Richard Britto | University of the Free State (original) (raw)

Papers by Richard Britto

Monthly Notices of the Royal Astronomical Society

We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J1... more We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J141922−083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog. Relatively bright flares (>3 magnitudes) were observed on 2015 February 21 (MJD 57074) and 2018 September 8 (MJD 58369) in the optical band with the MASTER Global Robotic Nettelescopes. Optical spectra obtained with the Southern African Large Telescopeon 2015 March 1 (MJD 57082), during outburst, and on 2017 May 30 (MJD 57903), during quiescence, showed emission lines at 5325 Å and at ≈3630 Å that we identified as the Mg ii 2798 Å and C iii] 1909 Å lines, respectively, and hence derived a redshift zzz = 0.903. Analysis of Fermi-Large Area Telescope (LAT) data was performed in the quiescent regime (5 yr of data) and during four prominent flaring states in 2014 February–April, 2014 October–November, 2015 February–March, and 2018 September. We present spectral and...

Astroparticle Physics, 2013

We present results of Monte Carlo simulations for the High Altitude Gamma Ray (HAGAR) telescope a... more We present results of Monte Carlo simulations for the High Altitude Gamma Ray (HAGAR) telescope array which detects very high energy gamma rays from astronomical sources. This telescope array, located at Hanle at an altitude of 4270 m in the Ladakh region of the Himalayas in India, is the highest altitude atmospheric Cherenkov experiment in the world. Taking advantage of the high altitude, this experiment could achieve relatively low energy threshold with a modest mirror area coverage. To understand the performance parameters of this telescope system, we have simulated large samples of extensive air showers initiated by gamma rays and various species of cosmic rays, using the CORSIKA package. Cherenkov photons produced in the atmosphere are sampled at ground level. These photons are then passed through the detector simulation program, which takes into account various design details and the data acquisition system of HAGAR. Night sky photons are also considered in the detector simulation program as performance of the telescope depends strongly on the level of night sky background (NSB) at the observation site. We have estimated various performance parameters like energy threshold and effective area for vertically incident showers as well as inclined showers. Details of these parameters, results obtained from simulations and comparison with the observed data are presented. It is shown that the energy threshold of the HAGAR telescope system is about 208 GeV, a factor of $4 less than for a similar set up at about 1000 m altitude, and it is able to detect Crab like sources at 5r significance in 17 h of observation without imposing additional criteria, like gamma-hadron separation, for further rejection of cosmic rays.

The Astrophysical Journal, 2022

The optical radiation emitted by blazars contains contributions from synchrotron radiation by rel... more The optical radiation emitted by blazars contains contributions from synchrotron radiation by relativistic electrons in the jets, as well as thermal radiation emitted mainly by the accretion disk (AD), the broad-line region (BLR), and the host galaxy. The unpolarized radiation components from the AD, BLR, and host galaxy present themselves by decreasing the total polarization in the optical/ultraviolet (UV) spectrum. A combined model for the spectral energy distribution (SED) and degree of optical/UV polarization is constructed, enabling the disentanglement of the synchrotron and AD components. Our model is applied to the multiwavelength SED and spectropolarimetry observations of the flat-spectrum radio quasar 4C+01.02 (z = 2.1) in its 2016 July–August flaring state and 2017 July–August quiescent state, using data from the Fermi Large Area Telescope, the Southern African Large Telescope, and the Las Cumbres Observatory network of telescopes. By constraining the AD component, the mas...

Proceedings of High Energy Astrophysics in Southern Africa 2021 — PoS(HEASA2021), 2022

Blazars represent a subset of AGN with relativistic jets, where the direction of the jet lies ver... more Blazars represent a subset of AGN with relativistic jets, where the direction of the jet lies very close to our line of sight. The highly Doppler boosted emission from the blazar's jet results in high apparent luminosities, and blazars display variability on periods from less than one day up to years. At optical wavelengths, the observed emission of the blazar is a superposition of the polarised non-thermal synchrotron emission, arising from the jet, and the unpolarised thermal emission, arising from the accretion disc, broad line region, torus and host galaxy. Polarimetry observations can serve as an important tool for diagnosing the emission from blazars. The RSS spectrograph, on SALT, can operate in spectropolarimetry mode and is currently being used to undertake spectropolarimetric observations of transient blazar sources. We present additional tools developed to work in conjunction with the current SALT spectropolarimetry reduction pipeline, polsalt, that aims to streamline the reduction of the SALT polarisation data, including the testing of the wavelength calibration of the individual O and E beams. This was applied to observations of 3C 279 during 2017.

Proceedings of The Golden Age of Cataclysmic Variables and Related Objects IV — PoS(GOLDEN 2017), 2018

We present optical photometric and spectroscopic observations of three SU UMa-type dwarf novae, i... more We present optical photometric and spectroscopic observations of three SU UMa-type dwarf novae, i.e. AR Pic, QW Ser and V521 Peg, conducted in 2016 and 2017. These sources were selected from the Catalina Real-Time Transient Survey and observed during quiescence, outburst (AR Pic and QW Ser) and superoutburst (V521 Peg). For AR Pic, strong flickering in the light curves and an asymmetric double-peaked Hβ emission line in the spectra, confirmed the presence of a very active hot spot. During outburst, detected on 18 February 2017, it exhibited a ∼ 3.3 magnitude brightening. The projected velocity of the inner edge of the accretion disc is ∼ 2000 km s −1. An outburst of QW Ser was detected on 8 August 2016. Absorption lines, characteristic of an optically thick disc during outburst, were detected and an outflow velocity of ∼ 1000 km s −1 was determined. Optical spectra during the superoutburst of V521 Peg were obtained for the first time, confirming the superoutburst nature of the outburst, as also observed in other SU UMa-type dwarf novae. The superoutburst was detected on 1 September 2017. Welldefined superhumps were observed, with an average superhump period of P sh ∼ 1.48 hr. A mass ratio of q ∼ 0.14 was determined from the period excess of ε ∼ 3%.

Proceedings of 5th Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA2017), 2018

The flat spectrum radio quasar 4C +01.02 became one of the brightest active galactic nuclei detec... more The flat spectrum radio quasar 4C +01.02 became one of the brightest active galactic nuclei detected at high redshift (z = 2.1) in gamma rays when it underwent a series of outbursts during several months in 2016. We monitored this source in gamma rays using the Large Area Telescope onboard of the Fermi spacecraft (Fermi-LAT), and in optical using the Las Cumbres Observatory (LCO). The highest peak flux detected was F(E > 100 MeV) = (2.8 ± 0.3) 10 −6 ph cm −2 s −1 on 10 July 2016 (MJD 57579, daily average). We also obtained optical spectropolarimetry with the Robert Stobie Spectrograph on the Southern African Large Telescope (SALT-RSS) and observed a degree of linear polarisation of up to 10% during flaring states, and ∼1% during a quiescent period. We report recent updates we obtained in our time-domain and spectral studies of this

Proceedings of Frontier Research in Astrophysics – II — PoS(FRAPWS2016), 2017

Blazars are known to be the most energetic sources (apart from gamma-ray bursts) we can observe i... more Blazars are known to be the most energetic sources (apart from gamma-ray bursts) we can observe in the Universe. They constitute the brightest type of radio-loud active galactic nuclei whose Doppler boosted relativistic jets are close to our line of sight. Blazars release a tremendous amount of non-thermal radiation throughout the whole electromagnetic spectrum, and often exhibit outbursts. These outbursts generally last between a few days and several months. The fast and frequent variability characteristics of many of these sources make possible detailed multiwavelength time-domain analysis. Also, we study the evolution of their spectral features in order to constrain physical mechanisms that drive the radiation production within the jets. After introducing the most common methods used to discriminate between leptonic and hadronic scenarios, we report studies on the flat spectrum radio quasar 3C 454.3 (one of the brightest blazars in gamma rays) to show how time-domain and spectral analysis can provide constraints on the size and the location of the gamma-ray emitting region. We also present our work with optical data obtained from South Africa based telescopes to carry out multiwavelength studies of blazars.

Proceedings of 4th Annual Conference on High Energy Astrophysics in Southern Africa — PoS(HEASA 2016), 2017

The Fermi-LAT telescope has provided an unprecedented view of the GeV gamma-ray sky since its lau... more The Fermi-LAT telescope has provided an unprecedented view of the GeV gamma-ray sky since its launch in 2008. The latest Fermi-LAT catalogue of Active Galactic Nuclei lists 1591 sources associated with AGN, of which 460 are classified as blazar candidates of uncertain type (BCU). The characterisation of the physical properties of these BCU sources is important for observational cosmology and fundamental physics, as these sources and their environments constitute a natural laboratory to study particle acceleration and matter/radiation interactions in extreme conditions. Of particular interest is the search for new and interesting/unusual sources that may be observable at very high energies by ground-based imaging atmospheric Cherenkov telescopes. Based on the observed gamma-ray properties, a number of machine learning techniques are being investigated to classify these sources. However, the classification of a blazar as a FSRQ or BL Lac depends on the optical spectral properties. Here we discuss the work that we have thus far undertaken to optically characterise a selection of sources as well as future plans to undertake classification to help calibrate an artificial neural network method.

Galaxies, 2017

2418 † based on observations made with the Southern African Large Telescope (SALT) under programm... more 2418 † based on observations made with the Southern African Large Telescope (SALT) under programme 2016-2-LSP-001 (PI: D. A. H. Buckley). ‡ for the Fermi-LAT collaboration.

The Astrophysical Journal Supplement Series

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area T... more We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of γ-ray sources. Based on the first 12 years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and b...

The Astrophysical Journal, 2016

A prominent outburst of the flat spectrum radio quasar 3C 454.3 was observed in 2014 June with th... more A prominent outburst of the flat spectrum radio quasar 3C 454.3 was observed in 2014 June with the Fermi Large Area Telescope. This outburst was characterized by a three-stage light-curve patternplateau, flare and post-flare-that occurred from 2014 May to July, in a similar pattern as observed during the exceptional outburst in 2010 November. The highest flux of the outburst reported in this paper occurred during 2014 June 7-29, showing a multiple-peak structure in the light-curves. The average flux in these 22 days was found to be F [E > 100 MeV] = (7.2 ± 0.2) × 10 −6 ph cm −2 s −1 , with a spectral index, for a simple power law, of Γ = 2.04 ± 0.01. That made this outburst the first γ-ray high state of 3C 454.3 ever to be detected by Fermi with such a hard spectrum over several days. The highest flux was recorded on 2014 June 15, in a 3 hr bin, at MJD 56823.5625, at a level of F [E > 100 MeV] = (17.6 ± 1.9) × 10 −6 ph cm −2 s −1. The rise time of one of the short subflares was found to be T r = 1200 ± 700 s at MJD = 56827, when the flux increased from 4 to 12 ×10 −6 ph cm −2 s −1. Several photons above 20 GeV were collected during this outburst, including one at 45 GeV on MJD 56827, constraining the γ-ray emission region to be located close to the outer boundary of the broad-line region, leading to fast flux variability.

We present the results of an extensive γ-ray data analysis of the emission from the blazar S5 071... more We present the results of an extensive γ-ray data analysis of the emission from the blazar S5 0716+714 with the primary motivation to study its temporal and spectral variability behavior. In this work, we extract a 10 days binned γ-ray light curve from 2008 August 4 to 2016 April 27 in the energy range of 0.1–300 GeV and identify six outburst periods with peak flux of >4 × 10−7 ph cm−2 s−1 from this highly variable source. The brightest flares are identified by zooming in these outburst periods to 1 day binning and using the Bayesian Blocks algorithm. The fastest variability timescale is found to be 1.5 ± 0.3 hr at MJD 57128.01 ± 0.01 with a peak flux above 100 MeV of (26.8 ± 6.9) × 10−7 ph cm−2 s−1. No hint of periodic modulations has been detected for the light curve of S5 0716+714. During the outburst phases, the γ-ray spectrum shows an obvious spectral break with a break energy between 0.93 and 6.90 GeV energies, which may be caused by an intrinsic break in the energy distri...

De l'emission gamma diffuse galactique est un sujet d'interet majeur en astronomie gamma ... more De l'emission gamma diffuse galactique est un sujet d'interet majeur en astronomie gamma au-dela du GeV. Elle permet l'etude des mecanismes d'acceleration et de propagation des rayonnements cosmiques charges de notre Galaxie. Face au peu de mesures de cette emission au-dessus de 10 GeV, nous avons entrepris de rechercher la composante gamma diffuse dans les donnees hors-source de CELESTE, premier telescope a effet Tcherenkov atmospherique a avoir un seuil en energie en dessous de 100 GeV. Situe dans les Pyrenees-Orientales, a 1650 m d'altitude, CELESTE a utilise, de 1999 a 2004, entre 40 et 53 heliostats de 54 m2 de l'ancienne centrale electro-solaire Themis, ainsi que la tour de 100 m au sommet de laquelle etaient installes une optique secondaire, des photomultiplicateurs et une electronique d'acquisition permettant l'echantillonnage du signal Tcherenkov au GigaHertz. Du fait d'un fond hadronique isotrope dominant a cause des gerbes initiees par ...

Flat spectrum radio quasars (FSRQs) are bright active galactic nuclei surrounded by gas clouds wi... more Flat spectrum radio quasars (FSRQs) are bright active galactic nuclei surrounded by gas clouds within a UV-visible intense radiation field that form the so-called broad line region (BLR). These objects emit relativistic jets from a region close to the central supermassive black hole and through the BLR. The Fermi-Large Area Telescope (Fermi-LAT) is sensitive to gamma-ray photons from sim\simsim30 MeV to more than 300 GeV. We have performed spectral analysis of bright FSRQs in a 5.5 year (2008-2014) data sample collected by Fermi-LAT, using the new Pass 8 event selection and instrument response function. Also, our study of flaring episodes in a limited time range brings interesting results while compared to the full 5.5 year data samples.

Flat Spectrum Radio Quasar 3C 454.3 is the brightest active galactic nucleus to have been detecte... more Flat Spectrum Radio Quasar 3C 454.3 is the brightest active galactic nucleus to have been detected in the γ-ray spectral band. It was observed in an outburst phase during 2014 May-July, and a large amount of data was collected with the Fermi-Large Area Telescope (Fermi-LAT) from 100 MeV to several tens of GeV. In its nominal sky-survey operating mode, Fermi-LAT can see the whole sky every 3 hours, which allowed us to draw detailed light curves during its flaring period, and perform spectral analysis during some specific sub-flares. During these periods, we observed the following features: a fast variability; a hard spectral index close to Γ = 2.0, by fitting spectral energy distributions with a single power law model > 100 MeV; the highest flux at a level of F[E>100 MeV] = 21.6±2.6×10−6 photons cm−2 s−1; several photons at high energies beyond 20 GeV. We will present upper limits on the estimation of the Doppler factor δ and discuss constraints on the location of the γ-ray emi...

The Fermi Gamma-Ray Space telescope has identified 1741 active galactic nuclei during its first f... more The Fermi Gamma-Ray Space telescope has identified 1741 active galactic nuclei during its first four years of observation (2008–2012) and detected 1145 blazars and 573 blazar candidates of uncertain type (BCUs) as listed in the Third Fermi-LAT Point Source Catalog (3FGL). Since Fermi typically operates in survey mode, sources from the whole sky are monitored almost continuously. Daily or sub-daily Fermi-LAT binned light-curves of bright blazars above 100 MeV can be produced for any given time range since August 2008. It is thus possible to identify flaring periods of blazars and trigger observations with South Africa-based optical telescopes to perform γ-ray versus optical correlation studies of flux variability. Also, the recently commissioned polarisation capability of the Robert Stobie Spectrograph (RSS) on the 10-meter class Southern African Large Telescope (SALT) at the South African Astronomical Observatory (SAAO), Sutherland, is expected to contribute to the characterisation ...

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015), Aug 18, 2016

Detection of 54 very high-energy (VHE) neutrinos by the IceCube Neutrino Observatory has opened a... more Detection of 54 very high-energy (VHE) neutrinos by the IceCube Neutrino Observatory has opened a new chapter in multi-messenger astronomy. However due to large errors in measuring the directions of the neutrino shower-type events, which dominate the current event list, it is difficult to identify their astrophysical sources. We perform cross-correlation study of IceCube neutrino events with extragalactic candidate sources using X-ray and gamma-ray selected source catalogues such as Swift-BAT, 3LAC and TeV-Cat. We apply different cuts on the X-ray and gamma-ray fluxes of the sources in these catalogs, and use different source classes in order to study correlation. We use invariant statistic and Monte Carlo simulations to evaluate statistical significance of any correlation.

Proceedings of High Energy Astrophysics in Southern Africa 2021 — PoS(HEASA2021)