arXiv subjects

Haocheng Zhang

Publications and source records attributed to Haocheng Zhang.

59 records · Page 4Linked to original sources

arXiv2015

Multi-wavelength study of flaring activity in BL Lac object S5 0716+714 during 2015 outburst

We present a detailed investigation of the flaring activity observed from a BL Lac object, S5 0716+714 , during its brightest ever optical state in the second half of January 2015. Observed almost simultaneously in the optical, X-rays and γ-rays, a significant change in the degree of optical polarization (PD) and a swing in the position angle (PA) of polarization were recorded. A detection in the TeV (VHE) was also reported by the MAGIC consortium during this flaring episode. Two prominent sub-flares, peaking about 5-days apart, were seen in almost all the energy bands. The multi-wavelength light-curves, spectral energy distribution (SED) and polarization are modeled using the time-dependent code developed by Zhang et al. (2014). This model assumes a straight jet threaded by large scale helical magnetic fields taking into account the light travel time effects, incorporating synchrotron flux and polarization in 3D geometry. The rapid variation in PD and rotation in PA are most likely due to re-connections happening in the emission region in the jet, as suggested by the change in the ratio of toroidal to poloidal components of magnetic field during quiescent and flaring states.

astro-ph.HE↗

arXiv2015

Polarization Swings Reveal Magnetic Energy Dissipation in Blazars

The polarization signatures of the blazar emissions are known to be highly variable. In addition to small fluctuations of the polarization angle around a mean value, sometimes large (> 180^o) polarization angle swings are observed. We suggest that such p henomena 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 of 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.

astro-ph.HE↗

arXiv2014

Magnetic Field Amplification and Flat Spectrum Radio Quasars

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/γ-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 emission appears to describe the available data better than a synchrotron self-Compton scenario, and in particular orphan optical flares are difficult to produce in the SSC framework. X-ray soft-excesses, γ-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 γ-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 >= 180 degree of the observed polarization angle, if light-travel-time effects are properly considered.

astro-ph.HE↗

arXiv2014

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.

astro-ph.HE↗

arXiv2013

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.

astro-ph.HE↗