612 research outputs found
X-Ray and Gamma-Ray Polarization in Leptonic and Hadronic Jet Models of Blazars
We present a theoretical analysis of the expected X-ray and gamma-ray
polarization signatures resulting from synchrotron self-Compton emission in
leptonic models, compared to the polarization signatures from proton
synchrotron and cascade synchrotron emission in hadronic models for blazars.
Source parameters resulting from detailed spectral-energy-distribution modeling
are used to calculate photon-energy-dependent upper limits on the degree of
polarization, assuming a perfectly organized, mono-directional magnetic field.
In low-synchrotron-peaked blazars, hadronic models exhibit substantially higher
maximum degrees of X-ray and gamma-ray polarization than leptonic models, which
may be within reach for existing X-ray and gamma-ray polarimeters. In
high-synchrotron-peaked blazars (with electron-synchrotron-dominated X-ray
emission), leptonic and hadronic models predict the same degree of X-ray
polarization, but substantially higher maximum gamma-ray polarization in
hadronic models than leptonic ones. These predictions are particularly relevant
in view of the new generation of balloon-borne X-ray polarimeters (and possibly
GEMS, if revived), and the ability of Fermi-LAT to measure gamma-ray
polarization at < 200 MeV. We suggest observational strategies combining
optical, X-ray, gamma-ray polarimetry to determine the degree of ordering of
the magnetic field and to distinguish between leptonic and hadronic high-energy
emission.Comment: Accepted for publication in The Astrophysical Journa
Synchrotron Polarization in Blazars
We present a detailed analysis of time- and energy-dependent synchrotron
polarization signatures in a shock-in-jet model for gamma-ray blazars. Our
calculations employ a full 3D radiation transfer code, assuming a helical
magnetic field throughout the jet. The code considers synchrotron emission from
an ordered magnetic field, and takes into account all light-travel-time and
other relevant geometric effects, while the relevant synchrotron self-Compton
and external Compton effects are taken care of with the 2D MCFP code. We
consider several possible mechanisms through which a relativistic shock
propagating through the jet may affect the jet plasma to produce a synchrotron
and high-energy flare. Most plausibly, the shock is expected to lead to a
compression of the magnetic field, increasing the toroidal field component and
thereby changing the direction of the magnetic field in the region affected by
the shock. We find that such a scenario leads to correlated synchrotron + SSC
flaring, associated with substantial variability in the synchrotron
polarization percentage and position angle. Most importantly, this scenario
naturally explains large PA rotations by > 180 deg., as observed in connection
with gamma-ray flares in several blazars, without the need for bent or helical
jet trajectories or other non-axisymmetric jet features.Comment: Submitted to Ap
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