A Multiwavelength Study of Flaring Activity in the High-energy Peaked BL Lac Object 1ES 1959+650 During 2015–2016

Abstract

We present the results from a multiwavelength study of the flaring activity in the high-energy peaked BL Lac object 1ES 1959+650 during 2015 January–2016 June. The source underwent two major outbursts, during 2015 March and 2015 October, across the whole electromagnetic spectrum (EMS). We used data from Fermi-LAT and Swift-XRT/UVOT, and optical data from Mt. Abu InfraRed Observatory and Steward Observatory to look for possible correlations between the emissions at different energies and the nature of the variability during the flaring state. During the 2015 October outburst, the nightly averaged V-band brightest magnitude, 14.45(0.03), and faintest magnitude, 14.45(0.03), were recorded. Apart from long-term flares, rapid and short-term variabilities were noticed at all energies. Our study suggests that the flaring activities at all frequencies, with diverse flare durations and time lags, are correlated. The magnetic field strength is estimated to be 4 G using the synchrotron-cooling timescale (2.3 hr), and the upper limits on the sizes of both emission regions, γ-ray and optical, are estimated to be of the order of 1016 cm. The quasi-simultaneous flux enhancements in 15 GHz and VHE γ-ray emissions indicate a fresh injection of plasma into the jet, which interacts with a standing submillimeter core, resulting in co-spatial emissions across the EMS. The synchrotron peak shifts to higher frequency in the spectral energy distribution while the γ-ray spectra softens during the flaring. The shape of the inverse-Compton spectra indicates a change in the particle energy distribution pre- and post-flare.

The γ-ray sky seen at X-ray energies

“…The emission is also detected in the γ-ray band (see, e.g., Aharonian et al 2005;Albert et al 2007;Giannios et al 2009;Tavecchio et al 2011;Wehrle et al 1998;Ackermann et al 2015) and shows strong variability. It also has flaring states in which spectral shape and/or luminosities change (see, e.g., Hartman et al 2001;Romero et al 2002;Böttcher et al 2007;Pandey et al 2017;Kaur et al 2017;Bruni et al 2018).…”

X-ray Observations of 1ES 1959+650 in its high activity state in 2016-2017 with AstroSat and Swift

“…These spectral changes result in a shift of the peak position of the X-ray part of the SED and hence in the position of the synchrotron peak. Such a shift of the synchrotron peak towards higher energies with increasing integrated flux is seen in most HBLs and is equivalent to the "bluer-when-brighter" trend seen in the optical continuum spectra of many BL Lac objects (Böttcher et al 2010;Dai et al 2011;Bhatta et al 2018;Kaur et al 2017). The best fit α and β values are used in equation A5 to derive the position of the peak of the synchrotron component.…”

“…The data were obtained using the EMCCD based optical camera installed at the f/13 cassegrain focus of the 1.2 m telescope. The data reduction and the photometry procedures adopted are discussed in Kaur et al (2017). Differential photometry, using several comparison stars in the same frame as the source, was used to minimize atmospheric seeing effects.…”

“…The source underwent a number of active states and an unprecedented X-ray flaring activity during August 2015 -January 2016 that was observed by Swift-XRT. The observed count rate was reported to vary by a factor of ∼5.7 with maximum value above 20 cts/s, with simultaneous high flux activity in TeV energy band (Kapanadze et al 2016a;Kaur et al 2017;Patel et al 2018;MAGIC Collaboration et al 2020). During this large flare, the synchrotron peak position of the SED showed a tendency to shift towards the higher X-ray energies accompanied by a hard X-ray spectral index.…”

Estudio observacional de la emisión óptica de blazares detectados a altas energías

“…En la literatura sólo existen registros de una tendencia BW B para esta fuente, lo cual está en acuerdo con los resultados que hemos encontrado. Sin embargo, estos ejemplos se corresponden a estudios que han comprendido campañas observacionales con escalas de tiempo del orden de meses a años (Ikejiri et al 2011;Kaur et al 2017). Durante un estado de muy alta actividad (en óptico, RX, UV y gamma) de 1ES 1959+650, registrado en el período enero 2015 y junio 2016, éste mostró una tendencia bien marcada BW B (Kaur et al 2017).…”