arXiv subjects

Tapasi Ghosh

Publications and source records attributed to Tapasi Ghosh.

16 recordsLinked to original sources

arXiv2026

Colloquium: Radio astronomy with the Arecibo 305-m telescope: In contemporaneous context

Most scientific research begins in the context of the then-contemporary state of knowledge of the field and moves toward a deeper understanding of the subject. This colloquium presents the Arecibo telescope$^{'}$s contribution to radio astronomy from the point of view of its contemporary impact and relevance and how that evolved over the 57 years of its long life. Sometimes, serendipitous discoveries at Arecibo and elsewhere brought revolutionary changes to the field. Further, significant upgrades to the reflector, optics, receiving, and data-taking systems helped clear a pathway for a leap ahead. Charting these movements through time and without any claim to completeness, this Colloquium presents a progression through Arecibo telescope$^{'}$s role in the history of radio astronomy.

astro-ph.GA↗

arXiv2023

Point Source C-Band Mueller Matrices for the Green Bank Telescope

C-Band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C138 and 3C286, were observed using the Spider program to steer the telescope across a broad range of Right Ascensions on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C-Band polarization measurements.

astro-ph.IM↗

arXiv2022

Regular Solar Radio Imaging at Arecibo: Space Weather Perspective of Evolution of Active Regions

The sudden release of magnetic energy on the Sun drives powerful solar flares and coronal mass ejections. The key issue is the difficulty in predicting the occurrence time and location of strong solar eruptions, i.e., those leading to the high impact space weather disturbances at the near-Earth environment. Solar radio imaging helps identify the magnetic field characteristics of active regions susceptible to intense flares and energetic coronal mass ejections. Mapping of the Sun at X-band (8.1 -- 9.3 GHz) with the 12-m radio telescope at the Arecibo Observatory allows monitoring of the evolution of the brightness temperature of active regions in association with the development of magnetic complexity, which can lead to strong eruptions. For a better forecasting strategy in the future, such ground-based radio observations of high-spatial and temporal resolution, along with a full polarization capability, would have tremendous potential not only to understand the magnetic activity of solar eruptions, but also for revealing the particle acceleration mechanism and additional exciting science.

astro-ph.IM↗

arXiv2022

Probing the Plasma Tail of Interstellar Comet 2I/Borisov

We present an occultation study of compact radio sources by the plasma tail of interstellar Comet 2I/Borisov (C/2019 Q4) both pre- and near-perihelion using the Arecibo and Green Bank radio telescopes. The interplanetary scintillation (IPS) technique was used to probe the plasma tail at P-band (302--352 MHz), 820 MHz, and L-band (1120--1730 MHz). The presence and absence of scintillation at different perpendicular distances from the central axis of the plasma tail suggests a narrow tail of less than 6~arcmin at a distance of $\sim$10~arcmin ($\sim$$10^6$~km) from the comet nucleus. Data recorded during the occultation of B1019+083 on 31 October 2019 with the Arecibo Telescope covered the width of the plasma tail from its outer region to the central axis. The systematic increase in scintillation during the occultation provides the plasma properties associated with the tail when the comet was at its pre-perihelion phase. The excess level of L-band scintillation indicates a plasma density enhancement of $\sim$15--20 times that of the background solar wind. The evolving shape of the observed scintillation power spectra across the tail from its edge to the central axis suggests a density spectrum flatter than Kolmogorov, and that the plasma-density irregularity scales present in the tail range between 10 and 700 km. The discovery of a high-frequency spectral excess, corresponding to irregularity scales much smaller than the Fresnel scale, suggests the presence of small-scale density structures in the plasma tail, likely caused by interaction between the solar wind and the plasma environment formed by the comet.

astro-ph.EP↗

arXiv2021

Dedicated Triggers for Displaced Jets using Timing Information from Electromagnetic Calorimeter at HL-LHC

In this paper, we study the prospect of ECAL barrel timing to develop triggers dedicated to long-lived particles decaying to jets, at the level-1 of HL-LHC. We construct over 20 timing based variables, and identify three of them which have better performances and are robust against increasing PU. We estimate the QCD prompt jet background rates accurately using the "stitching" procedure for varying thresholds defining our triggers, and compute the signal efficiencies for different LLP scenarios for a permissible background rate. The trigger efficiencies can go up to $\mathcal{O}(80\%)$ for the most optimal trigger for pair-produced heavy LLPs having high decay lengths, which degrades with decreasing mass and decay length of the LLP. We also discuss the prospect of including the information of displaced L1 tracks to our triggers, which further improves the results, especially for LLPs characterised by lower decay lengths.

hep-ph↗

arXiv2018

Serendipitous Detection of HI Absorption Sets the True Redshift of 4C +15.05 to $z=0.833$

4C+15.05, (also known as NRAO 91, PKS 0202+14 or J0204+15), is a quintessential blazar. It has a luminous, variable radio spectrum, a super-luminal jet, and gamma-ray detections. Arecibo observations with the 700-800 MHz receiver on the 305-m diameter William E. Gordon Telescope detected, serendipitously, HI in absorption against 4C+15.05 while using it as a bandpass calibrator for another object in an HI absorption project. Although the redshift we derive is different from that commonly in use in the literature (nominally $z=0.405$), it agrees very well with the value of $z=0.833$ determined by \cite{Stickel+96}. This absorption feature is best fitted by a sum of three Gaussians, which yield an average redshift of $z=0.8336 \pm 0.0004$, although without corresponding high resolution imaging it is not possible to say whether the components are parts of outflows or inflows. A total column density of $N(HI) = 2.39 \pm 0.13 \times 10^{21}$ cm$^{-2}$ is derived, relatively high compared to many radio-loud sources. These results are compared to various relationships in the literature.

astro-ph.GA↗

arXiv2018

Stringent constraints on fundamental constant evolution using conjugate 18 cm satellite OH lines

We have used the Arecibo Telescope to carry out one of the deepest-ever integrations in radio astronomy, targetting the redshifted conjugate satellite OH 18 cm lines at $z \approx 0.247$ towards PKS1413+135. The satellite OH 1720 and 1612 MHz lines are respectively in emission and absorption, with exactly the same line shapes due to population inversion in the OH ground state levels. Since the 1720 and 1612 MHz line rest frequencies have different dependences on the fine structure constant $α$ and the proton-electron mass ratio $μ$, a comparison between their measured redshifts allows one to probe changes in $α$ and $μ$ with cosmological time. In the case of conjugate satellite OH 18 cm lines, the predicted perfect cancellation of the sum of the line optical depths provides a strong test for the presence of systematic effects that might limit their use in probing fundamental constant evolution. A non-parametric analysis of our new Arecibo data yields $\left[ΔX/X \right] = (+0.97 \pm 1.52) \times 10^{-6}$, where $X \equiv μα^2$. Combining this with our earlier results from the Arecibo Telescope and the Westerbork Synthesis Radio Telescope, we obtain $\left[ΔX/X \right] = (-1.0 \pm 1.3) \times 10^{-6}$, consistent with no changes in the quantity $μα^2$ over the last 2.9~Gyr. This is the most stringent present constraint on fractional changes in $μα^2$ from astronomical spectroscopy, and with no evidence for systematic effects.

astro-ph.CO↗

arXiv2017

The gas and stellar mass of low-redshift damped Lyman-$α$ absorbers

We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope H{\sc i} 21cm spectroscopic studies of six damped and sub-damped Lyman-$α$ absorbers (DLAs and sub-DLAs, respectively) at $z \lesssim 0.1$, that have yielded estimates of their H{\sc i} column density, metallicity and atomic gas mass. This significantly increases the number of DLAs with gas mass estimates, allowing the first comparison between the gas masses of DLAs and local galaxies. Including three absorbers from the literature, we obtain H{\sc i} masses $\approx (0.24 - 5.2) \times 10^9 \: {\rm M}_\odot$, lower than the knee of the local H{\sc i} mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-$z$ DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, $\approx 10^7-10^{8.7} M_\odot$, in all cases, consistent with the hosts being dwarf galaxies. We obtain high H{\sc i} 21\,cm or CO emission line widths, $ΔV_{20} \approx 100-290$~km~s$^{-1}$, and high gas fractions, $f_{\rm HI} \approx 5-100$, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the H{\sc i} 21\,cm velocity spreads ($\gtrsim 100$~km~s$^{-1}$) appear systematically larger than the velocity spreads in typical dwarf galaxies.

astro-ph.GA↗

arXiv2017

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

physics.ins-det↗

arXiv2010

Probing fundamental constant evolution with redshifted conjugate-satellite OH lines

We report Westerbork Synthesis Radio Telescope and Arecibo Telescope observations of the redshifted satellite OH-18cm lines at $z \sim 0.247$ towards PKS1413+135. The "conjugate" nature of these lines, with one line in emission and the other in absorption, but with the same shape, implies that the lines arise in the same gas. The satellite OH-18cm line frequencies also have different dependences on the fine structure constant $α$, the proton-electron mass ratio $μ= m_p/m_e$, and the proton gyromagnetic ratio $g_p$. Comparisons between the satellite line redshifts in conjugate systems can hence be used to probe changes in $α$, $μ$, and $g_p$, with few systematic effects. The technique yields the expected null result when applied to Cen.A, a nearby conjugate satellite system. For the $z \sim 0.247$ system towards PKS1413+135, we find, on combining results from the two telescopes, that $[ΔG/G] = (-1.18 \pm 0.46) \times 10^{-5}$ (weighted mean), where $G = g_p [μα^2]^{1.85}$; this is tentative evidence (with $2.6 σ$ significance, or at 99.1% confidence) for a smaller value of $α$, $μ$, and/or $g_p$ at z~0.247, i.e. at a lookback time of ~2.9 Gyrs. If we assume that the dominant change is in $α$, this implies $[Δα/α] = (-3.1 \pm 1.2) \times 10^{-6}$. We find no evidence that the observed offset might be produced by systematic effects, either due to observational or analysis procedures, or local conditions in the molecular cloud.

astro-ph.CO↗

arXiv2010

A Radio Spectral Line Study of the 2-Jy IRAS-NVSS Sample: Part I

We present results from an on-going survey for the HI 21 cm line and the OH 18 cm lines in IR galaxies with the Arecibo 305 m Radio Telescope. The observations of 85 galaxies extracted from the 2 Jy IRAS-NVSS sample in the R.A. (B1950) range 20 h-00 h are reported in this paper. We detected the HI 21 cm line in 82 of these galaxies, with 18 being new detections, and the OH 18 cm lines in 7 galaxies, with 4 being new detections. In some cases, the HI spectra show the classic double-horned or single-peaked emission profiles. However, the majority exhibit distorted HI spectral features indicating that the galaxies are in interacting and/or merging systems. From these HI and OH observations, various properties of the sample are derived and reported.

astro-ph.CO↗

arXiv2009

Track fitting by Kalman Filter method for a prototype cosmic ray muon detector

We have developed a track fitting procedure based on Kalman Filter technique for an Iron Calorimeter (ICAL) prototype detector when the detector is flushed with single muon tracks. The relevant track parameters i.e., momentum, direction and charge are reconstructed and analyzed. This paper discusses the design of the prototype detector, its geometry simulation by Geant4, and the detector response with the cosmic ray muons. Finally we show the resolution of reconstructed momenta and also the charge identification efficiency of $μ^+$ and $μ^-$ events in the magnetized ICAL.

nucl-ex↗

arXiv2004

Conjugate 18cm OH Satellite Lines at a Cosmological Distance

We have detected the two 18cm OH satellite lines from the $z \sim 0.247$ source PKS1413+135, the 1720 MHz line in emission and the 1612 MHz line in absorption. The 1720 MHz luminosity is $L_{\rm OH} \sim 354 L_\odot$, more than an order of magnitude larger than that of any other known 1720 MHz maser. The profiles of the two satellite lines are conjugate, implying that they arise in the same gas. This allows us to test for any changes in the values of fundamental constants, without being affected by systematic uncertainties arising from relative motions between the gas clouds in which the different lines arise. Our data constrain changes in $G \equiv g_p [α^2/y]^{1.849}$, where $ y \equiv m_e/m_p$; we find $ΔG/G = 2.2 \pm 3.8 \times 10^{-5}$, consistent with no changes in $α$, $g_p$ and $y$.

astro-ph↗

arXiv2001

Mueller Matrix Parameters for Radio Telescopes and their Observational Determination

Modern digital crosscorrelators permit the simultaneous measurement of all four Stokes parameters. However, the results must be calibrated to correct for the polarization transfer function of the receiving system. The transfer function for any device can be expressed by its Mueller matrix. We express the matrix elements in terms of fundamental system parameters that describe the voltage transfer functions (known as the Jones matrix) of the various system devices in physical terms and thus provide a means for comparing with engineering calculations and investigating the effects of design changes. We describe how to determine these parameters with astronomical observations. We illustrate the method by applying it to some of the receivers at the Arecibo Observatory.

astro-ph↗

arXiv2001

All-Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope

We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in Stokes I the parameters include the beam width, ellipticity and its orientation, coma and its orientation, the point-source gain, the integrated gain (or, equivalently, the main beam efficiency); for the other Stokes parameters the beam parameters include beam squint and beam squash. For the first sidelobe ring in Stokes I the parameters include an 8-term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobe's fractional polarization. We illustrate the technique by applying it to the Arecibo telescope. The main beam width is smaller and the sidelobe levels higher than for a uniformly-illuminated aperture of the same effective area. These effects are modeled modestly well by a blocked aperture, with the blocked area equal to about 10% of the effective area (this corresponds to 5% physical blockage). In polarized emission, the effects of beam squint (difference in pointing direction between orthogonal polarizations) and squash (difference in beamwidth between orthogonal polarizations) do not correspond to theoretical expectation and are higher than expected; these effects are almost certainly caused by the blockage. The first sidelobe is highly polarized because of blockage.

astro-ph↗

arXiv2001

Detection of a multi-phase ISM at $z=0.2212$

We present sensitive Giant Metrewave Radio Telescope (GMRT) and high-resolution Arecibo HI 21-cm observations of the damped Lyman-$α$ absorber (DLA) at $z=0.2212$ towards OI 363. The spectra are in excellent agreement and yield a spin temperature $T_s = 890 \pm 160$ K, far higher than $T_s$ values measured in local spirals, but similar to values obtained in the majority of DLAs ($T_s \ga 1000$ K). The high velocity resolution of the Arecibo spectra enables us to obtain estimates of physical conditions in the absorbing clouds by fitting multiple Gaussians to the absorption profile. The spectra are well fit by a three-component model with two narrow and one wide components, with temperatures ${T_{k_1}} = 308 \pm 24$ K, ${T_{k_2}} = 180 \pm 30$ K and ${T_{k_3}} = 7600 \pm 1250$ K, respectively. The last of these is in excellent agreement with the expected temperatures for the WNM ($5000 - 8000$ K). The mere fact that components are seen with lower temperatures than the estimated $T_s$ implies that the absorber must have a multi-phase medium. We use the measured 21-cm optical depth and the above estimates of the kinetic temperature to obtain the HI column density in the various components. The total column density in the narrow components is found to be $\NHI (CNM) \le 1.9 \pm 0.25 \times 10^{20}$ cm, while that in the wide component is $\NHI (WNM) \ge 1.26 \pm 0.49 \times 10^{21}$ cm. Thus, the WNM contains at least 75% of the total HI in the $z = 0.2212$ DLA, unlike our Galaxy, in which the CNM and WNM have equitable contributions; this accounts for the difference in the spin temperatures of the $z = 0.2212$ system and local spirals, suggesting that the DLA is probably a dwarf or LSB type galaxy (abridged).

astro-ph↗