X-Ray Polarization Observations of BL Lacertae

Abstract

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus that the synchrotron emission is responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae (BL Lac) performed with the Imaging X-ray Polarimetry Explorer, from which an upper limit to the polarization degree Π X < 12.6% was found in the 2–8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton-synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.

X-Ray Polarization of BL Lacertae in Outburst

“…Our past measurements of Π X < Π O (Middei et al 2023) and the persistent low polarization at the upper end of the IXPE band therefore argues against a leptohadronic emission model. Furthermore, the detection of total polarization at low energies, with Π 2−4 keV ?…”

“…Two previous IXPE observations of BL Lacertae (Middei et al 2023), which occurred during a low state, where the X-ray spectrum was apparently dominated by SSC, produced upper limits on Π X that were significantly below the simultaneous optical polarization. However, in the data from the third epoch presented here, even the total (time and energy) averaged IXPE polarization signal is comparable to the optical values.…”

“…Optical polarization coverage was provided by the Calar Alto (Calar Alto Faint Object Spectrograph, CAFOS; R band), Higashi-Hiroshima Observatory (Kanata telescope) with the Hiroshima Optical and Near-InfraRed camera (HONIR; R and J band), NOT with the Alhambra Faint Object Spectrograph and Camera (ALFOSC; in BVRI), the 1.8 m Perkins Telescope (BVRI), and the Sierra Nevada Observatory (SNO; R band). A detailed description of the observing strategy and data reduction of the aforementioned telescopes can be found in Kawabata et al (1999), Akitaya et al (2014), Hovatta et al (2016, Marscher & Jorstad (2021), Liodakis et al (2022), andMiddei et al (2023), respectively, and references therein. The observations cover the entire duration of the IXPE observation, revealing high variability of the polarization degree, from ∼3% to ∼17%, with EVPA fluctuating about the direction of the jet axis, 10°± 2° (Weaver et al 2022).…”

IXPE and Multiwavelength Observations of Blazar PG 1553+113 Reveal an Orphan Optical Polarization Swing

“…The second hump is located in the GeV-TeV range, with the mechanism behind the emission still debated. Both hadronic and leptonic frameworks have been proposed (e.g., Maraschi et al 1992;Boettcher 2012;Cerruti et al 2015), although recent X-ray spectropolarimetric observations seem to favor a leptonic scenario in which synchrotron self-Compton emission plays a key role in shaping the SED of some blazars (see, e.g., Middei et al 2023;Peirson et al 2023). Depending on the frequency at which the synchrotron component peaks (ν peak ), blazars are classified as high synchrotron peak (HSP), intermediate synchrotron peak (ISP), or low synchrotron peak 66 Deceased.…”

“…Owing to the three gas pixel detectors (GPDs; Costa et al 2001;Bellazzini et al 2003;Baldini et al 2021) in its focal plane, IXPE provides an unprecedented opportunity to measure the X-ray polarization of cosmic sources. Since its launch on 2021 December 9, IXPE has observed different types of AGNs (e.g., Ehlert et al 2022;Marinucci et al 2022;Gianolli et al 2023;Ingram et al 2023;Tagliacozzo et al 2023;Ursini et al 2023), including blazars (e.g., Liodakis et al 2022;Middei et al 2023). PG 1553+113 is one of several HSP blazars observed by IXPE.…”