A model for upper kHz QPO coherence of accreting neutron star

{We investigate the coherence of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to the radial extent of the kHz QPO emission region, associated with the transitional layer at the magnetosphere-disk boundary, which corresponds to the coherence of upper kHz QPO. The theoretical model for Q-factor of upper kHz QPO is applied to the observational data of five Atoll and five Z sources, and the consistence is implied.Comment: accepted by A&

A two-component model for the high-energy variability of blazars. Application to PKS 2155-304

We study the production of VHE emission in blazars as a superposition of a steady component from a baryonic jet and a time- dependent contribution from an inner e-e+ beam launched by the black hole. Both primary relativistic electrons and protons are injected in the jet, and the particle distributions along it are found by solving a one-dimensional transport equation that accounts for convection and cooling. The short-timescale variability of the emission is explained by local pair injections in turbulent regions of the inner beam. For illustration, we apply the model to the case of PKS 2155-304, reproducing a quiescent state of emission with inverse Compton and synchrotron radiation from primary electrons, as well as proton-proton interactions in the jet. The latter also yield an accompanying neutrino flux that could be observed with a new generation km-scale detector in the northern hemisphere such as KM3NeT.Comment: 9 pages, 12 figures, published in A&A (final version

Fast TeV variability in blazars: jets in a jet

The fast TeV variability of the blazars Mrk 501 and PKS 2155--304 implies a compact emitting region that moves with a bulk Lorentz factor of Gamma_{em}~100 toward the observer. The Lorentz factor is clearly in excess of the jet Lorentz factors Gamma_j\simless 10 measured on sub-pc scales in these sources. We propose that the TeV emission originates from compact emitting regions that move relativistically {\it within} a jet of bulk Gamma_j~10. This can be physically realized in a Poynting flux-dominated jet. We show that if a large fraction of the luminosity of the jet is prone to magnetic dissipation through reconnection, then material outflowing from the reconnection regions can efficiently power the observed TeV flares through synchrotron-self-Compton emission. The model predicts simultaneous far UV/soft X-ray flares.Comment: Moderate changes to match the published version, MNRAS, 395, L29 (2009

Magnetospheric accretion-ejection processes in the classical T Tauri star AA

From a long time series of high resolution (R=115,000) HARPS spectra and simultaneous broad-band photometry, we report new evidence for magnetospheric accretion as well as ejection processes in the nearly edge-on classical T Tauri star AA Tau. AA Tau's light curve is modulated with a period of 8.22d. The recurrent luminosity dips are due to the periodic occultation of the central star by the magnetically-warped inner disk edge located at about 9 Rstar. Balmer line profiles exhibit a clear rotational modulation of high-velocity redshifted absorption components with a period of 8.22 days as well, with a maximum strength when the main accretion funnel flow passes through the line of sight. At the same time, the luminosity of the system decreases by about 1 mag, indicative of circumstellar absorption of the stellar photosphere by the magnetically-warped, corotating inner disk edge. The photospheric and HeI radial velocities also exhibit periodic variations, and the veiling is modulated by the appearance of the accretion shock at the bottom of the accretion funnel. Diagnostics of hot winds and their temporal behaviour are also presented. The peculiar geometry of the young AA Tau system (nearly edge-on) allows us to uniquely probe the acretion-ejection region close to the star. We find that most spectral and photometric diagnostics vary as expected from models of magnetically-channelled accretion in young stars, with a large scale magnetosphere tilted by 20 deg onto the star's spin axis. We also find evidence for time variability of the magnetospheric accretion flow on a timescale of a few rotational periods.Comment: 14 page

Morphological analysis on the coherence of kHz QPOs

We take the recently published data of twin kHz quasi-period oscillations (QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and investigate the morphology of the samples, which focuses on the quality factor, peak frequency of kHz QPOs, and try to infer their physical mechanism. We notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in general) and increase with the kHz QPO peak frequencies for both Z and Atoll sources. (2) The distribution of quality factor versus frequency for the lower kHz QPOs are quite different between Z and Atoll sources. For most Z source samples, the quality factors of lower kHz QPOs are low (usually lower than 15) and rise steadily with the peak frequencies except for Sco X-1, which drop abruptly at the frequency of about 750 Hz. While for most Atoll sources, the quality factors of lower kHz QPOs are very high (from 2 to 200) and usually have a rising part, a maximum and an abrupt drop. (3) There are three Atoll sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality factors for lower kHz QPOs. These three sources have been detected with the spin frequencies and sidebands, in which the source with higher spin frequency presents higher quality factor of lower kHz QPOs and lower difference between sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe

Energetic Constraints on a Rapid Gamma-Ray Flare in PKS 1222+216

We study theoretical implications of a rapid Very-High-Energy (VHE) flare detected by MAGIC in the Flat-Spectrum Radio Quasar PKS 1222+216. The minimum distance from the jet origin at which this flare could be produced is 0.5 pc. A moderate Doppler factor of the VHE source, D_{VHE} ~ 20, is allowed by all opacity constraints. The concurrent High-Energy (HE) emission observed by Fermi provides estimates of the total jet power and the jet magnetic field strength. Energetic constraints for the VHE flare are extremely tight: for an isotropic particle distribution they require a huge co-moving energy density in the emitting region and a very efficient radiative process. We disfavor hadronic processes due to their low radiative efficiency, as well as the synchrotron scenario recently proposed for the case of HE flares in the Crab Nebula, since the parameters needed to overcome the radiative losses are quite extreme. The VHE emission can be explained by the Synchrotron Self-Compton (SSC) mechanism for D_{VHE} ~ 20 or by the External Radiation Compton (ERC) mechanism involving the infrared radiation of the dusty torus for D_{VHE} ~ 50. After discussing several alternative scenarios, we propose that the extreme energy density constraint can be satisfied when the emission comes from highly anisotropic short-lived bunches of particles formed by the kinetic beaming mechanism in magnetic reconnection sites. By focusing the emitting particles into very narrow beams, this mechanism allows one to relax the causality constraint on the source size, decreasing the required energy density by 4 orders of magnitude.Comment: 12 pages, 2 figures, accepted for publication in MNRA

The Scientific Performance of the Microchannel X-ray Telescope on board the SVOM Mission

The Microchannel X-ray Telescope (MXT) will be the first focusing X-ray telescope based on a "Lobster-Eye" optical design to be flown on Sino-French mission SVOM. SVOM will be dedicated to the study of Gamma-Ray Bursts and more generally time-domain astrophysics. The MXT telescope is a compact (focal length ~ 1.15 m) and light (< 42 kg) instrument, sensitive in the 0.2--10 keV energy range. It is composed of an optical system, based on micro-pore optics (MPOs) of 40 micron pore size, coupled to a low-noise pnCDD X-ray detector. In this paper we describe the expected scientific performance of the MXT telescope, based on the End-to-End calibration campaign performed in fall 2021, before the integration of the SVOM payload on the satellite.Comment: 22 pages, 12 figures, accepted for publication in Experimental Astronom

H.E.S.S. observations of gamma-ray bursts in 2003-2007

Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray bursts (GRBs) in some scenarios. Exploring this photon energy regime is necessary for understanding the energetics and properties of GRBs. GRBs have been one of the prime targets for the H.E.S.S. experiment, which makes use of four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays. Dedicated observations of 32 GRB positions were made in the years 2003-2007 and a search for VHE gamma-ray counterparts of these GRBs was made. Depending on the visibility and observing conditions, the observations mostly start minutes to hours after the burst and typically last two hours. Results from observations of 22 GRB positions are presented and evidence of a VHE signal was found neither in observations of any individual GRBs, nor from stacking data from subsets of GRBs with higher expected VHE flux according to a model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from the GRB positions were derived. For those GRBs with measured redshifts, differential upper limits at the energy threshold after correcting for absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure