Stellar Systems Group

Paliya, Vaidehi S.; Ajello, M.; Ojha, R.; Angioni, R.; Cheung, C. C.; Tanada, K.; Pursimo, T.; Galindo, P.; Losada, I. R.; Siltala, L.; Djupvik, A. A.; Marcotulli, L.; Hartmann, D.
The Astrophysical Journal, Volume 871, Issue 2, article id. 211, 12 pp. (2019).
02/2019

ABSTRACT

High-redshift (z > 2) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and γ-ray detection, thereby making it difficult to characterize the energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant γ-ray emission from the high-redshift blazar DA 193 (z = 2.363). Its time-averaged γ-ray spectrum is soft (γ-ray photon index = 2.9 ± 0.1), and together with a relatively flat hard X-ray spectrum (14–195 keV photon index = 1.5 ± 0.4), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard γ-ray spectrum (photon index = 1.7 ± 0.2), which is somewhat unexpected because high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multifrequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories are presented, and this peculiar γ-ray flare is studied within the framework of a single-zone leptonic emission scenario.