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V. Gupta

Publications and source records attributed to V. Gupta.

At least 37 records · Page 2Linked to original sources
arXiv2019

Five new real-time detections of Fast Radio Bursts with UTMOST

We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between June 2017 and December 2018. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $μ$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering timescales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with 3 peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky$^{-1}$ day$^{-1}$ to a fluence limit of 8 Jy-ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index $α=-1.6$ ($F_ν\proptoν^α$). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio and X-ray followup has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.

astro-ph.HE↗
arXiv2018

Hunting for radio emission from the intermittent pulsar J1107-5907 at low frequencies

The rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ($\lesssim$ 300 MHz) coverage. We present the first low-frequency detection of the intermittent pulsar J1107-5907 with the Murchison Widefield Array (MWA) at 154 MHz and the simultaneous detection from the recently upgraded Molonglo Observatory Synthesis Telescope (UTMOST) at 835 MHz, as part of an on-going observing campaign. During a 30-minute simultaneous observation, we detected the pulsar in its bright emission state for approximately 15 minutes, where 86 and 283 pulses were detected above a signal-to-noise threshold of 6 with the MWA and UTMOST, respectively. Of the detected pulses, 51 had counterparts at both frequencies and exhibited steep spectral indices for both the bright main pulse component and the precursor component. We find that the bright state pulse energy distribution is best parameterised by a log-normal distribution at both frequencies, contrary to previous results which suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability, intensity modulations and to better constrain the signal propagation effects due to the interstellar medium and intermittency characteristics at these frequencies. The spectral index, extended profile emission covering a large fraction of pulse longitude, and the broadband intermittency of PSR J1107-5907 suggests that future low-frequency pulsar searches, for instance those planned with SKA-Low, will be in an excellent position to find and investigate new pulsars of this type.

astro-ph.HE↗
arXiv2018

Surface operators, dual quivers and contours

We study half-BPS surface operators in four dimensional N=2 SU(N) gauge theories, and analyze their low-energy effective action on the four dimensional Coulomb branch using equivariant localization. We also study surface operators as coupled 2d/4d quiver gauge theories with an SU(N) flavour symmetry. In this description, the same surface operator can be described by different quivers that are related to each other by two dimensional Seiberg duality. We argue that these dual quivers correspond, on the localization side, to distinct integration contours that can be determined by the Fayet-Iliopoulos parameters of the two dimensional gauge nodes. We verify the proposal by mapping the solutions of the twisted chiral ring equations of the 2d/4d quivers onto individual residues of the localization integrand.

hep-th↗
arXiv2018

The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey. The kinematic diversity of stellar halos and the relation between halo transition scale and stellar mass

In the hierarchical two-phase formation scenario, the extended halos of early type galaxies (ETGs) are expected to have different physical properties from those of the galaxies' central regions. This work aims at characterizing the kinematic properties of ETG halos using planetary nebulae (PNe) as tracers, which allow us to overcome the limitations of absorption line spectroscopy of continuum at low surface brightness. We present two-dimensional velocity and velocity dispersion fields for 33 ETGs, including both fast (FRs) and slow rotators (SRs), making this the largest kinematic survey to-date of extragalactic PNe. The velocity fields extend out to a median 5.6 effective radii (Re), with a range [3Re-13Re] for the PN.S ETGs. We complemented the PN kinematics with absorption line data from the literature. We find that ETGs typically show a kinematic transition between inner regions and halo. Estimated transition radii in units of Re anticorrelate with stellar mass. SRs have increased but still modest rotational support at large radii, while most of the FRs show a decrease in rotation, due to the fading of the stellar disk in the outer, more slowly rotating spheroid. 30% of the FRs are dominated by rotation also at large radii. Most ETGs have flat or slightly falling halo velocity dispersion profiles, but 15% of the sample have steeply falling profiles. 40% of the FRs show kinematic twists, misalignments, or rotation along two axes, indicating that they turn from oblate at the center to triaxial in the halo, consistently with the distribution of isophote twists from extended photometry. ETGs have more diverse kinematic properties in their halos than in the central regions. The observed kinematic transition to the halo and its dependence on stellar mass is consistent with LambdaCDM simulations and supports a two-phase formation scenario. [abridged]

astro-ph.GA↗
arXiv2018

A Second Chromatic Timing Event of Interstellar Origin toward PSR J1713+0747

The frequency dependence of radio pulse arrival times provides a probe of structures in the intervening media. Demorest et al. 2013 was the first to show a short-term (~100-200 days) reduction in the electron content along the line of sight to PSR J1713+0747 in data from 2008 (approximately MJD 54750) based on an apparent dip in the dispersion measure of the pulsar. We report on a similar event in 2016 (approximately MJD 57510), with average residual pulse-arrival times of approximately 3.0,-1.3, and -0.7 microseconds at 820, 1400, and 2300 MHz, respectively. Timing analyses indicate possible departures from the standard nu^-2 dispersive-delay dependence. We discuss and rule out a wide variety of potential interpretations. We find the likeliest scenario to be lensing of the radio emission by some structure in the interstellar medium, which causes multiple frequency-dependent pulse arrival-time delays.

astro-ph.HE↗
arXiv2018

Surface operators in 5d gauge theories and duality relations

We study half-BPS surface operators in 5d N=1 gauge theories compactified on a circle. Using localization methods and the twisted chiral ring relations of coupled 3d/5d quiver gauge theories, we calculate the twisted chiral superpotential that governs the infrared properties of these surface operators. We make a detailed analysis of the localization integrand, and by comparing with the results from the twisted chiral ring equations obtain constraints on the 3d and 5d Chern-Simons levels so that the instanton partition function does not depend on the choice of integration contour. For these values of the Chern-Simons couplings, we comment on how the distinct quiver theories that realize the same surface operator are related to each other by Aharony-Seiberg dualities.

hep-th↗
arXiv2017

Surface operators, chiral rings, and localization in N=2 gauge theories

We study half-BPS surface operators in supersymmetric gauge theories in four and five dimensions following two different approaches. In the first approach we analyze the chiral ring equations for certain quiver theories in two and three dimensions, coupled respectively to four- and five-dimensional gauge theories. The chiral ring equations, which arise from extremizing a twisted chiral superpotential, are solved as power series in the infrared scales of the quiver theories. In the second approach we use equivariant localization and obtain the twisted chiral superpotential as a function of the Coulomb moduli of the four- and five-dimensional gauge theories, and find a perfect match with the results obtained from the chiral ring equations. In the five-dimensional case this match is achieved after solving a number of subtleties in the localization formulas which amounts to choosing a particular residue prescription in the integrals that yield the Nekrasov-like partition functions for ramified instantons. We also comment on the necessity of including Chern-Simons terms in order to match the superpotentials obtained from dual quiver descriptions of a given surface operator.

hep-th↗
arXiv2016

Dibaryon decay sum rules and other multiquark states

The decays of the antisymmetric dibaryon octet $D(8_F)$ into two baryon octets are considered. Sum rules for these decays in first order broken SU(3) are given. An SU(4) extension of the analysis is commented upon. Possibilities for the experimental observation of multibaryon and anti-multibaryon states is pointed out.

hep-ph↗
arXiv2016

Decay coupling constants sum rules for dibaryon octet into two baryon octets with $λ_8$ first order SU(3) symmetry breaking

The SU(3) octet states with baryon number B = 2, hexaquark dibaryons, are considered. Decay coupling constants sum rules for dibaryon octet into two ordinary baryon octets with ?$λ_8$ first order SU(3) symmetry breaking are given. An SU(4) extension of the analysis is commented upon. Possibilities for the experimental observation of multibaryon and anti-multibaryon states are pointed out.

hep-ph↗
arXiv2013

Lepton flavor mixing in the Wolfenstein scheme

We mimic the Wolfenstein scheme of quark flavor mixing to describe neutrino oscillations in the standard model. We identify the parameter $λ$ with the experimentally measured value of the mixing element responsible for atmospheric neutrino oscillations. The matrix elements responsible for solar neutrino oscillations and the Chooz angle are taken to be proportional to $λ^2$ and $λ^3$, respectively. Using present world average data on neutrinos, we derive bounds on the other parameters $A$, $ρ$, and $η$, of the new scheme.

hep-ph↗
arXiv2013

Statistical comparison of quark mass matrices\\ in the physical basis

Using the four best measured moduli of the flavor mixing matrix ($|V_{\rm ud}|$, $|V_{\rm us}|$, $|V_{\rm cd}|$, $|V_{\rm cs}|$), the Jarlskog invariant $J(V)$, and the quark masses at $M_Z$ energy scale as experimental constraints, a statistical comparison of three different types of quark mass matrices in the physical basis is performed. The mass matrices in question are the Chaturvedi-Gupta-Sánchez-Colón (CGS), the Fritzsch and the Gupta-Rajpoot types. With nine parameters the best fits are obtained using a Gupta-Rajpoot type matrix while with seven parameters the best fits are obtained using the CGS type matrix. The stability of our analysis with respect to evolution of the quark masses is also presented.

hep-ph↗
arXiv2010

Transmission properties of the one-dimensional array of delta potentials

The problem of one-dimensional quantum wire along which a moving particle interacts with a linear array of N delta-function potentials is studied. Using a quantum waveguide approach, the transfer matrix is calculated to obtain the transmission probability of the particle. Results for arbitrary N and for specific regular arrays are presented. Some particular symmetries and invariances of the delta-function potential array for the N = 2 case are analyzed in detail. It is shown that perfect transmission can take place in a variety of situations.

quant-ph↗
arXiv2010

Recursive parametrization of Quark flavour mixing matrices

We examine quark flavour mixing matrices for three and four generations using the recursive parametrization of $U(n)$ and $SU(n)$ matrices developed by some of us in Refs.[2] and [3]. After a brief summary of the recursive parametrization, we obtain expressions for the independent rephasing invariants and also the constraints on them that arise from the requirement of mod symmetry of the flavour mixing matrix.

hep-ph↗
arXiv2008

Bounds on quark mass matrices elements due to measured properties of the mixing matrix and present values of the quark masses

We obtain constraints on possible structures of mass matrices in the quark sector by using as experimental restrictions the determined values of the quark masses at the $M_Z$ energy scale, the magnitudes of the quark mixing matrix elements $V_{\rm ud}$, $V_{\rm us}$, $V_{\rm cd}$, and $V_{\rm cs}$, and the Jarlskog invariant $J(V)$. Different cases of specific mass matrices are examined in detail. The quality of the fits for the Fritzsch and Stech type mass matrices is about the same with $χ^2/{\rm dof}=4.23/3=1.41$ and $χ^2/{\rm dof}=9.10/4=2.28$, respectively. The fit for a simple generalization (one extra parameter) of the Fritzsch type matrices, in the physical basis, is much better with $χ^2/{\rm dof}=1.89/4=0.47$. For comparison we also include the results using the quark masses at the 2 GeV energy scale. The fits obtained at this energy scale are similar to that at $M_Z$ energy scale, implying that our results are unaffected by the evolution of the quark masses from 2 to 91 GeV.

hep-ph↗
arXiv2004

The Effect of Ru substitution for Ni on the superconductivity in MgCNi3-xRux

The superconductor MgCNi3 has been chemically doped by partial substitution of Ru for Ni in the solid solution MgCNi3-xRux for 0<x<0.5. Magnetic and specific heat measurements show that the Sommerfeld parameter (gamma_exp) and TC decrease immediately on Ru substitution, but that a TC above 2K is maintained even for a relatively large decrease in gamma_exp. Ferromagnetism is not observed to develop through Ru substitution, and the normal state magnetic susceptibility is suppressed.

cond-mat.supr-con↗
arXiv2001

Constraining H0 from Chandra Observations of Q0957+561

We report the detection of the lens cluster of the gravitational lens (GL) system Q0957+561 from a deep observation with the Advanced CCD Imaging Spectrometer on-board the Chandra X-ray Observatory. Intracluster X-ray emission is found to be centered 4.3 +/- 1.3 arcsec east and 3.5(-0.6,+1.3) arcsec north of image B, nearer than previous estimates. Its spectrum can be modeled well with a thermal plasma model consistent with the emission originating from a cluster at a redshift of 0.36. Our best-fit estimates of the cluster temperature of T_e = 2.09(-0.54,+0.83) keV (90 percent confidence) and mass distribution of the cluster are used to derive the convergence parameter kappa, the ratio of the cluster surface mass density to the critical density required for lensing. We estimate the convergence parameter at the location of the lensed images A and B to be kappa_A = 0.22(+0.14,-0.07) and kappa_B = 0.21(+0.12,-0.07), respectively (90 percent confidence levels). The observed cluster center, mass distribution and convergence parameter kappa provide additional constraints to lens models of this system. Our new results break a mass-sheet degeneracy in GL models of this system and provide better constraints of ~ 29 percent (90 percent confidence levels) on the Hubble constant. We also present results from the detection of the most distant X-ray jet (z = 1.41) detected to date. The jet extends approximately 8 arcsec NE of image A and three knots are resolved along the X-ray jet with flux densities decreasing with distance from the core. The observed radio and optical flux densities of the knots are fitted well with a synchrotron model and the X-ray emission is modeled well with inverse Compton scattering of Cosmic Microwave Background photons by synchrotron-emitting electrons in the jet.

astro-ph↗