the HII region near l=24.4d, b=0.1d

We report multifrequency radio continuum and hydrogen radio recombination line observations of HII regions near l=24.8d b=0.1d using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n=172), 0.61 GHz (n=220) and the Very Large Array (VLA) at 1.42 GHz (n=166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l=24.83d and b=0.1d is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ~ 550 solar mass with a linear size of 7 pc and an rms electron density of ~ 110 cm^-3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l=24.83d and b=0.1d at all observed frequencies near Vlsr=100 km/s. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.

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arXiv2006

Radio recombination lines from the largest bound atoms in space

In this paper, we report the detection of a series of radio recombination lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon atoms detected in space. These atoms, which are more than a million times larger than the ground state atoms are undergoing delta transitions (n~1009, Delta n=4) in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A. Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels n~1500. Our data indicates that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise detections of alpha, beta and gamma transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in line widths with quantum number (proportional to n^5) due to pressure and radiation broadening of lines is much gentler than expected from existing models which assume a power law background radiation field. This discrepancy had also been noted earlier. The model line widths had been overestimated since the turnover in radiation field of Cassiopeia A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modeling, the slower increase in line width with quantum number is naturally explained.

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arXiv2006

[CI] 492 GHz mapping toward Cas A

We have mapped the [C I] emission at 492 GHz toward the supernova remnant Cas A. We detect [C I] emission from the periphery of the diffuse Photon Dominated Region (PDR) covering the disk of Cas A, as traced by the carbon recombination lines, as well as from the denser PDRs associated with the molecular clouds towards the south-east. [C I] emission is detected from both the Perseus and Orion arm molecular clouds, with the -47 km s^-1 Perseus arm feature being strong enough to be detected at all positions. We estimate the C/CO relative abundance to be 0.2 at the position of the identified CO clouds and >1 for most of the cloud. Here we show that the distribution of [C I] emitting regions compared to the C+ region and molecular cloud is consistent with a scenario involving PDRs. Using physical models for PDRs we constrain the physical properties of the [C I] line-forming regions. We estimate the densities of the [C I] emitting regions to be between 10^2 and 10^3 cm^-3. Based on rather high volume filling factors (~50%) we conclude that [C I] emission mainly arises from diffuse neutral gas in the Perseus arm.

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arXiv2005

Cosmic ray electron acceleration in the remnant of nova GK Persei?

Radio observations of the nova remnant of GK Per around 1985 (epoch 1) have shown it to have a spectral index of -0.4 (S propto nu^alpha) at frequencies <1.4 GHz and a spectral index of -0.7 at frequencies >1.4 GHz (Seaquist et al 1989, Biermann et al 1995). This spectrum is either due to secondary electrons from p-p interactions (Biermann et al 1995) or due to the presence of at least two electron populations which dominate the emission at different frequencies. Our radio observations (Anupama & Kantharia 2005) of the remnant of GK Persei using the Giant Metrewave Radio Telescope (GMRT) and archival Very Large Array (VLA) data show that the nova remnant has a low frequency (<1.3 GHz) spectral index alpha ~ -0.85 (in 2003; epoch 3) and a high frequency spectral index of alpha ~ -0.7 (1997; epoch 2). Moreover the higher frequencies show an annual secular decrease of ~ 2.1 % between epochs 1 and 2 whereas there is no change at 0.33 GHz. This suggests a significant evolution of the spectrum over 20 years. In this paper, we discuss a possible physical basis for this spectral evolution based on a comparison of the synchrotron loss and electron acceleration timescales.

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arXiv2005

GMRT Observations of the Group Holmberg 124: Evolution by Tidal Forces and Ram Pressure ?

We report radio continuum and 21cm HI observations using the Giant Metrewave Radio Telescope of the group Ho124 comprising four late-type galaxies, NGC2820, Mrk108, NGC2814 and NGC2805. Three of the galaxies have clearly undergone tidal interactions as seen from the various morphological tidal signatures whereas other features in the group members appear to be due to ram pressure. Here, we describe four interesting results:(a) detection of the tidal radio continuum bridge at 330 MHz connecting the galaxies NGC2820+Mrk108 with NGC2814 with a spectral index of -1.8. The radio bridge was discovered by van der Hulst & Hummel (1985) who reported a spectral index of -0.8.(b) Detection of various tidal features including a possible tidal dwarf galaxy.(c) Sharp truncation in the HI distribution in the south of NGC2820 and in the HI & radio continuum in the north of NGC2814. The optical disks in both the cases look undisturbed. (d) Detection of a large one-sided HI loop to the north of NGC 2820. No radio continuum emission or Halpha emission is detected. We discuss various scenarios for the origin of this loop. The central starburst scenario is not supported due to lack of ionized gas in the loop. For an electron density of 4x10^(-4) cm^-3 we estimate the ram pressure force of the IGrM to be comparable to the gravitational pull of the disk of NGC2820. We suggest that the loop could have formed by ram pressure stripping if tidal effects had reduced the surface density of HI in NGC2820. Finally we suggest that the evolution of the Ho124 group may be governed by both tidal forces due to the interaction and ram pressure due to motion of the member galaxies in the IGrM and that the IGrM densities might not be too low (>~4x10^(-4)cm^-3). This should be verified by other observations.

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arXiv2005

A multiwavelength study of the remnant of nova GK Persei

We present new observations of the nebular remnant of the old nova GK Persei 1901, in the optical using the 2m HCT and at low radio frequencies using the GMRT. The evolution of the nova remnant indicates shock interaction with the ambient medium, especially in the southwest quadrant. Application of a simple model for the shock and its evolution to determine the time dependence of the radius of the shell in the southwest quadrant indicates that the shell is now expanding into an ambient medium that has a lower density compared to the density of the ambient medium ahead of the shock in 1987.There are indications of a recent interaction of the nova remnant with the ambient medium in the northeast quadrant also. The nova remnant of GK Per is detected at all the observed radio frequencies and is of similar extent as the optical remnant. Putting together our radio observations with VLA archival data on GK Per from 1997, we obtain three interesting results: 1. The spectrum above 1.4 GHz follows a power law with an index -0.7 and below 1.4 GHz follows a power law with an index ~ -0.85. This could be due to the presence of at least two populations of electrons dominating the global emission at different frequencies. 2. We record an annual secular decrease of 2.1% in the flux density of the nova remnant at 1.4 and 4.9 GHz between 1984 and 1997 which has left the spectral index unchanged at -0.7. No such decrease is observed in the flux densities below 1 GHz. 3. We record an increase in the flux density at 0.33 GHz compared to the previous estimate in 1987. We conclude that the remnant of nova GK Per is similar to supernova remnants and in particular, to the young supernova remnant Cas A.

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arXiv2004

Associated HI absorption in the z=3.4 radio galaxy B2 0902+343 observed with the GMRT

We have made observations of the associated HI absorption of a high redshift radio galaxy 0902+34 at z=3.395 with the Giant Meterwave Radio Telescope in the $323\pm1$ MHz band. We find a narrow absorption line with a flux density of 11.5 mJy at a redshift of 3.397 consistent with that observed by Uson et al. (1991), Briggs et al. (1993) and de Bruyn (1996).A weak broad absorption feature reported by de Bruyn (1996) has not been detected in our observations. We also place an upper limit of 4 mJy (2 $σ$) on emission line strength at the position where Uson et al. (1991) claimed to have found a narrow emission line.

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arXiv2002

Carbon Recombination Lines near 327 MHz I: "Diffuse" CII regions in the Galactic Disk

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution -- 2°$\times$ 2°& 2°$\times$ 6\arcmin) of radio recombination lines (RRLs) near 327 MHz in the longitude range $l = $ 332°$\to$ 89°using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between $l = $ 358°$\to$ 20°and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in ``diffuse'' \CII regions. The \lv diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from \HII regions indicating that the distribution of the diffuse \CII regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the [CII] 158 micron emission from diffuse \CII regions and find that upto 95 % of the total observed [CII] 158 micron emission can arise in diffuse \CII regions if the temperature of the latter $\sim 80$ K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale $\sim$ 6\arcmin. Analysis of the dual-resolution observations toward a 2°wide field centered at $l = $ 13°.9 and toward the longitude range $l = $ 1°.75 to 6°.75 shows the presence of narrow ($ΔV \le $ 15 \kms) carbon line emitting regions extending over several degrees in $l$ and $b$. The physical size perpendicular to the line-of-sight of an individual diffuse \CII region in these directions is $>$ 200 pc.

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arXiv2001

Carbon Recombination Lines from the Galactic Plane at 34.5 & 328 MHz

We present results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C$575α$) made using the dipole array at Gauribidanur near Bangalore. Observations made towards 32 directions, led to detections of lines in absorption at nine positions. Followup observations at 328 MHz (C$272α$) using the Ooty Radio Telescope detected these lines in emission. A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10' for the angular size of the line forming region. The longitude-velocity distribution of the observed carbon lines indicate that the line forming region are located mainly between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near 76 MHz (\nocite{erickson:95} Erickson \et 1995) we obtain constraints on the physical parameters of the line forming regions. We find that if the angular size of the line forming regions is $\ge 4^{\circ}$, then the range of parameters that fit the data are: \Te $= 20-40$ K, \ne $\sim 0.1-0.3$ \cm3 and pathlengths $\sim 0.07-0.9$ pc which may correspond to thin photo-dissociated regions around molecular clouds. On the other hand, if the line forming regions are $\sim 2^{\circ}$ in extent, then warmer gas (\Te $\sim 60-300$ K) with lower electron densities (\ne $\sim 0.03-0.05$ \cm3) extending over several tens of parsecs along the line of sight and possibly associated with atomic \HI gas can fit the data. Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation regions.

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