Browsing by Author "Paliya, Vaidehi S."
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Item Detection of a Gamma-Ray Flare from the High-redshift Blazar DA 193(IOP, 2019-02-01) 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.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.Item The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004-447Gokus, Andrea; Paliya, Vaidehi S.; Wagner, Sarah M.; Buson, Sara; D'Ammando, Filippo; Edwards, Philip G.; Kadler, Matthias; Meyer, Manuel; Ojha, Roopesh; Stevens, Jamie; Wilms, JörnOn 2019 October 25, the Fermi-Large Area Telescope observed the first gamma-ray flare from the radio-loud narrow-line Seyfert 1 (NLSy 1) galaxy PKS 2004−447 (z=0.24). We report on follow-up observations in the radio, optical-UV, and X-ray bands that were performed by ATCA, the Neil Gehrels Swift observatory, XMM-Newton, and NuSTAR, respectively, and our multi-wavelength analysis. We study the variability across all energy bands and additionally produce γ-ray light curves with different time binnings to study the variability on short timescales during the flare. We examine the X-ray spectrum from 0.5−50 keV by describing the spectral shape with an absorbed power law. We analyse multi-wavelength datasets before, during, and after the flare and compare these with a low activity state of the source by modelling the respective SEDs with a one-zone synchrotron inverse Compton radiative model. Finally, we compare our results to gamma-ray flares previously observed from other γ-loud NLSy 1 galaxies. At gamma-ray energies (0.1−300 GeV) the flare reached a total maximum flux of (2.7±0.6)×10⁻⁶~ph~cm⁻²~s⁻¹ in 3-hour binning. With a photon index of Γ0.1−300GeV=2.42±0.09 during the flare, this corresponds to an isotropic gamma-ray luminosity of (2.9±0.8)×10⁴⁷ergs⁻¹. The γ-ray, X-ray, and optical-UV light curves covering the end of September to the middle of November show significant variability, and we find indications for flux-doubling times of ∼2.2~hours at γ-ray energies. During the flare, the SED exhibits large Compton dominance. While the increase in the optical-UV range can be explained by enhanced synchrotron emission, the elevated γ-ray flux can be accounted for by an increase in the bulk Lorentz factor of the jet, similarly observed for flaring gamma-ray blazars.Item Supermassive black holes at high redshifts(arXiv, 2019-03-14) Paliya, Vaidehi S.; Ajello, Marco; Marcotulli, Lea; Tomsick, John; Perkins, Jeremy S.; Prandini, Elisa; D'Ammando, Filippo; De Angelis, Alessandro; Thompson, David; Li, Hui; Dominguez, Alberto; Beckmann, Volker; Guiriec, Sylvain; Wadiasingh, Zorawar; Coppi, Paolo; Harding, J. Patrick; Petropoulou, Maria; Hewitt, John W.; Ojha, Roopesh; Marcowith, Alexandre; Doro, Michele; Castro, Daniel; Baring, Matthew; Hays, Elizabeth; Orlando, Elena; Guiriec, Sylvain; Bozhilov, Vladimir; Agudo, Ivan; Venters, Tonia; McEnery, Julie; The, Lih-Sin; Hartmann, Dieter; Buson, Sara; Longo, Francesco; Gasparrini, DarioMeV blazars are the most luminous persistent sources in the Universe and emit most of their energy in the MeV band. These objects display very large jet powers and accretion luminosities and are known to host black holes with a mass often exceeding 10⁹M⊙. An MeV survey, performed by a new generation MeV telescope which will bridge the entire energy and sensitivity gap between the current generation of hard X-ray and gamma-ray instruments, will detect >1000 MeV blazars up to a redshift of z=5−6. Here we show that this would allow us: 1) to probe the formation and growth mechanisms of supermassive black holes at high redshifts, 2) to pinpoint the location of the emission region in powerful blazars, 3) to determine how accretion and black hole spin interplay to power the jet.