arXiv subjects

Abhishek Desai

Publications and source records attributed to Abhishek Desai.

14 recordsLinked to original sources

arXiv2026

A New Measurement of the Extragalactic Background Light using 15\,yr of {\it Fermi}-Large Area Telescope Data

The extragalactic background Light (EBL) from ultraviolet to infrared comprises the emission from all stars, galaxies, and actively accreting black holes in the observable Universe. A precise measurement of the EBL is critically important to probe models of star formation and galaxy evolution. The EBL can be measured via the absorption imprint left on the spectra of gamma-ray blazars. In this work, we rely on 15 years of {\it Fermi}-LAT data and 1576 blazars to measure the EBL optical depth in the $0<z<4.3$ range. We detect the EBL attenuation with $\sim23σ$ significance and measure the optical depth in 19 redshift bins, extending the coverage and improving on our previous results. This allows us to reconstruct the EBL evolution and find general consistency with recent EBL models. These results represent the most precise determination of the EBL with GeV $γ$ rays to date.

astro-ph.HE↗

arXiv2025

A multi-wavelength view of the multi-messenger sources NGC 1068 and PKS 1502+1061

Multi-messenger astronomy offers a powerful approach to studying high-energy radiative processes in astrophysical sources. A notable example was seen in 2017, when the IceCube Neutrino Observatory detected a high-energy neutrino event that was found to coincide with a gamma-ray flare from a blazar. Since then, numerous multi-messenger studies combining neutrino and photon data have been conducted, yet the origin of neutrinos from active galactic nuclei (AGN) remains uncertain. In this work, we present the results of an X-ray observing program targeting two AGNs, NGC 1068 and PKS 1502+106. The multi-wavelength dataset includes new observations from NICER and NuSTAR from the observing proposal along with gamma-ray data collected using Fermi-LAT, and one archival observation from Chandra. Additionally, we derive the neutrino fluxes for both AGNs using ten years of IceCube data and neutrino spectra predicted by theoretical models. These results demonstrate the value of combining multi-messenger data in building and constraining theoretical models. They also highlight the importance of testing model predictions against observational data to refine measurements of both the neutrino flux and spectral shape.

astro-ph.HE↗

arXiv2025

IceCube population constraints on neutrino emission by Fermi-LAT detected active galactic nuclei

Gamma-ray-bright active galactic nuclei (AGN) have been one of the most promising source classes of high-energy astrophysical neutrinos detected by IceCube. The first evidence of an IceCube point source was a blazar detected by the Fermi Large Area Telescope (LAT), TXS ~0506+056. Previous analyses have ruled out GeV-bright blazars as the predominant contributor to the high-energy astrophysical neutrino flux under simple correlation assumptions about the relationship between the fluxes of gamma rays and neutrinos. We present results from a more general and more sensitive search for correlation between neutrinos and GeV-selected AGN using improvements in the IceCube statistical methods and 13 years of data. We detect no correlation and set stringent constraints on neutrino emission by populations of GeV-detected AGN. These include constraints on the neutrino emission from subclasses of GeV-detected AGN, including BL Lacs, flat-spectrum radio quasars (FSRQ) and non-blazar AGN, using stacking analyses testing a variety of hypothesized relationships between neutrino and gamma-ray flux. We also present results from an analysis that is sensitive to a wider range of relationships between the gamma-ray and neutrino signal.

astro-ph.HE↗

arXiv2024

Constraints on the origins of the Galactic neutrino flux

Galactic and extragalactic objects in the universe are sources of high-energy neutrinos {that may contribute to the astrophysical neutrino signal seen by IceCube.} Recently, a study done using cascade-like events seen by IceCube reported neutrino emission from the Galactic plane with $>$4$σ$ significance. In this work, we put a lower limit on the number of Galactic sources required to explain this emission. To achieve this, we {use} a simulation package created to simulate point sources in the Galaxy along with the neutrino and gamma-ray flux emissions originating from them. Along with {using} past IceCube discovery potential curves, we also account for Eddington bias effects due to Poisson fluctuations in the number of detected neutrino events.{We present a toy Monte Carlo simulation to show that there should be at least 8 sources, each with luminosities $1.6 \times 10^{35}$erg/s responsible for the Galactic neutrino emission. } Our results constrain the number of individual point-like emission regions, which applies both to discrete astrophysical sources and to individual points of diffuse emission.

astro-ph.HE↗

arXiv2023

Exploring the Galactic neutrino flux origins using IceCube datasets

Astrophysical neutrinos detected by the IceCube observatory can be of Galactic or extragalactic origin. The collective contribution of all the detected neutrinos allows us to measure the total diffuse neutrino Galactic and extragalactic signal. In this work, we describe a simulation package that makes use of this diffuse Galactic contribution information to simulate a population of Galactic sources distributed in a manner similar to our own galaxy. This is then compared with the sensitivities reported by different IceCube data samples to estimate the number of sources that IceCube can detect. We provide the results of the simulation that allows us to make statements about the nature of the sources contributing to the IceCube diffuse signal.

astro-ph.HE↗

arXiv2022

Modeling the Extragalactic Background Light and the Cosmic Star Formation History

We present an updated model for the extragalactic background light (EBL) from stars and dust, over wavelengths approximately 0.1 to 1000 $μ$m. This model uses accurate theoretical stellar spectra, and tracks the evolution of star formation, stellar mass density, metallicity, and interstellar dust extinction and emission in the universe with redshift. Dust emission components are treated self-consistently, with stellar light absorbed by dust reradiated in the infrared as three blackbody components. We fit our model, with free parameters associated with star formation rate and dust extinction and emission, to a wide variety of data: luminosity density, stellar mass density, and dust extinction data from galaxy surveys; and $γ$-ray absorption optical depth data from $γ$-ray telescopes. Our results strongly constraint the star formation rate density and dust photon escape fraction of the universe out to redshift $z=10$, about 90% of the history of the universe. We find our model result is, in some cases, below lower limits on the $z=0$ EBL intensity, and below some low-$z$ $γ$-ray absorption measurements.

astro-ph.GA↗

arXiv2021

An observational determination of the evolving extragalactic background light from the multiwavelength HST/CANDELS survey in the Fermi and CTA era

The diffuse extragalactic background light (EBL) is formed by ultraviolet (UV), optical, and infrared (IR) photons mainly produced by star formation processes over the history of the Universe, and contains essential information about galaxy evolution and cosmology. Here, we present a new determination of the evolving EBL spectral energy distribution using a novel approach purely based on galaxy data aiming to reduce current uncertainties on the higher redshifts and IR intensities. Our calculations use multiwavelength observations from the UV to the far-IR of a sample of approximately 150,000 galaxies detected up to $z\sim 6$ in the five fields of the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) from the Hubble Space Telescope. This is one of the most comprehensive and deepest multi-wavelength galaxy datasets ever obtained. These unprecedented resources allow us to derive the overall EBL evolution up to $z\sim 6$ and its uncertainties. Our results agree with cosmic observables estimated from galaxy surveys and $γ$-ray attenuation such as monochromatic luminosity densities, including those in the far-IR, and star formation rate densities, also at the highest redshits. Optical depths from our EBL approximation, which will be robust at high redshifts and for $γ$ rays up to tens of TeV, will be reported in a companion paper.

astro-ph.CO↗

arXiv2021

Realtime Follow-up of Astrophysical Transients with the IceCube Neutrino Observatory

Realtime analyses are necessary to identify the source of high energy neutrinos. As an observatory with a 4$π$ steradian field of view and near-100% duty cycle, the IceCube Neutrino Observatory is a unique facility for investigating transients. In 2016, IceCube established a pipeline that uses low-latency data to rapidly respond to astrophysical events that were of interest to the multi-messenger observational community. Here, we describe this pipeline and summarize the results from all of the analyses performed since 2016. We focus not only on those analyses which were performed in response to transients identified using other messengers such as photons and gravitational waves, but also on how this pipeline can be used to constrain populations of astrophysical neutrino transients by following up high-energy neutrino alerts.

astro-ph.HE↗

arXiv2021

Testing the AGN Radio and Neutrino correlation using the MOJAVE catalog and 10 years of IceCube Data

On 22 September 2017, IceCube reported a high-energy neutrino event which was found to be coincident with a flaring blazar, TXS 0506+056. This multi-messenger observation hinted at blazars contributing to the observed high-energy astrophysical neutrinos and raised a need for extensive correlation studies. Recent work shows that the internal absorption of gamma rays, and their interactions intrinsic to the source and with the extragalactic background, will cause a lack of energetic gamma-ray and neutrino correlation while hinting towards a correlation between neutrinos and lower photon energy observations in the X-ray and radio bands. Studies based on published IceCube alerts and radio observations report a possible radio-neutrino correlation in both gamma-ray bright and gamma-ray dim active galactic nuclei (AGN). However, they have marginal statistical significance due to limited available data. We present a correlation analysis between 15 GHz radio observations of AGN reported in the MOJAVE XV catalog and 10 years of IceCube detector data and discuss the results derived from a time-averaged stacking analysis.

astro-ph.HE↗

arXiv2020

The Diffuse Supernova Neutrino Background

The Diffuse Supernova Neutrino Background (DSNB) in the MeV regime represents the cumulative cosmic neutrino emission, predominantly due to core collapse supernovae. We estimate the DSNB flux for different Star Formation Rate Density (SFRD) models. We find that the DSNB flux estimated using the SFRD derived from Fermi-LAT Collaboration et al. (2018) is significantly higher ($\approx$ 32$\%$) relative to the flux estimated using the SFRD from Madau & Fragos (2017). This depicts the sensitivity between the DSNB flux and the SFRD estimates which shows that future detection of the DSNB can be used as a valuable tool to constrain the SFRD.

astro-ph.HE↗

arXiv2019

A new measurement of the Hubble constant and matter content of the Universe using extragalactic background light $γ$-ray attenuation

The Hubble constant $H_{0}$ and matter density $Ω_{m}$ of the Universe are measured using the latest $γ$-ray attenuation results from Fermi-LAT and Cherenkov telescopes. This methodology is based upon the fact that the extragalactic background light supplies opacity for very high energy photons via photon-photon interaction. The amount of $γ$-ray attenuation along the line of sight depends on the expansion rate and matter content of the Universe. This novel strategy results in a value of $H_{0}=67.4_{-6.2}^{+6.0}$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.14_{-0.07}^{+0.06}$. These estimates are independent and complementary to those based on the distance ladder, cosmic microwave background (CMB), clustering with weak lensing, and strong lensing data. We also produce a joint likelihood analysis of our results from $γ$ rays and these from more mature methodologies, excluding the CMB, yielding a combined value of $H_{0}=66.6\pm 1.6$~km~s$^{-1}$~Mpc$^{-1}$ and $Ω_{m}=0.29\pm 0.02$.

astro-ph.CO↗

arXiv2019

A GeV-TeV Measurement of the Extragalactic Background Light

The Extragalactic Background Light (EBL) can be probed via the absorption imprint it leaves in the spectra of gamma-ray sources ($γγ\rightarrow e^-e^+$). We recently developed a dedicated technique to reconstruct the EBL, and its evolution with redshift, from $γ$ ray optical depth data using a large sample of blazars detected by the $Fermi$ Large Area Telescope. Here, we extend this dataset to the TeV regime using ground-based Cherenkov observations of 38 blazars and report the first homogeneous measurement of the EBL spectral intensity covering the ultraviolet to infrared wavelengths ($\sim$0.1-100$\mathrm{μm}$). A minimal EBL throughout the wavelength range with respect to integrated galaxy light is found, allowing little additional unresolved emission from faint or truly diffuse populations setting an upper limit of $\lesssim 4~{\rm nW\cdot m^{-2}sr^{-1}}$ at 1.4 ${\rm μm}$. In particular, the cosmic optical background (COB) at $z=0$ is found to be $27.8_{-2.0}^{+2.1}~{\rm nW\cdot m^{-2}sr^{-1}}$. This work lays the foundation for accurate gamma-ray measurements of the EBL across its whole spectral range using a combination of GeV and TeV data.

astro-ph.HE↗

arXiv2019

Identifying the 3FHL catalog: CTIO results

Active galactic Nuclei (AGNs) with their relativistic jets pointed towards the observer, form a subclass of luminous gamma-ray sources commonly known as blazars. The study of blazars is essential to improve our understanding on the AGNs emission mechanisms and evolution, as well as to map the extragalactic background light. To do so, however, one needs to correctly classify and measure a redshift for a large sample of these sources. The Third {\it Fermi}--LAT Catalog of High-Energy Sources (3FHL) contains $\approx1160$ blazars reported at energies greater than $10$\,GeV. However $\sim$25\% of these sources are unclassified and $\sim$50\% lack of redshift information. To increase the spectral completeness of the 3FHL catalog, we are working on an optical spectroscopic follow up campaign using 4--m and 8--m telescopes. In this paper, we present the results of the second part of this campaign, where we observed 23 blazars using the 4\,$m$ telescope at CTIO in Chile. We report all the 23 sources to be classified as BL Lacs, a confirmed redshift measurement for 3 sources, a redshift lower limit for 2 sources and a tentative redshift measurement for 3 sources.

astro-ph.HE↗

arXiv2017

Probing the EBL evolution at high redshift using GRBs detected with the Fermi-LAT

The extragalactic background light (EBL), from ultraviolet to infrared wavelengths, is predominantly due to emission from stars, accreting black holes and reprocessed light due to Galactic dust. The EBL can be studied through the imprint it leaves, via $γ$-$γ$ absorption of high-energy photons, in the spectra of distant $γ$-ray sources. The EBL has been probed through the search for the attenuation it produces in the spectra of BL Lacertae (BL Lac) objects and individual $γ$-ray bursts (GRBs). GRBs have significant advantages over blazars for the study of the EBL especially at high redshifts. Here we analyze a combined sample of twenty-two GRBs, detected by the Fermi Large Area Telescope between 65 MeV and 500 GeV. We report a marginal detection (at the ~2.8 $σ$ level) of the EBL attenuation in the stacked spectra of the source sample. This measurement represents a first constraint of the EBL at an effective redshift of ~1.8. We combine our results with prior EBL constraints and conclude that Fermi-LAT is instrumental to constrain the UV component of the EBL. We discuss the implications on existing empirical models of EBL evolution.

astro-ph.HE↗