Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from less than 10 keV to more than 1 TeV. Photon-level data and high-level data products are made publically available in near-real time, and efforts continue to improve the response time. This long-duration, all-sky monitoring has enabled...
Blazars exhibit strong variability, and abrupt changes in their flux are observed at high energies down to hour-, or even minute-time scales. Regular monitoring and prompt identication of these variations is key to organise quick follow-up observations. Thanks to its allsky monitoring capabilities, the Fermi-LAT is a very powerful instrument to survey the high energy sky and reveal such...
Many multi-wavelength campaigns have been carried out in the last years to study the correlation between the very high energy (VHE) gamma-ray emission and the X-ray emission in blazars. A linear (even quadratic) correlation has been predicted as consequence of leptonic mechanisms being responsible for the VHE gamma-ray emission. Although the activity in these two energy ranges seems to be...
Multi-frequency monitorings are an essential tool for investigating the possible connection between the different emission bands, allowing us to discern among the various emission mechanisms producing the observed radiation. In the case of blazars, a strong and significant correlarion was found between radio emission and gamma-rays between 100 MeV and 100 GeV, by using both concurrent and...
Variability power spectral densities (PSDs) of blazar light curves, crudly represented as $P(f) = A f^{-\beta}$, where A is the normalization and $\beta$ is the slope, indicate that the variability is generated due to the underlying {\it stochastic} processes (i.e., $\beta \simeq 1-3$, characteristic of flicker/red noise). We present the results of our power spectral analysis on blazar sources...
We report on the first systematic VLBI and gamma-ray monitoring study of a representative sample of radio galaxies with strong compact radio emission, with the aim of exploring the intrinsic relationship between high-energy emission and pc-scale jet properties in active galactic nuclei (AGN). While a number of studies have firmly established a close relationship between the gamma-ray and radio...
IceCube is the current largest neutrino observatory with an instrumented detection volume of 1 km³ in the ice-sheet below the antarctic South Pole station. With a 4π field of view and an uptime of >99% it is constantly monitoring the full sky to find astrophysical neutrinos. With the detection of an astrophysical neutrino flux in 2013, IceCube opened a new observation window to the...
The Astrophysical Multimessenger Observatory Network (AMON) was founded to tie the world’s high-energy and multimessenger observatories into a single network, with the purpose to discover multimessenger sources, to exploit these sources for purposes of astrophysics, fundamental physics, and cosmology, and to explore project datasets for evidence of multimessenger source populations. Successes...
The flat spectrum radio quasar CTA 102 (z=1.032) has exhibited a tremendous phase of its existence. Since early 2016 the gamma-ray flux level has been significantly higher than in previous years. It was topped by a 4-month long giant outburst, where peak fluxes were more than 100 times higher than the quiescence level. Similar trends are observable in optical and X-ray energies. We have...
BL Lacertae objects (BL Lacs) constitute a rare class of active galactic nuclei (AGNs) with the extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. The spectral energy distribution (SED) of these sources, extending over 17-19 orders of the frequency from radio to the TeV energy range, is of non-thermal...
I will highlight recent observational results
concerning the timing characteristics of AGN at gamma-ray
energies with a focus on indications for log-normality
and QPOs. The findings will be discussed in the context
of theoretical approaches to understand the physical
origin of variability in AGN.
The broadband spectral energy distribution (SED) of blazars has two well-separated bumps, one of low energy, peaking at soft X-rays and the other of high energy, peaking at hundreds of GeVs. The SED in most of blazars is well understood through the standard one-zone Self-Synchroton Compton (SSC) emission. However, if that is the case, a strong correlation between X-ray and TeV-emission is...
Blazars are a sub-category of radio-loud Active Galactic Nuclei having their jet pointing towards us and are known for their emission covering practically all frequencies of the electromagnetic spectrum. These sources, in some cases, exhibit a correlation between gamma-ray and radio emission, especially during flaring episodes. In this work, we construct a one zone leptonic model in order to...
I will present a suite of synthetic full Stokes single dish light curves generated from the Turbulent Extreme Multi-Zone (TEMZ) model of blazar emission. These synthetic light curves are created via ray-tracing through the TEMZ jet model and include the effects of optical depth, relativistic aberration, Faraday rotation, Faraday conversion, slow-light interpolation, and beam convolution. We...
The light curves of black hole X-ray binaries show variability on timescales from milliseconds to months. The variability is from X-ray-bright matter in the inner region of curved spacetime surrounding the black hole. We use Fourier techniques to identify timing signals such as quasi-periodic oscillations (QPOs) and intrinsic broadband/band-limited noise. These signals are not just due to...
To provide a detailed understanding of blazar and its environment, we study variability over diverse timescales using various statistical methods. As optical flux variations in blazars are often followed by spectral changes, thus we examine their colour – magnitude relationship on diverse timescales which helps us to understand the origin of variability. Presence or absence of correlation...
Most gamma-ray emitting AGN are variable and multiwavelength temporal studies provide insights into acceleration and radiation mechanisms, source size, radiative re-processing and source structure.
The gamma-ray band is very wide and is explored with very different techniques. In different energy
bands very different biases affect temporal studies. The biases have significant implication for...
Four hundred years ago Galileo revolutionised the way we see the universe through his telescope. Since then fascinating yet bizarre astrophysical phenomena reveal our capricious universe. Using electromagnetic observations we have been discovering exciting events at different wavelengths. Surprisingly, the universe is eager to send us information through different kinds of waves which probe...
On August 17th, 2017 the LIGO and Virgo detectors observed gravitational waves consistent with a binary neutron star (BNS) coalescence, with spatial and temporal coincidence with a gamma-ray burst (GRB) detected by the Fermi Gamma-ray Burst Monitor. Subsequent emission in the optical, ultraviolet, infrared, X-ray and radio bands was also observed. This was a milestone in multi-messenger...
The First G-APD Cherenkov Telescope (FACT) has been monitoring blazars at TeV energies for more than six years. Because of the automatic operations and the usage of robust solid state photosensors (SiPM, aka G-APDs), it has been possible to collect a large and unbiased data sample of more than 11,000 hours. One of the closest and brightest blazars in the gamma-ray/X-ray sky, Mrk 421, is...
The locations of emission of gamma-ray radiation in active galactic nuclei jet are highly debated and it range from light-hours to a few light-year in quasar jets. The situation is more complex in the case of flat spectrum radio quasars, where the gamma-rays photons above 10 GeV may interact with the UV radiation from broad line region and get absorbed. I will be talking about the recent...
PKS 1510-089 is one of only a handful of flat spectrum radio quasars detected in very high energy (VHE, E > 100 GeV) gamma rays. Since the first detection in 2009, the source has been monitored VHE. Here, we present one special event that is a direct result of the monitoring effort. In May 2016, a major VHE gamma-ray flare was observed from PKS 1510-089 by the H.E.S.S. and MAGIC telescopes....
Blazars are known to show variability on time scales from minutes to
years. This complicates the measurement of their ground state. For
this, long-term monitoring is important to increase the chance to study
the source in an all-time low state.
The First G-APD Cherenkov Telescope (FACT) is monitoring bright TeV
blazars since more than six years and has collected between 1500 and
3000 hours of...
Working with multi-messenger data comes with a variety of challenges. Ideally, one would like to take the prior information into account that the scientific object looks similar in neighbouring frequency channels. Moreover, data from radio telescopes has different statistical properties compared to data generated by gamma-ray telescopes: the former can be assumed to have Gaussian noise and the...
Active Galactic Nuclei emit radiation over the whole electromagnetic spectrum up to TeV energies. Blazars are one subtype with their jets pointing towards the observer. One of their typical features is extreme variability on timescales from minutes to years.
The fractional variability is an often used parameter for investigating the degree of variability of a light curve. By using public data...
The AstroSat, space-based Indian multi-wavelength observatory, provides an unique platform to enable the access of a very broad energy band (E~ 0.012 - 120.0 keV). It has also displayed the capability of observing hard X-ray polarization (e.g. 100-380 keV for Crab) for bright objects like bright GRBs, Crab, Cyg-1 etc. However, the timing capability of AstroSat has been displayed for a number...
This talks reviews general theoretical aspects of modeling the broadband
spectral variability of blazars. Both time-dependent leptonic and lepto-
hadronic models will be discussed. Recent applications to the modeling of
coordinated and unccoordinated (orphan flares) multi-wavelength variability
of several prominent Fermi-LAT blazars will be discussed.
Flaring in blazar jets has been found to occur at TeV energies on rapid timescales as short as minutes, implying the emission originates from a very compact region within the jet. Whilst the physical origin powering such flares is yet to be established, recent particle-in-cell (PIC) simulations have indicated that magnetic reconnection can plausibly produce plasmoids small enough to...
Most research in blazar variability focuses on individual flares to explain acceleration
and radiation mechanisms and improve on current models. These short-time events (minutes,
hours or days) might not be representative of the underlying mechanisms causing small-
amplitude variability and/or continuous emission present most of the time. We therefore
investigate long-term (month to years)...
Because of their brightness and proximity (z=0.03), Mrk421 and Mrk501 are among the very-high-energy gamma-ray objects that can be studied with the greatest level of detail. This makes them excellent astrophysical high-energy physics laboratories to study the nature of blazars. Since 2009, there has been an unprecedentedly long and dense monitoring of the radio to very-high-energy gamma-ray...
In this contribution, I will present the results of a recently published paper on the Flat Spectrum Radio Quasar 3C 279. We use light curves that cover a time-frame of six years, at different wavelengths: Gamma-rays, X-rays, UV 3000 Å continuum, optical V band, Near-Infrared (NIR) JHK bands, 1mm, as well as optical spectropolarimetry.
By applying cross-correlation analysis, We find that the...
The BL Lac object OJ 287 exhibits a regular ~12 years quasi-periodic outburst in optical band. The latest of this outburst occurred in December 2015 and since then till July 2017, it has exhibited intense multi-wavelength (MW) activity with many new features never seen before. The overall MW activity can be divided into two phases: November 2015 – May 2016, exhibiting strong variability from...
Active galactic nuclei (AGN) present rapid variability across the electromagnetic spectrum. In most cases, this can be explained with relativistic boosting along the line of sight. A very valuable approach to probe the nature of these objects is to study their radio morphology at the highest resolution available at present, provided by very-long-baseline interferometry (VLBI). While...
In September 2012, the blazar Mrk421 showed a remarkable and distinct radio flare most prominent at cm wavelengths, following a similar flare at gamma-ray energies that occured about 40 days earlier. The radio flaring bahavior indicates the injection of fresh plasma into the jet, which may lead to the formation of a new jet component on parsec scales. This can be verified in analysis of the...
In order to investigate the high energy emission and jet formation in blazars, we study a sample of gamma-ray bright AGN in a combined 7mm / 3mm Vlbi monitoring program. Here we present total and linearly polarized GMVA images of a sample of blazars from the VLBA-BU-BLAZAR program, obtained from May 2016 to March 2017. The lower opacity at 3 mm and high angular resolution, of the order of 50...
Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the blazar populations best. We use a Bayesian hierarchical code called Magnetron to model the light curves of 973 sources observed by the Owens...
The High-Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view instrument under operations since March 2015 and located in the state of Puebla, México. HAWC observes two thirds of the sky daily at energies between 0.1 and 100 TeV with a duty cycle greater than 95%. This capability allows us to monitor unbiasedly known sources as the Mrk 421 blazar, to search blindly for transient...
The COMPTEL experiment aboard the Compton Gamma-Ray Observatory (CGRO) explored
the MeV sky (0.75 - 30 MeV) for more than 9 years between April 1991 and June 2000,
providing many discoveries. Now, more than 18 years after the deorbit of CGRO, the
COMPTEL data are still the forefront of our knowledge on the non-thermal soft gamma-ray
sky (1 - 30 MeV), because no successor is yet operating.
The...
After 5 years of polarimetric monitoring of blazars, the RoboPol project has uncovered several key characteristics of polarimetric rotations in the optical for these most variable sources. The most important of these is that polarization properties of the synchrotron emission in the optical appear to be directly linked with gamma-ray activity. I will discuss the evidence for this connection,...
The prototype blazar BL Lac is monitored in the frame of the Blazar Optical Sky Survey (BOSS) Project at the University of Athens Observatory (UOAO), during the period of 2014-2018. BL Lac is continuously observed on a daily basis, in order to achieve dense temporal coverage in optical wavelengths, and study the short time-scale flux variability. Several long-runs have been conducted, where...
ATOM is an optical telescope located at the H.E.S.S. site in Namibia. It monitors optical flux of roughly 300 known gamma-ray emitters. In the beginning of 2018, a new instrument has been installed with the aim of enhancing the capabilities of ATOM – including measuring sub-second variability and polarisation. I will give a short overview of the new instrument's design and present first results.
Magnetic field strength and geometry are fundamental properties which control the formation and evolution of relativistic jets, and their observed emission via the field's impact on particle acceleration. At centimeter-band, where the emission is well-known to be produced by the synchrotron process, magnetic field properties can be constrained using a wealth of data from both single-dish...
Blazars are extremely variable objects emitting radiation across the electromagnetic spectrum and showing variability on time scales from minutes to years. Simultaneous multi-wavelength observations are crucial for understanding the emission mechanisms. From radio via optical, X-ray to gamma rays, a variety of instruments, as Fermi and OVRO, are already monitoring blazars. At TeV energies,...
The Cherenkov Telescope Array (CTA) is the next-generation ground-based gamma-ray observatory, currently in the prototyping and testing phase. CTA will consist of two arrays of imaging atmospheric Cherenkov telescopes, one in the Northern and one in the Southern hemisphere, reaching a sensitivity roughly five to ten times higher than existing instruments and covering an energy range from 20...
