I use multi-wavelength observations to study blazars, with a slight focus on X-ray and gamma-ray data as I am specifically interested in high-energy processes. I am a member of the TELAMON collaboration and an external collaborator to the German consortium of the eROSITA collaboration. Within TELAMON we monitor TeV-detected blazars and candidate neutrino-associated AGN at radio frequencies from 6 to 40 GHz. In case of high radio activity, we report our findings to the astronomical community via Astronomer's Telegram or GCN within a few days. The eROSITA X-ray telescope is an all-sky survey instrument, which is sensitive between 0.2 and 5 keV.

In the past, I was a member of the Fermi-LAT collaboration and as part of that I frequently took part in data-analysis shifts to look at real-time data from gamma-ray sources.

Below, you can find my main projects, both current and past, on blazars and active galactic nuclei. A full list of my publications is given in my ORCID.


The FACT telescope (Credit: José Luis Lemus)

TeV and X-ray flare observation of Mrk 421

Blazars - Jets - Multi-wavelength flares

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. You can read the full story of the flare in 2019 and a few results regarding multi-wavelength variability 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, in press

SED of PKS 2004–447 during its gamma-ray flare. Credit: Gokus et al., A&A, 649, 77, 2021, reproduced with permission, ©ESO

The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004–447

Narrow-line Seyfert 1 galaxies - Gamma-ray flares - Multi-wavelength observations

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 (you can also find unrestricted access to the paper on arXiv)

Past projects

Systematic study of simultaneous observations by XMM-Newton and NuSTAR

Internship at ESAC - X-ray satellites - Active galaxies

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.

Correlations between gamma-ray and radio light curves of PKS 0537–441

Bachelor's thesis - Blazars - Radio observations - GeV observations

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.