TY - JOUR A1 - Zhang, Haocheng A1 - Chen, Xuhui A1 - Böttcher, Markus A1 - Guo, Fan A1 - Li, Hui T1 - Polarization swings reveal magnetic energy dissipation in blazars JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The polarization signatures of blazar emissions are known to be highly variable. In addition to small fluctuations of the polarization angle around a mean value, large (greater than or similar to 180 degrees) polarization angle swings are sometimes observed. We suggest that such phenomena can be interpreted as arising from light travel time effects within an underlying axisymmetric emission region. We present the first simultaneous fitting of the multi-wavelength spectrum, variability, and time-dependent polarization features of a correlated optical and gamma-ray flaring event of the prominent blazar 3C279, which was accompanied by a drastic change in its polarization signatures. This unprecedented combination of spectral, variability, and polarization information in a coherent physical model allows us to place stringent constraints on the particle acceleration and magnetic field topology in the relativistic jet of a blazar, strongly favoring a scenario in which magnetic energy dissipation is the primary driver of the flare event. KW - galaxies: active KW - galaxies: jets KW - gamma-rays: galaxies KW - radiation mechanisms: nonthermal KW - relativistic processes Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/804/1/58 SN - 0004-637X SN - 1538-4357 VL - 804 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhang, Haocheng A1 - Chen, Xuhui A1 - Boettcher, Markus T1 - Synchrotron polarization in blazars JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - 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 three-dimensional 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 handled with the two-dimensional Monte-Carlo/Fokker-Planck (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 + synchrotron-self-Compton flaring, associated with substantial variability in the synchrotron polarization percentage and position angle. Most importantly, this scenario naturally explains large polarization angle rotations by greater than or similar to 180 degrees, as observed in connection with gamma-ray flares in several blazars, without the need for bent or helical jet trajectories or other nonaxisymmetric jet features. KW - galaxies: active KW - galaxies: jets KW - gamma rays: galaxies KW - radiation mechanisms: non-thermal KW - relativistic processes Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/789/1/66 SN - 0004-637X SN - 1538-4357 VL - 789 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Chen, Xuhui A1 - Chatterjee, Ritaban A1 - Zhang, Haocheng A1 - Pohl, Martin A1 - Fossati, Giovanni A1 - Boettcher, Markus A1 - Bailyn, Charles D. A1 - Bonning, Erin W. A1 - Buxton, Michelle A1 - Coppi, Paolo A1 - Isler, Jedidah A1 - Maraschi, Laura A1 - Urry, Meg T1 - Magnetic field amplification and flat spectrum radio quasars JF - Monthly notices of the Royal Astronomical Society N2 - We perform time-dependent, spatially resolved simulations of blazar emission to evaluate several flaring scenarios related to magnetic-field amplification and enhanced particle acceleration. The code explicitly accounts for light-travel-time effects and is applied to flares observed in the flat spectrum radio quasar (FSRQ) PKS 0208-512, which show optical/gamma-ray correlation at some times, but orphan optical flares at other times. Changes in both the magnetic field and the particle acceleration efficiency are explored as causes of flares. Generally, external Compton (EC) emission appears to describe the available data better than a synchrotron self-Compton (SSC) scenario, and in particular orphan optical flares are difficult to produce in the SSC framework. X-ray soft-excesses, gamma-ray spectral hardening, and the detections at very high energies of certain FSRQs during flares find natural explanations in the EC scenario with particle acceleration change. Likewise, optical flares with/without gamma-ray counterparts can be explained by different allocations of energy between the magnetization and particle acceleration, which may be related to the orientation of the magnetic field relative to the jet flow. We also calculate the degree of linear polarization and polarization angle as a function of time for a jet with helical magnetic field. Tightening of the magnetic helix immediately downstream of the jet perturbations, where flares occur, can be sufficient to explain the increases in the degree of polarization and a rotation by a parts per thousand yen180A degrees of the observed polarization angle, if light-travel-time effects are properly considered. KW - radiation mechanisms: non-thermal KW - galaxies: active KW - galaxies: jets KW - quasars: individual: PKS 0208-512 Y1 - 2014 U6 - https://doi.org/10.1093/mnras/stu713 SN - 0035-8711 SN - 1365-2966 VL - 441 IS - 3 SP - 2188 EP - 2199 PB - Oxford Univ. Press CY - Oxford ER -