arXiv subjects

Neeraj Gupta

Publications and source records attributed to Neeraj Gupta.

50 records · Page 3Linked to original sources

arXiv2010

Geometric Design and Stability of Power Networks

From the perspective of the network theory, the present work illustrates how the parametric intrinsic geometric description exhibits an exact set of pair correction functions and global correlation volume with and without the inclusion of the imaginary power flow. The Gaussian fluctuations about the equilibrium basis accomplish a well-defined, non-degenerate, curved regular intrinsic Riemannian surfaces for the purely real and the purely imaginary power flows and their linear combinations. An explicit computation demonstrates that the underlying real and imaginary power correlations involve ordinary summations of the power factors, with and without their joint effects. Novel aspect of the intrinsic geometry constitutes a stable design for the power systems.

stat.AP↗

arXiv2009

HI gas in the rejuvenated radio galaxy 4C29.30

We report the results of our observations of HI absorption towards the central region of the rejuvenated radio galaxy 4C29.30 (J0840+2949) with the Giant Metrewave Radio Telescope (GMRT). The radio source has diffuse, extended emission with an angular size of $\sim$520 arcsec (639 kpc) within which a compact edge-brightened double-lobed source with a size of 29 arcsec (36 kpc) is embedded. The absorption profile which is seen towards the central component of the inner double is well resolved and consists of six components; all but one of which appears to be red-shifted relative to the optical systemic velocity. The neutral hydrogen column density is estimated to be $N$(HI)=4.7$\times10^{21}$($T_s$/100)($f_c$/1.0) cm$^{-2}$, where $T_s$ and $f_c$ are the spin temperature and covering factor of the background source respectively. This detection reinforces a strong correlation between the occurrence of HI absorption and rejuvenation of radio activity suggested earlier, with the possibility that the red-shifted gas is fuelling the recent activity.

astro-ph.CO↗

arXiv2009

Constraining fundamental constants of physics with quasar absorption line systems

We summarize the attempts by our group and others to derive constraints on variations of fundamental constants over cosmic time using quasar absorption lines. Most upper limits reside in the range 0.5-1.5x10-5 at the 3sigma level over a redshift range of approximately 0.5-2.5 for the fine-structure constant, alpha, the proton-to-electron mass ratio, mu, and a combination of the proton gyromagnetic factor and the two previous constants, gp(alpha^2/mu)^nu, for only one claimed variation of alpha. It is therefore very important to perform new measurements to improve the sensitivity of the numerous methods to at least <0.1x10-5 which should be possible in the next few years. Future instrumentations on ELTs in the optical and/or ALMA, EVLA and SKA pathfinders in the radio will undoutedly boost this field by allowing to reach much better signal-to-noise ratios at higher spectral resolution and to perform measurements on molecules in the ISM of high redshift galaxies.

astro-ph.CO↗

arXiv2009

Science with the Australian Square Kilometre Array Pathfinder (ASKAP)

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

astro-ph.CO↗

arXiv2007

Energy and System Size Dependence of Photon Production at Forward Rapidities at RHIC

The energy and system size dependence of pseudorapidity ($η$) and multiplicity distributions of photons are measured in the region -2.3 $\leq$ $η$ $\leq$ -3.7 for Cu + Cu collisions at $\sqrt{s_{NN}}$ = 200 and 62.4 GeV. Photon multiplicity measurements at forward rapidity have been carried out using a Photon Multiplicity Detector (PMD) in the STAR experiment. Photons are found to follow longitudinal scaling for Cu + Cu collisions for 0-10% centrality. A Comparison of pseudorapidity distributions with HIIJING model is also presented.

nucl-ex↗

arXiv2006

Detecting cold gas at intermediate redshifts: GMRT survey using Mg II systems

Intervening HI 21-cm absorption systems at z > 1.0 are very rare and only 4 confirmed detections have been reported in the literature. Despite their scarcity, they provide interesting and unique insights into the physical conditions in the interstellar medium of high-z galaxies. Moreover, they can provide independent constraints on the variation of fundamental constants. We report 3 new detections based on our ongoing Giant Metrewave Radio Telescope (GMRT) survey for 21-cm absorbers at 1.10< z_abs< 1.45 from candidate damped Lyman_alpha systems. The 21-cm lines are narrow for the z_abs = 1.3710 system towards SDSS J0108-0037 and z_abs = 1.1726 system toward SDSS J2358-1020. Based on line full-width at half maximum, the kinetic temperatures are <= 5200 K and <=800 K, respectively. The 21-cm absorption profile of the third system, z_abs =1.1908 system towards SDSS J0804+3012, is shallow, broad and complex, extending up to 100 km/s. The centroids of the 21-cm lines are found to be shifted with respect to the corresponding centroids of the metal lines derived from SDSS spectra. This may mean that the 21-cm absorption is not associated with the strongest metal line component.

astro-ph↗

arXiv2006

HI gas in rejuvenated radio galaxies: GMRT observations of the DDRG J1247+6723

We report the detection of HI absorption towards the inner double of the double-double radio galaxy (DDRG) J1247+6723 with the Giant Metrewave Radio Telescope (GMRT). The inner double is a Giga-hertz peaked spectrum (GPS) source with a linear size of 14 pc while the overall size defined by the outer double is 1195 kpc, making it a giant radio source. The absorption profile is well resolved and consists of a number of components on either side of the optical systemic velocity. The neutral hydrogen column density is estimated to be N(HI)=6.73*10^{20}(T_s/100)(f_c/1.0) cm^{-2}, where T_s and f_c are the spin temperature and covering factor of the background source respectively. We explore any correlation between the occurrence of HI absorption and rejuvenation of radio activity and suggest that there could be a strong relationship between them.

astro-ph↗

arXiv2006

Multiwavelength investigation of a near-solar metallicity sub-DLA at z =1.3647 towards PKS 0237-233

We searched for 21-cm absorption associated with the z_abs = 1.3647 absorption system toward PKS 0237-233 using the GMRT. A high quality UVES spectrum shows that C I and C I* are detected at this redshift together with C II*, Mg I, Mg II, Si II, Al II, Fe II and Mn II. The complex profiles, spread over ~300 km/s, are fitted with 21 Voigt profile components. None of these components are detected in 21-cm absorption down to a detection limit of τ(3σ)\le 3x10^{-3} (or N(HI)/T_S <10^{17} cm ^-2 K^-1). We derive log N(HI)<19.30$\pm$0.30 using the Lyman alpha absorption line detected in the IUE spectrum of the quasar. Mg II, Si II and Al II column densities are consistent with near solar metallicity and we measure [O/H]>-0.33. Using photoionization models constrained by the fine-structure excitations of C I and C II, and the 21-cm optical depth, we show that the C I absorption arises predominantly either in WIM or WNM in ionization and thermal equilibrium with the meta-galactic UV background dominated by QSOs and star forming galaxies. The estimated thermal pressure of the gas is of the same order of magnitude over different velocity ranges through the absorption profile (2.6\le log [P/k cm^{-3} K]\le 4.0). The gas-phase metallicity corrected for ionization is Z>0.5 Z_\odot with a signature of Fe co-production elements being under abundant compared to α-process elements by ~0.5 dex. At z>1.9, C I absorption is usually associated with H_2 absorption arising from cold gas in DLAs. This system and the z=2.139 toward Tol 1037-270 are the only two systems known which show that C I absorption can also be detected in warm gas provided the metallicity is high enough. Interestingly, both the systems are part of unusual concentrations of absorption lines.

astro-ph↗

arXiv2006

Probing radio source environments via HI and OH absorption

We present the results of HI and OH absorption measurements towards a sample of radio sources using the Arecibo 305-m telescope and the GMRT. In total, 27 radio sources were searched for associated 21-cm HI absorption. One totally new HI absorption system was detected against the radio galaxy 3C258, while five previously known HI absorption systems, and one galaxy detected in emission, were studied with improved frequency resolution and/or sensitivity. Our sample included 17 GPS and CSS objects, 4 of which exhibit HI absorption. This detection rate of ~25% compares with a value of ~40% by Vermeulen et al. for similar sources. We detected neither OH emission nor absorption towards any of the sources that were observed at Arecibo, and estimate a limit on the abundance ratio of N(HI)/N(OH)>4x10^6 for 3C258. We have combined our results with those from other available HI searches to compile a heterogeneous sample of 96 radio sources consisting of 27 GPS, 35 CSS, 13 flat spectrum and 21 large sources. The HI absorption detection rate is highest (~45%) for the GPS sources and least for the large sources. We find HI column density to be anticorrelated with source size, as reported earlier by Pihlström et al. The HI column density shows no significant dependence on either redshift or luminosity, which are themselves strongly correlated. These results suggest that the environments of radio sources on GPS/CSS scales are similar at different redshifts. Further, in accordance with the unification scheme, the GPS/CSS galaxies have an HI detection rate of ~40% which is significantly higher than the detection rate (~20%) towards the GPS/CSS quasars. Also, the principal (strongest) absorption component detected towards GPS sources appears blue-shifted in ~65% of the cases, in agreement with the growing evidence for jet-cloud interactions.

astro-ph↗

arXiv2006

Unification scheme and the distribution of neutral gas in compact radio sources

We examine the consistency of HI properties with the unification scheme for radio galaxies and quasars, and any correlation with the symmetry parameters for a sample of CSS and GPS sources. In our sample, 15 out of 23 galaxies exhibit 21-cm absorption as against 1 out of 9 quasars, which is broadly consistent with the unification scheme. Also there is a tendency for the detection rate as well as the column density for galaxies to increase with core prominence, f_c, a statistical indicator of the orientation of the jet axis to the line of sight. This can be understood in a scenario where radio sources are larger than the scale of the circumnuclear HI disk so that the lines of sight to the lobes at very large inclinations do not intersect the disk. The sources in our sample also exhibit the known anti-correlation between HI column density and source size. This suggests that small linear size, along with intermediate values of core prominence, is a good recipe for detecting 21-cm absorption in CSS and GPS objects. Some of the absorption could also be arising from clouds which may have interacted with the radio jet. The HI column density and velocity shift of the primary absorption component, however, show no dependence on the degree of misalignment and the separation ratio of the radio sources.

astro-ph↗

arXiv2006

Associated 21-cm HI absorption towards the radio galaxy 3C452 (J2245+3941)

We report the detection of 21-cm HI absorption towards the core of the Fanaroff-Riley II radio galaxy 3C452 (J2245+3941). The absorption profile is well resolved into three components; the strongest and narrowest component being coincident with the velocity corresponding to [O III] emission lines while the other two components are blue-shifted with respect to it by ~30 and 115 km\s. If the systemic velocity of the host galaxy is determined from low-ionization lines, which are red-shifted with respect to the [O III] doublet by about ~200 km\s, then both the [O III] emission and 21-cm absorption lines are associated with outflowing material. The neutral hydrogen column density is estimated to be N(HI) = 6.39X10^{20}(T_s/100)(1.0/f_c) cm^{-2}, where T_s and f_c are the spin temperature and partial coverage of the background source respectively. If the 21-cm absorber is also responsible for the nuclear extinction at infrared wavelengths and x-ray absorption, then for a spin temperature of ~8000 K, the absorber occults only ~10 per cent of the radio core.

astro-ph↗

arXiv2005

Outflowing material in the CSS quasar 3C48: evidence of jet-cloud interaction?

We report the detection of a zabs=0.3654 associated absorption-line system in the UV spectrum of the CSS quasar 3C48. The absorbing material is blue shifted with respect to the quasar emission-line redshift, zem=0.3700, suggesting an outflow velocity of ~1000 km/s. We detect absorption lines over a range of ionization states from Ly-beta, Ly-gamma, CIV, NIV, SVI to OVI and possibly OIV and NeVIII. The kinematical properties of the absorption-line system are similar to the blue-shifted emission line gas seen in [OIII]\lambda5007 (Chatzichristou, Vanderriest & Jaffe 1999), which is believed to have interacted with the radio jet. We study the properties of the absorbing material using CLOUDY and find that photoionization models with Solar abundance ratios (with overall metallicity in the range 0.1<Z/Z$_\odot$<1.3) are enough to explain the observed column densities of all the species except NeVIII, detection of which requires confirmation. Since the cooling and recombination time for the gas is ~10^5 yr, the consistency with the photoionization models suggests that any possible interaction of absorbing material with the jet must have taken place before ~10^5 yr. The abundance ratio of nitrogen to carbon is close to Solar values, unlike in the case of most quasars, especially at high-redshifts, which have super-Solar values. We observed 3C48 with the Giant Metrewave Radio Telescope (GMRT) to search for redshifted 21cm HI absorption. However, we did not detect any significant feature in our spectra and estimate the 3-sigma upper limit to the optical depth to be in the range 0.001 to 0.003. However, due to the diffuse nature of the radio source, optical depths as high as 0.1 towards individual knots or compact components cannot be ruled out.

astro-ph↗

arXiv2003

Outflowing material in the zem = 4.92 BAL QSO SDSS~J160501.21-011220.0

We present the analysis of two BALs seen in the spectrum of the QSO SDSS J160501.21-011220.0. Our UVES spectrum shows two well detached BALs at z(abs)= 4.685 and 4.855. The system at z(abs)= 4.855 covers the background source completely suggesting that the gas is located outside the BLR. On the contrary the system at z(abs)= 4.685 has a covering factor of ~0.9. The observed H I absorption line together with the limits on C II and Si II absorptions suggest that 16<log N(H I) (cm^{-2})<~17 in z(abs) = 4.855 absorption system. Comparison with models show that the observed column densities of N V, Si IV and C IV in this system require that N is underabundant by more than a factor 3 compared to Si if the ionizing radiation is similar to a typical QSO spectrum. This is contrary to what is usually derived for the emission line gas in QSOs. We show that the relative suppression in the N V column density can be explained for Solar abundance ratios or abundance ratios typical of Starburst abundances if an ionizing spectrum devoid of X-rays is used instead. Thus, if the composition of BAL is like that of BLR then it is most likely that the cloud sees a spectrum devoid of X-rays similar to what we observe from this QSO. This is consistent with the fact that none of our models have high Compton optical depth to remove X-rays from the QSO. Similar arguments lead to the conclusion that the system at z(abs)= 4.685 as well is not Compton thick. The estimated black hole mass is 8 10^8 solar mass suggesting the accretion onto the seed black hole must have started as early as z = 11.

astro-ph↗

arXiv2003

Polarization asymmetry in CSS sources: evidence of AGN fuel?

The compact steep spectrum and gigahertz peaked spectrum sources are widely believed to be young radio sources, with ages <10^6 yr. If the activity in the nucleus is fuelled by the supply of gas, one might find evidence of this gas by studying the structural and polarization characteristics of CSS sources as these evolve through this gas. We present polarization observations of a sample of CSS sources, and combine our results with those available in the literature, to show that CSS sources are more asymmetric in the polarization of the outer lobes compared with the more extended ones. We suggest that this could be possibly due to interaction of the jets with infalling material, which fuels the radio source. We also investigate possible dependence of the polarization asymmetry of the lobes on redshift, since this might be affected by more interactions and mergers in the past. No such dependence is found for the CSS sources, suggesting that the environments on the CSS scales are similar at different redshifts. However, the polarization asymmetry of the oppositely-directed lobes is larger at higher redshifts for the more extended sources, possibly reflecting the higher incidence of interactions in the past.

astro-ph↗