arXiv subjects

A. Pandey

Publications and source records attributed to A. Pandey.

At least 19 recordsLinked to original sources

arXiv2026

The exceptional 2017 gamma-ray flare of the radio galaxy NGC 1275: VERITAS and Multiwavelength Observations

The radio galaxy NGC 1275 is the Brightest Cluster Galaxy in the Perseus cluster. It is well-studied across all wavebands, including Very High Energy (VHE; E>100GeV gamma-rays, and with radio observations over the last 20 years tracking an unusual radio component, "C3". NGC 1275 was observed in an exceptional VHE flaring state between 2016 December 31 and 2017 January 3. The flare peak reached ~1.5 Crab units as measured by the MAGIC observatory. We report on the observations of NGC~1275 conducted by VERITAS and multi-wavelength data collected during this flaring state, and for context, data taken between 2009 and 2017 inclusive. VERITAS detected the declining state of the flare on 2017 January 2 (MJD 57755) and 3 (MJD 57756) at an average flux state of 0.5 Crab units. VERITAS spectra show an overall long-term trend of harder-when-brighter. During the flare, the gamma-ray spectrum obtained from the combined Fermi-LAT, MAGIC, and VERITAS observations, changes from a power law with an exponential cut-off on January 1 to a log-parabola on January 2. To study the evolution of the flare in more detail, multi-band spectral energy distributions (SEDs) were constructed for the nights of 2017 January 1 and 2 corresponding to the shift from the peak to the decline of the flare. A blob-in-jet modeling of the SEDs results in support for a two-component model with a jet angle of 10 degrees to the line of sight and the gamma-ray emission zone located in the vicinity of the C3 radio component.

astro-ph.HE↗

arXiv2026

Timing performance of large prototype based on $\upmu$RWELL- PICOSEC detector technology with $10 \times 10\ \mathrm{cm}^{2}$ active area

The $\upmu$RWELL-PICOSEC detector, which combines a $\upmu$RWELL gaseous amplification structure with a Cherenkov radiator and photocathode, is a novel approach to acheive fast and precise timing in gaseous detectors. With timing precision at the level of tens of picoseconds, this technology is particularly suited for time-of-flight (TOF) applications in particle physics and potentially medical imaging. Beam tests with a 150~GeV/$c$ muon beam have been carried out on a large-area (10~$\times$~10~cm$^{2}$) prototype equipped with a cesium iodide (CsI) photocathode. Using an oscilloscope-based single-channel readout, timing measurements on two individual pads of the detector have yielded resolutions of $\approx$ 48 ps and $\approx$ 52 ps under different biasing conditions respectively.

physics.ins-det↗

arXiv2026

Investigation of the Microquasar SS 433 with VERITAS

Microquasars such as SS 433 are considered potential contributors to cosmic rays up to the knee of the cosmic ray energy spectrum ($\sim4\,\mathrm{PeV}$), where a transition in the dominant acceleration processes is expected. The SS 433 system, located within the W50 supernova remnant, is a Galactic microquasar with relativistic jets interacting with the surrounding medium over parsec scales, providing an example for studying jet-driven particle acceleration. A deep morphological and spectral study of SS 433 is performed using more than 150 hours of observations with VERITAS, sensitive to $\gamma$-ray energies $>100\,\mathrm{GeV}$. With an angular resolution better than $0.1^\deg$, extended TeV $\gamma$-ray emission is resolved from both the eastern and western jet lobes, located tens of parsecs from the central binary. The emission appears elongated along the jet axis and coincides with regions where the jets interact with the surrounding supernova remnant. No TeV emission is detected from the central binary, nor is significant emission observed between the central binary and the jet lobes. Phase-resolved analyses show no evidence for variability with orbital or precessional phase, supporting a steady emission scenario. The observed morphology and spectra are consistent with scenarios where particles are accelerated in the lobes of the jets, possibly through shocks or alternative processes such as magnetic reconnection. The extended TeV emission from the jet lobes of SS 433 favors a leptonic origin in the VERITAS energy range, suggesting any hadronic acceleration is subdominant. The results offer valuable constraints on how microquasar jets may contribute to the Galactic cosmic-ray population toward the knee.

astro-ph.HE↗

arXiv2026

VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broad-band emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$\gamma$ interactions and optically thin $\gamma$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.

astro-ph.HE↗

arXiv2025

Design and Performance of a 96-channel Resistive PICOSEC Micromegas Detector for ENUBET

The PICOSEC-Micromegas (PICOSEC-MM) detector is a fast gaseous detector that achieves picosecond-level timing by coupling a Cherenkov radiator, typically an MgF2 crystal, to a Micromegas-based photodetector with a photocathode. This configuration allows the fast photoelectron-induced signal to suppress the intrinsic time jitter of gaseous detectors, enabling sub-20 ps timing precision while preserving the robustness and scalability of micro-pattern gaseous detector technologies. The 96-pad PICOSEC-MM detector is a large-area demonstrator optimized for precision timing in high-energy physics, building on research and development insights from earlier 7-pad resistive prototypes to validate scalability, uniformity, and robustness for the ENUBET project. It employs a 2.5 nm diamond-like carbon photocathode and a Micromegas board with a surface resistivity of 10 megaohms per square, and was characterized using 150 GeV/c muons at the CERN SPS beamline, with one-third of the active area instrumented per run. A dedicated alignment procedure for multi-pad PICOSEC-MM systems was used to reconstruct pad centers and merge measurements across regions, yielding a timing resolution of 43 ps and uniform signal arrival time distributions over the tested area. Mechanical flatness was identified as a key factor, with planarity tolerances within 10 micrometers required to maintain good timing resolution, and the successful operation of the 96-pad demonstrator confirms the scalability of the PICOSEC-MM concept toward robust, high-granularity, picosecond-level gaseous timing detectors for monitored neutrino beam experiments such as ENUBET.

physics.ins-det↗

arXiv2025

Performance Optimization and Characterization of 7-pad Resistive PICOSEC Micromegas Detectors

We present a comprehensive characterization of resistive PICOSEC Micromegas detector prototypes, tested under identical conditions, constant drift gap, field configurations, and photocathode at the CERN SPS H4 beam line. This work provides a proof of concept for the use of resistive layer technology in gaseous timing detectors, demonstrating that robustness can be improved without compromising the excellent timing performance of PICOSEC Micromegas. Different resistive architectures and values were explored to optimize stability and ensure reliable long-term operation in challenging experimental environments. The prototype with a 10M{\Omega} resistive layer achieved the best overall performance, with a timing resolution of 22.900 {\pm} 0.002 ps and a spatial resolution of 1.190 {\pm} 0.003 mm, while charge sharing across multiple pads enabled combined timing resolutions below 28 ps. A lower-resistivity (200k{\Omega}) configuration exhibited enhanced charge spread, leading to minor systematic offsets in reconstructed pad centers, yet maintained robust timing and spatial performance. Capacitive charge-sharing architectures improved spatial resolution in some regions but suffered from signal attenuation and nonuniform charge distributions, resulting in slightly degraded timing (33.300 {\pm} 0.002 ps) and complex localization patterns. Mechanical precision, particularly readout planarity and photocathode alignment, was identified as critical for uniform detector response. These studies benchmark the potential of resistive layers for gaseous timing detectors and provide a foundation for scalable designs with optimized timing and spatial resolution across diverse experimental applications.

physics.ins-det↗

arXiv2025

Probing a cosmogenic origin of astrophysical neutrinos and cosmic rays using gamma-ray observations of TXS 0506+056

In September 2017, a high-energy neutrino event detected by the IceCube Neutrino Observatory (IceCube-170922A) was associated, at the $3\sigma$ level, with a gamma-ray flare from the blazar TXS 0506+056. Cosmic rays that are accelerated in astrophysical sources can escape from their jets and interact with background radiation fields. Interactions with the extragalactic background light can produce pions and hence neutrinos, while interactions with the cosmic microwave background predominantly drive inverse Compton scattering, contributing to electromagnetic cascades in intergalactic space. The resulting secondary gamma-ray emission can be detected with high-energy gamma-ray telescopes. Here, we report on a new search for such cosmogenic cascade emission from the blazar TXS 0506+056, using a combined data set from the Fermi-Large Area Telescope and VERITAS. We compare the gamma-ray spectrum and neutrino observations with the predictions of cosmic-ray induced cascades in intergalactic space. The observed gamma-ray spectrum is modeled as a combination of the primary spectrum and the cascade spectrum. We apply a Monte Carlo simulation with a $\Delta\chi^2$-based likelihood analysis to jointly determine the best-fit parameters of a proton emission spectrum describing the data and derive constraints on the proton escape luminosity. Assuming a log-parabola primary photon spectrum, we find consistency with a proton injection spectral index of $\alpha_{p} \simeq 2.0$ and a cutoff energy of $E_{p,\text{max}} \simeq 1.3 \times 10^{16}$ eV, and constrain the isotropic proton escape luminosity to $1 \times 10^{44}$ erg s$^{-1}$ $\lesssim L_{p, esc} \lesssim 3 \times 10^{45}$ erg s$^{-1}$ at the 90 % confidence level.

astro-ph.HE↗

arXiv2025

Constraints on Axion-Like Particles from VERITAS Observations of a Flaring Radio Galaxy in the Perseus Cluster

Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV gamma-ray sources. Data: In 2017, the VERITAS gamma-ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength O(10$\mu$G) through which escaping photons traverse, making it an excellent target to study ALPs. Methods: We analyze the VERITAS data of NGC 1275's 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the CLs method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity. Results: No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2 \times 10^{-7}$eV $\lesssim m_a \lesssim 4 \times 10^{-7}$eV and at the coupling strength of $g_{a\gamma} \gtrsim 3 \times 10^{-11}$ GeV$^{-1}$ up to 80% confidence level, which are consistent with previous studies. Conclusions: We find the CLs method to be a trustworthy approach, and advocate for its...

astro-ph.HE↗

arXiv2025

Multi-wavelength Study of HESS J0632+057: New Insights into Pulsar-Disk Interaction

We present an analysis of new multi-wavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023--2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source's X-ray spectrum, including increased X-ray absorption at the orbital phase interval of $\phi\approx0.3\textrm{--}0.4$. The source's X-ray flux within this phase interval seems to have exhibited a significant change on an orbital timescale. Additionally, occasional short-term variations in the X-ray band on a timescale of less than 3 days have been observed. The measured duration of the increased absorbing column density and the flux variability timescales can provide clues about the interaction between the putative pulsar and the Be companion's disk if, as previously suggested, the pulsar crosses the disk at this phase interval. Moreover, the new contemporaneous X-ray and TeV observations around the pulsar-crossing phases revealed independent variability in the X-ray and TeV fluxes, contrary to a previous observation of concurrent flux increases. While these observations alone cannot provide definitive conclusions, we discuss our results in the context of pulsar-disk interaction and intrabinary shock emission scenarios.

astro-ph.HE↗

arXiv2025

Measurement of the photosphere oblateness of $\gamma$ Cassiopeiae via Stellar Intensity Interferometry with the VERITAS Observatory

We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at Fred Lawrence Whipple Observatory (FLWO) as a light collector to obtain measurements of the rapid rotator star $\gamma$ Cassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and projected orientation of the photosphere. Fitting the data with a uniform ellipse model yields a minor-axis angular diameter of $0.43\pm0.02$ mas, a major-to-minor-radius ratio of $1.28\pm0.04$, and a position angle of $116^\circ\pm5^\circ$ for the axis of rotation. A rapidly-rotating stellar atmosphere model that includes limb and gravity darkening describes the data well with a fitted angular diameter of $0.604^{+0.041}_{-0.034}$ mas corresponding to an equatorial radius of 10.9$^{+0.8}_{-0.6}~R_\odot$, a rotational velocity with a $1~\sigma$ lower limit at $97.7\%$ that of breakup velocity, and a position angle of $114.7^{+6.4}_{-5.7}$ degrees. These parameters are consistent with H$\alpha$ line spectroscopy and infrared-wavelength Michelson interferometric measurements of the star's decretion disk. This is the first measurement of an oblate photosphere using intensity interferometry.

astro-ph.SR↗

arXiv2025

Multiwavelength observation of a candidate pulsar halo LHAASO J0621+3755 and the first X-ray detection of PSR J0622+3749

Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755 with VERITAS and XMM-Newton in the TeV and X-ray bands, respectively. For this work, we developed a novel background estimation technique for imaging atmospheric Cherenkov telescope observations of such extended sources. No halo emission was detected with VERITAS (0.3--10 TeV) or XMM-Newton (2--7 keV) within 1 degree and 10 arcmin around PSR J0622+3749, respectively. Combined with the LHAASO-KM2A and Fermi-LAT data, VERITAS flux upper limits establish a spectral break at ~1--10 TeV, a unique feature compared with Geminga, the most studied pulsar halo. We model the gamma-ray spectrum and LHAASO-KM2A surface brightness as inverse Compton emission and find suppressed diffusion around the pulsar, similar to Geminga. A smaller diffusion suppression zone and harder electron injection spectrum than Geminga are necessary to reproduce the spectral cutoff. A magnetic field <= 1 uG is required by our XMM-Newton observation and synchrotron spectral modeling, consistent with Geminga. Our findings support slower diffusion and lower magnetic field around pulsar halos than the Galactic averages, hinting at magnetohydrodynamic turbulence around pulsars. Additionally, we report the detection of an X-ray point source spatially coincident with PSR J0622+3749, whose periodicity is consistent with the gamma-ray spin period of 333.2 ms. The soft spectrum of this source suggests a thermal origin.

astro-ph.HE↗

arXiv2025

PICOSEC Micromegas Precise-timing Detectors: Development towards Large-Area and Integration

PICOSEC Micromegas (MM) is a precise timing gaseous detector based on a Cherenkov radiator coupled with a semi-transparent photocathode and an MM amplifying structure. The detector conceprt was successfully demonstrated through a single-channel prototype, achieving sub-25 ps time resolution with Minimum Ionizing Particles (MIPs). A series of studies followed, aimed at developing robust, large-area, and scalable detectors with high time resolution, complemented by specialized fast-response readout electronics. This work presents recent advancements towards large-area resistive PICOSEC MM, including 10 $\times$ 10 $\text{cm}^2$ area prototypes and a 20 $\times$ 20 $\text{cm}^2$ prototype, which features the jointing of four photocathodes. The time resolution of these detector prototypes was tested during the test beam, achieved a timing performance of around 25 ps for individual pads in MIPs. Meanwhile, customized electronics have been developed dedicated to the high-precision time measurement of the large-area PICOSEC MM. The performance of the entire system was evaluated during the test beam, demonstrating its capability for large-area integration. These advancements highlight the potential of PICOSEC MM to meet the stringent requirements of future particle physics experiments.

physics.ins-det↗

arXiv2024

Associative ionization in a dilute ultracold $^7$Li gas probed with a hybrid trap

The formation of Li$_2^+$ and subsequently Li$^+$ ions, during the excitation of $^7$Li atoms to the $3S_{1/2}$ state in a $^7$Li magneto optical trap (MOT), is probed in an ion-atom hybrid trap. Associative ionization occurs during the collision of Li($2P_{3/2}$) and Li($3S_{1/2}$) ultracold atoms, creating Li$_2^+$ ions. Photodissociation of Li$_2^+$ by the MOT lasers is an active channel for the conversion of Li$_2^+$ to Li$^+$. A fraction of the Li$_2^+$ ions is long lived even in the presence of MOT light. Additionally, rapid formation of Li$^+$ from Li$_2^+$ in the absence of MOT light is observed. Resonant excitation of ultracold atoms, resulting in intricate molecular dynamics, reveals important processes in ultracold dilute gases.

physics.atom-ph↗

arXiv2024

PICOSEC-Micromegas Detector, an innovative solution for Lepton Time Tagging

The PICOSEC-Micromegas (PICOSEC-MM) detector is a novel gaseous detector designed for precise timing resolution in experimental measurements. It eliminates time jitter from charged particles in ionization gaps by using extreme UV Cherenkov light emitted in a crystal, detected by a Micromegas photodetector with an appropriate photocathode. The first single-channel prototype tested in 150 GeV/c muon beams achieved a timing resolution below 25 ps, a significant improvement compared to standard Micropattern Gaseous Detectors (MPGDs). This work explores the specifications for applying these detectors in monitored neutrino beams for the ENUBET Project. Key aspects include exploring resistive technologies, resilient photocathodes, and scalable electronics. New 7-pad resistive detectors are designed to handle the particle flux. In this paper, two potential scenarios are briefly considered: tagging electromagnetic showers with a timing resolution below 30 ps in an electromagnetic calorimeter as well as individual particles (mainly muons) with about 20 ps respectively.

physics.ins-det↗

arXiv2024

Application of the FRADO model of BLR formation to the Seyfert galaxy NGC 5548 and the first step toward determining the Hubble constant

The dynamical and geometric structures of the Broad Line Region (BLR), along with the origins of continuum time delays in active galaxies, remain topics of ongoing debate. In this study, we aim to reproduce the observed broadband spectrum, the H$\beta$ line delay, and the continuum time delays using our newly developed model for the source NGC 5548. We adopt the standard accretion disk model, with the option of an inner hot flow, and employ the lamp-post model to account for disk irradiation. Additionally, we model the BLR structure based on radiation pressure acting on dust. The model is parameterized by the black hole mass, $M_{\text{BH}}$ (which is fixed), the accretion rate, the viewing angle, the height of the lamp-post, the cloud density, and the cloud covering factor. The resulting continuum time delays arise from a combination of disk reprocessing and the reprocessing of a fraction of radiation by the BLR. Our model reasonably reproduces the observed broad-band continuum, the H$\beta$ time delay, and the continuum inter-band time delays measured during the observational campaign. When the accretion rate is not constrained by the known distance to the source, our approach allows for a direct estimation of the distance. The resulting Hubble constant, $H_0$ = $66.9^{+10.6}_{-2.1}$ km s$^{-1}$ Mpc$^{-1}$, represents a significant improvement over previously reported values derived from continuum time delays in the literature. This pilot study demonstrates that, with sufficient data coverage, it is possible to disentangle the time delays originating from the accretion disk and the BLR. This paves the way for effectively using inter-band continuum time delays as a method for determining the Hubble constant. Additionally, the findings provide strong support for the adopted model for the formation of the H$\beta$ line.

astro-ph.GA↗

arXiv2024

Photocathode characterisation for robust PICOSEC Micromegas precise-timing detectors

The PICOSEC Micromegas detector is a~precise-timing gaseous detector based on a~Cherenkov radiator coupled with a~semi-transparent photocathode and a~Micromegas amplifying structure, targeting a~time resolution of tens of picoseconds for minimum ionising particles. Initial single-pad prototypes have demonstrated a~time resolution below 25 ps, prompting ongoing developments to adapt the concept for High Energy Physics applications, where sub-nanosecond precision is essential for event separation, improved track reconstruction and particle identification. The achieved performance is being transferred to robust multi-channel detector modules suitable for large-area detection systems requiring excellent timing precision. To enhance the robustness and stability of the PICOSEC Micromegas detector, research on robust carbon-based photocathodes, including Diamond-Like Carbon (DLC) and Boron Carbide (B4C), is pursued. Results from prototypes equipped with DLC and B4C photocathodes exhibited a~time resolution of approximately 32 ps and 34.5 ps, respectively. Efforts dedicated to improve detector robustness and stability enhance the feasibility of the PICOSEC Micromegas concept for large experiments, ensuring sustained performance while maintaining excellent timing precision.

physics.ins-det↗

arXiv2024

A Novel Diamond-like Carbon based photocathode for PICOSEC Micromegas detectors

The PICOSEC Micromegas (MM) detector is a precise timing gaseous detector based on a MM detector operating in a two-stage amplification mode and a Cherenkov radiator. Prototypes equipped with cesium iodide (CsI) photocathodes have shown promising time resolutions as precise as 24 picoseconds (ps) for Minimum Ionizing Particles. However, due to the high hygroscopicity and susceptibility to ion bombardment of the CsI photocathodes, alternative photocathode materials are needed to improve the robustness of PICOSEC MM. Diamond-like Carbon (DLC) film have been introduced as a novel robust photocathode material, which have shown promising results. A batch of DLC photocathodes with different thicknesses were produced and evaluated using ultraviolet light. The quantum efficiency measurements indicate that the optimized thickness of the DLC photocathode is approximately 3 nm. Furthermore, DLC photocathodes show good resistance to ion bombardment in aging test compared to the CsI photocathode. Finally, a PICOSEC MM prototype equipped with DLC photocathodes was tested in muon beams. A time resolution of around 42 ps with a detection efficiency of 97% for 150 GeV/c muons were obtained. These results indicate the great potential of DLC as a photocathode for the PICOSEC MM detector.

physics.ins-det↗

arXiv2024

Single channel PICOSEC Micromegas detector with improved time resolution

This paper presents design guidelines and experimental verification of a single-channel PICOSEC Micromegas (MM) detector with an improved time resolution. The design encompasses the detector board, vessel, auxiliary mechanical parts, and electrical connectivity for high voltage (HV) and signals, focusing on improving stability, reducing noise, and ensuring signal integrity to optimize timing performance. A notable feature is the simple and fast reassembly procedure, facilitating quick replacement of detector internal components that allows for an efficient measurement strategy involving different detector components. The paper also examines the influence of parasitics on the output signal integrity. To validate the design, a prototype assembly and three interchangeable detector boards with varying readout pad diameters were manufactured. The detectors were initially tested in the laboratory environment. Finally, the timing performance of detectors with different pad sizes was verified using a Minimum Ionizing Particle (MIP) beam test. Notably, a record time resolution for a PICOSEC Micromegas detector technology with a CsI photocathode of 12.5$\pm$0.8 ps was achieved with a 10 mm diameter readout pad size detector.

physics.ins-det↗