Andrea Gokus

The gamma-ray satellite Fermi-LAT has enabled scientists to study the variability of high-energy objects with continuous data sampled since 2008. Among the objects observed by Fermi-LAT, blazars show the most variable behaviour as their gamma-ray emission originates from their highly relativistic jets, whose emission gets boosted towards us. I am interested in measuring the dynamics of the particle acceleration processes in the jet and radiation zone near the central black hole. Together with Prof. Rebecca Phillipson at Villanova University, I want explore the highly energetic variability of 59 blazars with recurrence analysis, in particular with a tool called 'recurrence plots'.
Since this a new project with a start day in September 2024 (funded by NASA), we have not yet published our findings in a publication, but you can find an overview with some preliminary results in a poster created to introduce the project at gamma-ray focused conferences in 2024.

Detecting blazars at high redshifts (z > 3) at gamma-ray energies is challenging, and only slightly more than a dozen have been reported so far based on data obtained with Fermi-LAT. My collaborators and I are running an automated pipeline to monitor more than 80 high-z blazars with public, real-time data from Fermi-LAT. In February 2022, we detected increasing gamma-ray emission from TXS 1508+572, a blazar at z = 4.31, which indicated that the blazar is undergoing a flare. We acquired multiwavelength data from multiple facilities in order to study the broadband SED of the source. The results are presented in a publication with 'The Astrophysical Journal' (Paper I). In addition, we launched a VLBI campaign that took images of TXS 1508+572 over the course of 10 months at frequencies between 15 GHz and 86 GHz using the VLBA, Effelsberg, and the Greenbank Telescope. Our publication in 'Astronomy & Astrophysics' (Paper II) shows, among other things, a kinematic analysis of the observed changes in the jet.
Paper I: Gokus et al., 2024, ApJ, in press
Paper II: Benke et al., 2024, A&A, 689, A43
Poster: See a compressed summary of the project on a poster

Mrk 421 is one of the closest blazars and shows a compex variability behaviour. Although this source has been part of several multi-wavelength campaigns, deep pointed observations during flares are rare. I am working on multi-wavelength data covering such a flare by Mrk 421 that happened in 2019, which we were able to obtain thanks to a monitoring campaign led by the TeV Cherenkov telescope FACT. These simultaneous data include, e.g., X-ray data obtained with XMM-Newton, INTEGRAL, and Swift. The full story of the flare in 2019 and a few results regarding multi-wavelength variability are given in my proceedings published for the 37th International Cosmic Ray Conference (ICRC). A refereed paper published with MNRAS focuses on the timing analysis of XMM-Newton data taken during the exceptionally bright X-ray flare that coincided with the TeV flare in 2019.
Proceedings: Gokus et al., 2021, Proceedings of Science, 869
Paper: Gokus et al., 2024, MNRAS, 529, 1450

PKS 2004–447 is an interesting source, because it is classified as a narrow-line Seyfert 1 (NLSy1) galaxy, but other than the typical NLSy1 AGN, PKS 2004–447 shows gamma radiation. In October 2019, this source even showed a gamma-ray flare (see ATel 13229), which was detected by Fermi-LAT. Because of this discovery we started a multi-wavelength follow-up campaign with Swift, XMM-Newton, NuSTAR, and ATCA. We analysed the variability as well as the broad band spectral energy distributions before, during and after the flare.
Paper: Gokus et al., 2021, A&A, 649, 77 (open access on arXiv)

Using different instruments for multi-wavelength observation requires good knowledge of the cross-calibration between these. The two X-ray satellites XMM-Newton and NuSTAR are ideal for simultaneous use as XMM-Newton covers the soft X-ray regime (0.5 - 10 keV), while NuSTAR is sensitive up to 80 keV and at the same time reaches down to 3 keV, therefore providing an overlap with the soft X-ray regime. This enables a systematic study on the cross-calibration between those two instruments. In this 5-month internship I studied simultaneous observations of active galaxies with both Focal Plane Modules onboard NuSTAR and the EPIC-pn detector onboard XMM-Newton. Concluding from the analysis of the observations available at that time, the difference for the flux normalisation reached up to 10%, while the photon index seems to be about 0.1 larger in observations taken by NuSTAR (see left image), therefore indicating softer X-ray spectra.
The report on this work is included in my Master's thesis, which also explores the topic of numerical calculation of synchrotron emission.

In my Bachelor's thesis I studied the correlation between gamma-ray and radio radiation of the blazar PKS 0537–441. The availability of continous gamma-ray data has dramatically increased with the launch of the Fermi satellite, which monitors the whole sky each day since 2008. Radio monitoring campaigns for several AGN, like the one studied in my Bachelor's thesis, have been going even longer, however the data sampling is not that dense, making a cross-correlation analysis difficult. In this work, I used the Discrete Correlation Function (DCF) by Edelson & Krolik (1988) as well as the Interpolated Correlation Function (ICF) by White & Peterson (1994) on gamma-ray data from Fermi-LAT and radio data obtained with the Ceduna telescope and the VLBI networks like USNO and TANAMI (see left image for a VLBI image of PKS 0537–441). The found time lag, in which the gamma-ray emission leads the radio emission, is in agreement with previous results, however, the quite large time lag of nearly 100 days has not been seen so far and could be a hint for the necessity of denser radio monitoring.