Description
AGN - GRBs - SNR/PWNe - Future Missions/Instruments - Analysis Techniques - Pulsars - Binaries - Neutrinos - Diversity and Equity - Dark Matter - Solar System Gravitational Waves
Both observational evidence and theoretical considerations from MHD simulations of jets suggest that the relativistic jets of active galactic nuclei (AGN) are radially stratified, with a fast inner spine surrounded by a slower-moving outer sheath. The resulting relativistic shear layers are a prime candidate for the site of relativistic particle acceleration in the jets of AGN and gamma-ray...
Blazars are among the brightest objects in the γ-ray sky. Nonetheless, even after decades of γ-ray and multiwavelength observations, they are far from being understood.
In this contribution, we introduce a multiwavelength data set of the archetypal blazar, Mrk 501. The data set spans the period from 2017 to 2020 and is complemented by a 12-year data set that starts in 2008 for some of the...
We report the detection of a transient quasi-periodic flux modulation in the 6-hour binned $\gamma$-ray light curve of blazar PKS 0903-57 observed by Fermi-LAT. Several independent periodicity search analyses revealed a periodicity of $\sim$6 days that lasted for 5 full cycles during its outburst in March-April 2020. Using Monte-Carlo light curve simulation technique, we found the QPO...
We present the results of the multiwavelength study of several blazars at redshift z >~1 that have been observed by the High Energy Stereoscopic System (H.E.S.S.) since 2016 in target-of-opportunity observations, triggered by gamma-ray flaring states detected by the Fermi Large Area Telescope (LAT). We collect data from the Fermi-LAT, SWIFT and H.E.S.S. telescopes and model the broadband...
Blazars are a special kind of active galactic nuclei (AGNs) with jets oriented at small angles to our line of sight. Due to the relativistic motion of plasma along the jet, it constitutes one of the most rapidly varying classes of objects over a broad energy band (radio to γ-ray). S5 1044+71 (z = 1.15) is a known distant blazar observed in the GeV energy band. In the latest Fermi-LAT source...
Blazars are among the most powerful cosmic particle accelerators in our Universe. Strong multi-wavelength variability suggests extreme particle acceleration locally in the blazar emission region. Previous studies find that shock, magnetic reconnection, and turbulence can be the underlying particle acceleration mechanisms in the emission region. However, most theoretical models involve...
Polarimetry is a direct measurement of the magnetic field in astrophysical objects. With the launch of IXPE, as well as future MeV polarimeters under development, such as COSI and AMEGO-X, multi-wavelength polarimetry will shed light on the magnetic field and particle acceleration in blazars. Here we present our theoretical predictions of multi-wavelength blazar polarization signatures. We...
Blazars are active galactic nuclei with jets aligned very closely to our line of sight. The optical emission of blazars is often dominated by the polarised, non-thermal emission arising in the jet, with an underlying unpolarised, thermal emission component arising from the host galaxy, dusty torus, and accretion disk components. As the emission of blazars varies between flaring/high states and...
Active Galactic Nuclei (AGN) are compact
and highly luminous regions at the centre of galaxies. Relativistic
jets from radio loud AGN have been shown to exhibit variability over
various time-scales and frequencies. A contributing component to the
formation of variability results from the injection and propagation of
blob/shock structures within the jet. We have investigated this
through...
The extragalactic gamma-ray sky observed by Fermi-LAT is dominated by blazars, with only a handful of narrow-line Seyfert 1 (NLS1) galaxies detected in 10 years of observation. Flares from this elusive source class are among the rarest events that the Fermi-LAT has seen so far, and we are presenting the analysis on one such event from the radio- and gamma-ray loud source PKS 2004–447.
On...
Active galaxies are complex astrophysical objects in that many physical processes occur almost simultaneously. They emit at all electromagnetic frequencies. Active galactic nuclei (AGNs) are the parent class of blazars which are the brightest of active galaxies. There are a number of models that can explain the spectral energy distribution (SED) of blazars. However, the complexity of blazars...
Despite occupying ∼40% of the local Universe, Low Luminosity Active Galactic Nuclei (LLAGNs) are less explored due to their faintness. Detection of a few in gamma rays by Fermi-LAT allows us to constrain the physical parameters of the jet by modeling their broadband spectral energy distributions. While a one-zone model explains the broadband emission up to a few GeV, another component is...
Blazars show variability across the electromagnetic spectrum on a variety of time scales. In some cases, flaring events in one frequency band are not accompanied by flaring in other bands. Such events are termed ”orphan flares”. The causes of this variability and conditions in and location of the high-energy emission region are not entirely understood. The hadronic synchrotron mirror model is...
Blazars are a subclass of Active Galactic Nuclei (AGN) seen almost along the relativistic jet, which emanate from very close to the central super massive black hole. As these jets are close to our line of sight, blazars represent a unique sample to study the extreme particle energisation, nature of the magnetic field and many other physical properties of jets. Blazars are well known to show...
Gamma-ray bursts are the most energetic cosmic explosions in the Universe, covering a spectral domain from all the way radio to gamma-ray up to tens of GeV. Recently, the detection of very high energy emissions (z ~ 0.0785 to 1.1) associated with the afterglows of a few GRBs by HESS and MAGIC telescopes has provided new insights into the research area of these fascinating objects. This work...
A wealth of Gamma-Ray Burst (GRB) data is available today with known redshifts (observed up to z =9.4), provided by different instruments with well-measured prompt gamma-ray flux and spectral information. In order to estimate redshifts of GRBs using a theoretical estimate (so-called pseudo-redshifts) from spectral relations, several phenomenological relations have been developed. Amati...
Gamma-Ray Burst Monitor (GBM) on board Fermi, designed to detect short transient events, has been continuously observing the entire sky in gamma-ray since its launch in 2008. Over the 14 years, GBM has produced the largest database of all-sky observations in gamma rays with high time resolution continuous data (2 µs; CTTE since July 2010). Such continuous data contain a wealth of relatively...
The composition of the jet and the nature of the prompt non-thermal emission are open questions in gamma-ray bursts astrophysics. In this work, we study the degree of magnetisation of the jet and the prompt emission for 14 Fermi LAT GRBs with sub-dominant black-body components. We first carry out the joint spectral analysis of these GRBs with the GBM and LAT data using multi-component...
Gamma-ray bursts (GRBs) comprise of short, bright, energetic flashes of emission from extragalactic sources followed by a longer afterglow phase of decreased brightness. Recent discoveries of GRB 180720B and GRB 190829A afterglow emission up to very-high-energy $\gamma$-rays by H.E.S.S. have raised questions regarding the emission mechanism responsible. We interpret these observed afterglows...
TeV halos have become a new class of astrophysical objects which were not predicted before their recent observation. They offer evidence that diffusion around sources (concretely, pulsars) is not compatible with the effective average diffusion that our models predict for the Galaxy. This directly impacts Galaxy formation, our knowledge of the propagation process throughout the Galaxy and our...
HESS J1702-420 is an unidentified multi-TeV gamma-ray source with a peculiar energy-dependent morphology which most naturally can be explained as a composition of two independent emission components with significantly different spatial and energy distributions. Here we propose an alternative interpretation assuming that we deal with a single hadronic accelerator injecting protons with...
Supernovae remnants (SNRs) are widely believed to be one of the prime sources of Galactic cosmic rays. They are known to be efficient particle accelerators which is indirectly confirmed by detection of non-thermal emission across the whole electromagnetic spectrum from radio to very-high-energy gamma-rays. Protons and electrons can be accelerated to very high energies of at least several tens...
The first catalog of Fermi-LAT sources below 100 MeV (1FLE catalog, Principe et 2018) covers the energy range from 30 to 100 MeV. The catalog contains 198 sources of different nature, having a detection significance above 3 sigma. Most of these sources are of extragalactic nature with flat spectrum radio quasars (FSRQs) being the majority. The COMPTEL experiment aboard the Compton Gamma-Ray...
Fermi’s observations of gamma-ray bursts (GRBs) allow us to model the next generation of GRB instruments, including CubeSats such as the soon-to-be launched BurstCube. Over the first 14 years of its operation, GBM has studied nearly 3500 GRBs, many in stunning detail, which is a significant sample for simulating aspects of BurstCube’s observations when combined with detailed simulations of the...
Next-generation radio surveys from the Square Kilometre Array (SKA) mid-frequency telescope and its precursors will observe the universe with high spectral precision. The 21-cm neutral hydrogen (HI) emission line detected from these surveys is ideal for obtaining more accurate constraints on cosmological parameters. However, the HI line is intrinsically faint and difficult to detect at high...
We present a catalog of LAT information on VHE sources. The purpose of the catalog is a compilation of complementary information from LAT and ground-based gamma-ray arrays on sources detected in the VHE energy band (E> 100 GeV)
Magnetars are isolated, young ($<10^5$ years), highly magnetic ($>10^{14}$ G) neutron stars with long spin periods (2-12 s). All magnetars emit short repeated bursts in soft gamma rays; however, only a handful of magnetars show radio pulsations. After the detection of magnetar-like bursts from highly magnetic ($>10^{13}$ G) Rotation Powered Pulsars (RPPs), the question of whether magnetar-like...
Millisecond pulsars are old recycled pulsars with a rotation period of less than 30 ms. The pulsed GeV emission mechanism of the MSPs is not fully understood. The Fermi LAT Third Pulsar Catalogue (3PC) will present light curves of unprecedented quality for more than 275 gamma-ray pulsars, with more than 100 of them being MSPs. As a part of our project, we are studying the morphology of the...
After the detection of neutrinos from blazar TXS 0506+056 in a flaring state, there have been advancements in neutrino astronomy including a detection of neutrinos from a tidal disruption event. Blazar TXS 0506+056 and other neutrino sources were detected by the first-generation neutrino telescopes during transient states. The spectral energy distributions of blazars jet emissions can be...
One of the most pressing questions for modern physics is the nature of dark matter (DM). Several efforts have been made to model this elusive kind of matter, whose presence has been assessed only by gravitational effects so far. The largest fraction of DM cannot be made of any of the known particles of the Standard Model (SM). The ground-based gamma-ray telescope system MAGIC could potentially...
Sustained gamma-ray emission (SGRE) events observed by FERMI/LAT at >100 MeV energies are associated with fast and wide coronal mass ejections (CMEs) from the Sun. These CMEs are similar to those that cause ground level enhancement in solar energetic particle (SEP) events. CME-driven shocks have been suggested to be the acceleration site of the >300 MeV protons producing the >100 MeV gamma-ray...
