Although gamma ray bursts (GRBs) have been detected for many decades, the lack of knowledge regarding the radiation mechanism that produces the energetic flash of radiation, or prompt emission, from these events has prevented the full use of GRBs as probes of high-energy astrophysical processes. While there are multiple models that attempt to describe the prompt emission, each model can be tuned to account for observed GRB characteristics in the gamma and X-ray energy bands. One energy range that has not been fully explored for the purpose of prompt emission model comparison is that of the optical band, especially with regard to polarization. Here, we use an improved Monte Carlo radiation transfer code to calculate the expected photospheric optical and gamma-ray polarization signatures (Πoptand Π
Probing shock acceleration in BL Lac jets through X-ray polarimetry: the time-dependent view
ABSTRACT Polarimetric measurements, especially if extended at high energy, are expected to provide important insights into the mechanisms underlying the acceleration of relativistic particles in jets. In a previous work, we have shown that the polarization of the synchrotron X-ray emission produced by highly energetic electrons accelerated by a mildly relativistic shock carries essential imprints of the geometry and the structure of the magnetic fields in the downstream region. Here, we present the extension of our analysis to the non-stationary case, especially suitable to model the highly variable emission of high-energy emitting BL Lacs. We anticipate a large ($\Pi \approx 40{{\ \rm per\ cent}}$), almost time-independent degree of polarization in the hard/medium X-ray band, a prediction soon testable with the upcoming mission IXPE. The situation in other bands, in particular in the optical, is more complex. A monotonic decrease of the optical degree of polarization is observed during the development of a flare. At later stages, Π reaches zero and then it starts to increase, recovering large values at late times. The instant at which Π = 0 is marked by a rotation of the polarization angle by 90°. However, at optical frequencies, it is likely that more than one region contribute to the observed emission, potentially making it difficult to detect the predicted behaviour.
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- Award ID(s):
- 1716567
- NSF-PAR ID:
- 10291262
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 498
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 599 to 608
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract γ , respectively) from a set of two relativistic hydrodynamic long GRB simulations, which emulate a constant and variable jet. We find that time-resolved Πoptcan be large (∼75%) while time-integrated Πoptcan be smaller due to integration over the asymmetries in the GRB jet where optical photons originate; Πγ follows a similar evolution as Πoptwith smaller polarization degrees. We also show that Πoptand Πγ agree well with observations in each energy range. Additionally, we make predictions for the expected polarization of GRBs based on their location within the Yonetoku relationship. While improvements can be made to our analyses and predictions, they exhibit the insight that global radiative transfer simulations of GRB jets can provide with respect to current and future observations. -
Abstract Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength polarization properties of blazars. The recent launch of the Imaging X-Ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2–8 keV) to probe the properties of the jet synchrotron emission in high-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Π x = 15% ± 2% and electric vector position angle ψ x = 35° ± 4°) from the jet of the HSP Mrk 421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of Π x compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.more » « less
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ABSTRACT We present an X-ray spectropolarimetric analysis of the bright Seyfert galaxy NGC 4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM–Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree Π = 4.9 ± 1.1 per cent and angle Ψ = 86° ± 7° east of north (68 per cent confidence level) are measured in the 2–8 keV energy range. The spectropolarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires, however, a reflection with a very large (>38 per cent) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ∼4 to ∼8 per cent is found. The observed polarization degree excludes a ‘spherical’ lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC 4151 is the first active galactic nucleus with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry.more » « less
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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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/staa2457
