Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userExamining High Energy Radiation Mechanisms of Knots and Hotspots in Active Galactic Nucleus Jets
147
0
0.0
(
0
)
Authors
Jin Zhang
Ask ChatGPT about the research
No Arabic abstract
We compile the radio-optical-X-ray spectral energy distributions (SEDs) of 65 knots and 29 hotspots in 41 active galactic nucleus jets to examine their high energy radiation mechanisms. Their SEDs can be fitted with the single-zone leptonic models, except for the hotspot of Pictor A and six knots of 3C 273. The X-ray emission of one hotspot and 22 knots is well explained as synchrotron radiations under the equipartition condition; they usually have lower X-ray and radio luminosities than the others, which may be due to a lower beaming factor. An inverse Compton (IC) process is involved for explaining the X-ray emission of the other SEDs. Without considering the equipartition condition, their X-ray emission can be attributed to the synchrotron-self-Compton (SSC) process, but the derived jet power (P_jet) are not correlated with L_k and most of them are larger than L_k with more than three orders of magnitude, where L_k is the jet kinetic power estimated with their radio emission. Under the equipartition condition, the X-ray emission is well interpreted with the IC process to the cosmic microwave background photons (IC/CMB). In this scenario, the derived P_jet of knots and hotspots are correlated with and comparable to L_k. These results suggest that the IC/CMB model may be the promising interpretation of their X-ray emission. In addition, a tentative knot-hotspot sequence in the synchrotron peak-energy--peak-luminosity plane is observed, similar to the blazar sequence, which may be attributed to their different cooling mechanisms of electrons.
rate research
Read More
The emission mechanisms in extragalactic jets include synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Compton of cosmic microwave background photons or synchrotron emission from a second, high-energy population of electrons. Here we discuss optical polarimetry as a method for diagnosing the mechanism for the high-energy emission in quasar jets, as well as revealing the jets three-dimensional energetic and magnetic field structure. We then discuss high-energy emission mechanisms for powerful jets in the light of the HST polarimetry of PKS 1136-135.
In this chapter we review some aspects of X-ray binaries, particularly those presenting steady jets, i.e. microquasars. Because of their proximity and similarities with active galactic nuclei (AGN), galactic jet sources are unique laboratories to test astrophysical theories of a universal scope. Due to recent observational progress made with the new generation of gamma-ray imaging atmospheric Cherenkov telescopes and in view of the upcoming km3-size neutrino detectors, we focus especially on the possible high-energy gamma radiation and neutrino emission. In connection with this, we also comment about astrophysical jets present in young stellar objects, and we briefly discuss similarities and differences with extragalactic AGN and gamma-ray bursters.
We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.
The relationship between observed variability time and emission region geometry is explored for the case of emission by relativistic jets. The approximate formula for the jet-frame size of the emission region, $R=DcDelta t_{rm obs}$ is shown to lead to large systematic errors when used together with observed luminosity and assumed or estimated Doppler factor $D$ to estimate the jet-frame photon energy density. These results have implications for AGN models in which low-energy photons are targets for interaction of high energy particles and photons, e.g. synchrotron-self Compton models and hadronic blazar models, as well as models of intra-day variable sources in which the photon energy density imposes a brightness temperature limit through Compton scattering. The actual relationship between emission region geometry and observed variability is discussed for a variety of geometries including cylinders, spheroids, bent, helical and conical jet structures, and intrinsic variability models including shock excitation. The effects of time delays due to finite particle acceleration and radiation time scales are also discussed.
We present the results of a multi-wavelength follow up campaign for the luminous nuclear transient Gaia16aax, which was first identified in January 2016. The transient is spatially consistent with the nucleus of an active galaxy at z=0.25, hosting a black hole of mass $rm sim6times10^8M_odot$. The nucleus brightened by more than 1 magnitude in the Gaia G-band over a timescale of less than one year, before fading back to its pre-outburst state over the following three years. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the $rm Halpha$ and $rm Hbeta$ emission lines develop a secondary peak. We also report on the discovery of two transients with similar light curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disk. We consider variability in the accretion flow in the inner part of the disk, or a tidal disruption event of a star $geq 1 M_{odot}$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
