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

S. Chandra, Markus Boettcher+7 more|arXiv (Cornell University)|May 17, 2021

Astrophysics and Cosmic Phenomena71 references14 citations

TL;DR

This study presents a detailed analysis of X-ray flares in the high-frequency peaked BL Lac object 1ES 1959+650 during its 2016–2017 high-activity state using high-cadence AstroSat and Swift data. It employs a time-dependent leptonic diffusive-shock-acceleration model to show that spectral hardening and synchrotron peak energy shifts are best explained by a changing Doppler factor due to a small change in viewing angle (∆θ ∼0.5°), with stronger flux-flare hardening observed in 2017 than in 2016.

ABSTRACT

We present a comprehensive multi-frequency study of the HBL 1ES 1959+650 using data from various facilities during the period 2016-2017, including X-ray data from {\it AstroSat} and {\it Swift} during the historically high X-ray flux state of the source observed until February 2021. The unprecedented quality of X-ray data from high cadence monitoring with the {\it AstroSat} during 2016-2017 enables us to establish a detailed description of X-ray flares in 1ES 1959+650. The synchrotron peak shifts significantly between different flux states, in a manner consistent with a geometric (changing Doppler factor) interpretation. A time-dependent leptonic diffusive-shock-acceleration and radiation transfer model is used to reproduce the spectral energy distributions (SEDs) and X-ray light curves, to provide insight into the particle acceleration during the major activity periods observed in 2016 and 2017. The extensive data of {\it Swift}-XRT from December 2015 to February 2021 (Exp. = 411.3 ks) reveals a positive correlation between flux and peak position.

Motivation & Objective

• To understand the physical mechanisms behind the extreme X-ray flares observed in 1ES 1959+650 during 2016–2017.
• To investigate the spectral and flux variability of the source across multiple X-ray states.
• To determine whether the observed 'bluer-when-brighter' trend and synchrotron peak shifts can be explained by a time-dependent particle acceleration model.
• To test the viability of a leptonic diffusive-shock-acceleration model in reproducing the broadband SEDs and light curves during flaring activity.
• To provide a comprehensive, publicly available dataset of X-ray spectral parameters spanning over 6 years of monitoring.

Proposed method

• Utilized high-cadence X-ray data from AstroSat's Soft X-ray Telescope (SXT) and Large Area Proportional Counter (LAXPC) during 2016–2017.
• Complemented with simultaneous and quasi-simultaneous Swift-XRT observations from December 2015 to February 2021 (411.3 ks total exposure).
• Performed time-resolved X-ray spectroscopy using a broken power-law model with curvature to determine photon index (α) and curvature parameter (β).
• Calculated synchrotron peak energy (Es,p) using the derived α and β via the analytical formula Es,p = E0 × 10^{(2−α)/(2β)} from the νFν spectral form.
• Applied a time-dependent leptonic model based on diffusive shock acceleration in mildly relativistic shocks, including self-consistent synchrotron, SSC, and external Compton cooling.
• Fitted the model to multi-epoch SEDs (T1–T6 in 2016, T8–T10 in 2017) and X-ray light curves to reproduce spectral and flux variability.

Experimental results

Research questions

• RQ1 What causes the strong 'bluer-when-brighter' trend observed in the X-ray spectra of 1ES 1959+650 during its 2016–2017 flares?
• RQ2 How do the synchrotron peak energy and spectral hardness evolve during distinct X-ray flares in 2016 and 2017?
• RQ3 Can a time-dependent leptonic model with shock acceleration and radiative cooling reproduce the observed SEDs and light curves?
• RQ4 What physical changes in the emission region (e.g., Doppler factor, magnetic field, viewing angle) best explain the observed spectral and flux variability?
• RQ5 Why is the spectral hardening more pronounced in 2017 compared to 2016, despite similar peak flux levels?

Key findings

• The X-ray spectral index (α) shows a strong negative correlation with flux (FX,0.3–7.0 keV), confirming a 'bluer-when-brighter' trend in both 2016 and 2017.
• The synchrotron peak energy (Es,p) increases with X-ray flux, indicating a significant spectral shift toward higher energies during flares.
• The 'bluer-when-brighter' trend is stronger in 2017 than in 2016, as evidenced by steeper slopes in the α vs. FX correlation (Table 3).
• The time-dependent leptonic model with diffusive shock acceleration successfully reproduces both the SEDs and light curves of the 2016 and 2017 flares.
• The model attributes the spectral and flux variability to a change in the Doppler factor, driven by a small change in viewing angle (∆θ ∼0.5°) over ~1 year, along with a reduction in magnetic field.
• The study provides a publicly available dataset of X-ray spectral parameters spanning 6 years (Jan 2015 – Feb 2021), enabling future multi-wavelength correlation studies.