2. Research interests

Current research interests and plans (since 2014):

Multiwavelength studies of quasars and blazars

focused on the optical and gamma-ray bands and spectropolarimetry observations

Introduction: Active galactic nuclei and blazars

The physics of Active Galactic Nuclei (AGNs) is a very active branch of contemporary astrophysics. AGNs constitute a major population of distant extragalactic objects. Located at the centre of massive galaxies, they are understood as supermassive black holes (SMBHs, 106 − 1010 solar mass) surrounded by an accretion disk of hot gas powering strong thermal emission, mainly at ultraviolet and blue wavebands. Radio-loud AGNs possess a pair of twin jets of relativistic particles and radiation, which are oriented perpendicularly to the accretion disk. Formed close to the SMBH (~ 0.1 pc), these jets carry a large amount of radiation within a narrow opening angle. When the Earth direction is found within the opening angle of one of the jets, these radio-loud AGNs are called blazars. In this privileged orientation, due to the Doppler boosting of the emission from the jet, a relatively large flux of non-thermal radiation is detected by both space and earth-based telescopes at all wavelengths, from radio waves to gamma rays (Urry & Padovany, 1995, PASP, 107, 80).

The Spectral Energy Distributions (SEDs) of blazars are characterised by a double hump structure. The radio to optical/X-ray component is understood to be produced by synchrotron radiation from relativistic electrons, whereas the second, higher energy component, from X-ray to gamma-ray energies, can be produced through either leptonic (inverse Compton) or hadronic processes (e.g. Böttcher et al, 2013, ApJ, 768, 54B). Blazars are classified into two main subclasses that relate to their morphological properties: Flat Spectrum Radio Quasars (FSRQs) and BL Lacs. FSRQs exhibit an optical spectrum of broad emission lines, which is due to the presence of a relatively intense radiation field – called the broad-line region (BLR) – surrounding the SMBH at a distance of ~ 1 pc. On the other hand, BL Lacs are on average less luminous and their optical spectra are either featureless or have weak and narrow emission lines. Blazars are found at all redshifts from z ≤ 0.1 up to z > 6 (S. Belladitta et al. 2020, A&A 635, L7), and so can be used as probe of galaxy evolution throughout the history of the Universe. Characterisation of their physical properties is of definitive importance for observational cosmology and for fundamental physics, as these sources and their environment constitute natural laboratories to study particle accelerations and matter/radiation interactions in extreme conditions, up to energies beyond the limit of the Large Hadron Collider (LHC) at CERN.

Launched in June 2008, the Fermi Gamma Ray Space Telescope has amassed a large amount of data from the whole sky. Operating mainly in survey mode, the whole sky is observed every three hours, which allows a continuous monitoring of any bright object – up to sub-day intervals in several instances. Its main instrument, the Large Area Telescope (LAT) is sensitive to photons from 20 MeV to several hundreds of GeVs (Atwood, et al., 2009, ApJ, 697, 1071). The Fourth Fermi-LAT AGN Catalog (4FGL-DR3, Abdollahi et al, 2022, ApJS, 260, 53) lists 3814 blazars (1458 BL Lacs and 792 FSRQ) as well as 1493 sources which are labelled as blazar candidates of uncertain type – or BCUs. These sources are expected to be classified either as FSRQs or as BL Lacs sources without plausible counterparts at other wavelengths.

Recent and current research focus

Since 2014, I have been a postdoctoral member of the Fermi-LAT Collaboration. Most of my current research activities over these past 10 years are based on the observations and the modelling of FSRQs (Britto et al., 2016sf2a.conf...93B), including the study of their optical polarisation during flaring activity (FSRQ 4C+01.02 – Schutte, Britto, Böttcher et al., 2022, ApJ 925:139) and source classification (FSRQ NVSS J141922-083930 – Buckley, Britto, Chandra et al., 2022, MNRAS 517:5791). However, I am also interested in the observation of other AGN types such as the high energy misaligned AGN emitters (Chiaro, Alvarez Crespo, Britto et al., 2018, arXiv:1808.05881 [astro-ph.HE]). Bright AGNs can be studied up to several tens of GeV with Fermi-LAT.

I also worked on the study of the optical afterglow of gamma-ray burst GRB191221B, which was the first GRB afterglow observed by the Southern African Large Telescope (SALT), using the Robert Stobie Spectrograph (RSS). We reported on the following observations: optical spectropolarimetry with SALT-RSS and VLT-FORS2, optical photometry with the MASTER Global Robotic Network, and radio data with MeerKAT (Buckley, Bagnulo, Britto et al, 2021, MNRAS 506, 4621).

Multiwavelength monitoring of AGN is crucial to probe intrinsic properties of these sources (Britto, PoS(SSC2015)032). When AGNs are significantly variable, analysis in the time domain are fundamental to study the emission regions. As a part of an international team involved in the multiwavelenth monitoring of AGNs, I perform the analysis of Fermi-LAT data (> 100 MeV) from several blazars and we coordinate optical follow-ups and X-ray target-of-opportunity observations of these sources during flaring episodes (Britto et al. PoS(FRAPWS2016)055). Some of our main observation campaigns are related to FSRQs, e.g. 4C +01.02, NVSS J141922-083830, PKS 2023-07, 3C 279. For several of these targets, data were collected by Fermi-LAT, Las Cumbres Observatory (LCO), MASTER-Net and Swift-XRT. In addition to flux monitoring, we obtain spectropolarimetry with SALT-RSS, that allows us to measure the degree of linear polarisation and the position angle of blazars for different flaring and quiescent episodes (Britto et al, HEASA 2017), and to perform broad-band SED modelling using the wavelength dependence of the polarisation. We could derive constraints on the degree of order of the magnetic fields around the central region of the AGN, that brings an additional constraint on the mass of the central SMBH (Schutte, Britto, Böttcher et al. 2022, ApJ 925:139).

The broad-band SED modelling of blazars allows the measurement or the estimate of physical quantities of these active galactic nuclei (AGNs), such as the mass of the supermassive black hole (SMBH), the magnetic field, etc. Several physics parameters involved in the radiation emission processes within plasma jets and interaction of the jet with the surrounding media can then be constrained. These parameters are, e.g. the spectral parameters of the electron distribution at the origin of the Synchrotron and inverse Compton (IC) processes, the accretion disk luminosity. However, this technique is subject to several limitations. First, the broad-band SED is not continuously sampled, as we do not have instruments which are sensitive at all wavelengths and are not able to cover the whole electromagnetic spectrum from radio waves to gamma rays, though all bands (radio, IR, visible, UV, X-rays, gamma rays) remain at least partly covered. Also, when one studies specific flaring events, statistics is limited by the duration of the event and data are to be collected within a period of a few days or a few weeks. In addition, some models (more than one single emission leptonic models and hadronic models) require multiple free parameters, which makes parameters of the SED fits to be correlated and hinders significant constraints on the quantities that we try to estimate. However, the large number of sources that were studied, and the large amount of multiwavelength data that were collected, have allowed to provide interesting results within the framework of the AGN unified model. Still, many challenges remain in understanding the production of radiation within blazar jets.

Moreover, hadronic models that could account for the high energy emission observed from some blazars may involve synchrotron emission from protons and muons, and also cascades initiated by these particles. Since these models require more parameters than one-emission-zone leptonic models, the discrimination between them on scarcely sampled SED are often non conclusive, so, in order to discriminate between leptonic and hadronic processes at the origin of the photon production that account for the high energy bump of the blazar SEDs, a good sampling of the X-ray to very and ultra high energy gamma rays is needed, both in time and energy range. Such work requires the use of observations from soft and hard X-ray instruments such as Swift-XRT and NuStar, as well as Fermi-LAT for the MeV and GeV domain, and also the Cherenkov imaging arrays HESS, MAGIC, VERITAS, and the Cherenkov Telescope Array Observatory (CTAO) whose Large-Sized Telescopes (LSTs) have started operating. Detection of FSRQ OP 313 by LST-1 in 2023 shows the potential of this observatory since it was the first time that such a distant AGNs (z ~ 1) was detected at energies above 100 GeV (Cortina, 2023, ATel #16381). Long term observations from HAWC and LHASSO are also well designed to constrain TeV signal from bright blazars.

My main research project is to continue to study samples of AGNs whose data are available at various wavelengths. For each of these sources, various investigations will be performed in order to constrain or measure one or more physical quantities of these AGNs. Data will be taken from public data sets (archival, SDSS, Swift, Fermi-LAT, CTAO, etc.) and from dedicated observations from our observation proposals.

Previous research experience

(2008-2013)

• Work on the HAGAR experiment

The High Altitude GAmma-Ray (HAGAR) array is a wavefront sampling array of 7 telescopes, each one built with 7 para-axially mounted 0.9 m-diameter mirrors, set-up at Hanle, at 4270 m amsl, in the Ladakh region of the Himalayas. It constitutes the first phase of the HImalayan Gamma-Ray Observatory (HIGRO) project. HAGAR is the first array of atmospheric Cherenkov telescopes established at such a high altitude, and was designed to reach a relatively low threshold (currently around 200 GeV) with quite a low mirror area (31 m2). Using the atmospheric Cherenkov sampling technique, HAGAR performs observations of the UV to visible Cherenkov light caused by the relativistic secondary particles of the atmospheric showers induced by incoming cosmic gamma rays. Data are recorded for each event using 12-bit Time to Digital Converters (TDCs), 12 bit charge to Digital Converters (QDCs) and 8-bit Flash-ADCs at 1Ghz.

In order to reject the dominant cosmic ray flux, observations are performed by pairs of data, in such a way that signal can be extracted by getting an excess of events in the on-source data set compared to the off-source data set where only cosmic ray background is expected. As charged cosmic rays also produce atmospheric showers causing Cherenkov light, a robust analysis method had to be developed to allow the extraction of the gamma-ray signal. Regular calibration and source data are taken with the complete HAGAR array since 2008. I was mainly working on the quality check of raw data during the first months of my postdoctoral fellowship at TIFR, with an emphasis on the study of the Charge-to-Digital Converters (QDCs) pedestal distributions and Latch information, in collaboration with engineers of our group. I also simulated the detector response by updating parameters like cable attenuation, NSB, etc. By varying some parameters like discriminator values, I obtained the energy threshold of the experiment versus trigger rate. Later, my work in the HAGAR collaboration was mainly dedicated to the analysis of data from Crab nebula, to the study of systematics effects due to the background light, and to data selection by characterisation of several parameters of raw data.

In order to estimate the level of statistical fluctuations due to our observation strategy (source and background not taken at the same time) and our analysis method, I performed the analysis of dark regions pairs, from which we expect significance of the excess to be compatible with zero, if systematics are not dominant. This study was necessary for optimisation of our analysis cuts (normalisation and rejection cuts). Also, the Crab nebula is sitting on the sky at 67.5' from a blue star of magnitude 3. This means that HAGAR photomultipliers get a significant amount of light coming from this star during the Crab observations. Results for a configuration with 4 and 5 telescopes suggest that there is an artefact of signal. This indicates that systematic effects induced by a much brighter field of view inducing hardware rate differences during observations are to be considered in our data/analysis, and should be balanced by an appropriate event selection. I have reported results on the calibration and Crab nebula throughout the different steps of the development of the analysis and on different data sets.

• Work with the PICASSO collaboration

The Project In CAnada to Search for Supersymmetric Objects (PICASSO) is a set up of superheated droplet detectors (SDDs), located in the SNOlab underground laboratory in Sudbury, Ontario, Canada. PICASSO is designed for the direct search for supersymmetric dark matter particles, called WIMP (for Weakly Interacting Massive Particles), using 32 detectors of capacity 4.5 liters and containing droplet emulsions of superheated liquid C4F10 inside a polymerised gel. Energy deposited during the passage of a particle through a drop of superheated liquid can induce bubble nucleation in the drop. The bubble expands if the energy deposited is greater than a certain critical amount over a certain critical length scale. The expansion of the bubble is accompanied by emission of an acoustic pulse which can be recorded by appropriate devices (piezoelectric sensors or microphones). By the study of the detection thresholds as a function of the temperature and the characterisation of the acoustic pulses, one can discriminate between the events generated by different types of particles. 

In 2012, I joined the PICASSO collaboration and I participated in the development of the analysis procedure by looking for analysis strategies for reducing the background events dominated by alpha particles induced in the detector. Beside the regular set up of the underground PICASSO detectors, calibration runs are also conducted using the regular detectors as well as new prototypes of SDDs (different droplet sizes, active liquids and sound recording devices). The signature of WIMPs induced nucleation is estimated to be similar to what is observed for neutrons. Alpha particles constitute the main intrinsic background of this type of detection. 

By comparing calibration run taken with alpha and neutron sources respectively, I have constructed discriminating variables based on the characterisation of acoustic pulses (amplitudes, frequencies, etc.) in order to study the alpha/neutron discrimination for data taken with detectors with different droplet sizes and at different temperatures. Preliminary results indicated promising efficiency for rejecting a significant fraction of alpha particles. Also, this discrimination capability can be used to improve the sensitivity of present and future dark matter search experiments using the superheated droplet technique or bubble chambers.