Detection of X-Ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-Ray Polarimetry Explorer

Abstract

Observations of linear polarization in the 2–8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating nonthermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray-selected BL Lac–type blazar 1ES 1959+650 on 2022 May 3–4 showed a significant linear polarization degree of Πx = 8.0% ± 2.3% at an electric-vector position angle ψ x = 123° ± 8°. However, on 2022 June 9–12, only an upper limit of Πx ≤ 5.1% could be derived (at the 99% confidence level). The degree of optical polarization at that time, ΠO ∼ 5%, is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray-selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.

Multiwavelength Polarization Observations of Mrk 501

“…The polarization model polconst assumes constant polarization parameters within the operating energy range and has only two free parameters, Π X and ψ X . For the spectropolarimetric fit, we fixed the values of N 0 and Γ X obtained through fitting the I spectra, with only Π X and ψ X being free parameters (see also Errando et al 2024). The results of the spectropolarimetric fits are also presented in Table 1.…”

“…To investigate whether there are energy-dependent variations in X-ray polarization, we separated the entire 2-8 keV energy range of IXPE into two smaller ranges (i.e., 2-5 keV and 5-8 keV) and performed spectropolarimetric fits to the Stokes I, Q, and U spectra, with only Π X and ψ X as free parameters (see Section 2.1 and Errando et al 2024). As a cross-check, we also utilized the software ixpeobssim to estimate the polarization parameters in the two smaller ranges for the six IXPE observations.…”

“…9 The program details can be found in Jorstad & Marscher (2016), Jorstad et al (2017), andWeaver et al (2022). The ratio of P peak to I peak calculated using the values provided on the website can serve as a representative indicator of polarization degree (Π R ) for the radio core (see Errando et al 2024;Kim et al 2024), where I peak and P peak are the peaks of the total and polarized intensity (Jorstad et al 2005). Comparing with the X-ray polarization parameters, Π R is lower than Π X , while the polarization angles remain consistent in both bands, as displayed in Figure 3.…”