The Vanishing of the Primary Emission Region in PKS 1510–089

Abstract

In 2021 July, PKS 1510−089 exhibited a significant flux drop in the high-energy γ-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy γ-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.

Optical Spectropolarimetric Variability Properties in Blazars PKS 0637–75 and PKS 1510–089

“…After this observation in 2021, our SALT data display significantly lower levels of polarization (less than 4%). In Aharonian et al (2023), the polarization level reached a maximum of 12.5% and minimum of 2.2% in 2021, similar to the values we obtained from our SALT observations at similar times (within ±1-2 days).…”

“…Using the SED + spectropolarimetry model of Schutte et al (2022), Aharonian et al (2023) were able to show that contributions from the synchrotron jet, AD, and BLR can explain the observed emission and polarization levels in 2021, whereas for 2022, the drop in polarization level to being consistently below 2% is consistent with no polarization in the blazar, such that the AD and BLR flux are sufficient to fully explain the optical spectrum. The decrease in polarization levels after mid-2021 July we observe gives support to the above suggestion that the optical-UV spectrum became dominated by the thermal AD and BLR, while low levels of polarization (4%) between 2021 mid-July and August may be consistent with only interstellar polarization and no intrinsic polarization in the source, in agreement with what was found by Aharonian et al (2023) in their later observations; see the bottom panels (c) and (d) in Figure 5 where we see in more detail the decline in polarization levels of PKS 1510-089.…”

“…From the beginning of 2021 April to mid-June, there exists an overlap of SALT spectropolarimetry observations of PKS 1510-089 between this observing campaign and that of Aharonian et al (2023), which used H.E.S.S., ATOM, and SALT observations to better understand the primary emission region of PKS 1510-089 (see panels (c) and (d) of Figure 5). In this study, we observed a drop in the optical continuum polarization and an increase in the EW of broad emission lines Hγ and Hβ during the quiescent stages of PKS 1510-089, most notably after the optical flaring event around 2021 July 14.…”

“…During this time, the Hβ and Hγ line polarization is nominally 0% within their errors. analysis of PKS 1510-089 has been done by Aharonian et al (2023), so we refer to that work for a detailed study of its SED.…”

“…FSRQs are extremely well studied in time series observations across a range of wavelengths, but a relative paucity of spectropolarimetric observations of blazars exist (e.g., Böttcher et al 2017;Patiño-Álvarez et al 2018;Schutte et al 2022;Aharonian et al 2023), with such studies becoming prominent only recently. Since we aim to increase the characterization of blazar spectropolarimetric properties, we focus on FSRQs as they generally show prominent, quasar-like emission lines in contrast to BL Lacs.…”