On the anomalously large extension of the Pulsar Wind Nebula HESS J1825-137

The very high energy (VHE) gamma-ray emission reported from a number of pulsar wind nebulae (PWNe) is naturally explained by the inverse Compton scattering of multi-TeV electrons. However, the physical dimensions of some gamma-ray-emitting PWNe significantly exceed the scales anticipated by the standard hydrodynamical paradigm of PWN formation. The most "disturbing" case in this regard is HESS J1825-137, which extends to distances $r\approx70\rm\,pc$ from the central pulsar PSR J1826-1334. If the gamma-ray emission is indeed produced inside the PWN, but not by electrons that escaped the nebula and diffuse in the interstellar medium (ISM), the formation of such an anomalously extended plerion could be realized, in a diluted environment with the hydrogen number density $n_{ism}\le10^{-2}\rm\,cm^{-3}$. In this paper, we explore an alternative scenario assuming that the pulsar responsible for the formation of the nebula initially had a very short rotation period. In this case, the sizes of both the PWN and the surrounding supernova remnant depend on the initial pulsar period, the braking index, and the ISM density. To check the feasibility of this scenario, we study the parameter space that would reproduce the size of HESS J1825-137. We show that this demand can be achieved if the braking index is small, $n\leq2$ and the pulsar birth period is short, $P_{\rm b}\simeq1\rm\,ms$. This scenario can reproduce the wind termination position, which is expected at $R_{ts}\simeq0.03\rm\,pc$, only in a dense environment with $n_{ism}\geq\rm1\,cm^{-3}$. The requirement of the dense surrounding gas is supported by the presence of molecular clouds found in the source vicinity.Comment: 15 pages, 6 figures, ApJ accepte

Propeller outflows from an MRI disc

We present the results of axisymmetric simulations of MRI-driven accretion onto a rapidly rotating, magnetized star accreting in the propeller regime. The stellar magnetosphere corotates with the star, forming a centrifugal barrier at the disc-magnetosphere boundary which inhibits matter accretion onto the star. Instead, the disc matter accumulates at the disc-magnetosphere interface and slowly diffuses into the inner magnetosphere where it picks up angular momentum and is quickly ejected from the system as an outflow. Due to the interaction of the matter with the magnetosphere, this wind is discontinuous and is launched as discrete plasmoids. If the ejection rate is lower than the disc accretion rate, the matter accumulates at the disc-magnetosphere boundary faster than it can be ejected. In this case, accretion onto the star proceeds through the episodic accretion instability in which episodes of matter accumulation are followed by simultaneous accretion and ejection. During the accretion phase of this instability in which matter flows onto the star in funnel streams, we observe a corresponding rise in the outflow rate. Both the accretion and ejection processes observed in our simulations are highly non-stationary. The stars undergo strong spin-down due to the coupling of the stellar field with the disc and corona and we measure the spin-down timescales of around 1 Myr for a typical CTTS in the propeller regime.Comment: 13 pages, 10 figures, submitted to MNRA

Hydrodynamics of interaction of pulsar and stellar winds and its impact on the high energy radiation of binary pulsar systems

The hydrodynamics of the interaction of pulsar and stellar winds in binary systems harboring a pulsar and its impact on the nonthermal radiation of the binary pulsar PSR B1259-63/SS2883 is discussed. The collision of an ultrarelativistic pulsar wind with a nonrelativistic stellar outflow results in significant bulk acceleration of the shocked material from the pulsar wind. Already at distances comparable to the size of the binary system, the Lorentz factor of the shocked flow can be as large as $\gamma$~4. This results in significant anisotropy of the inverse Compton radiation of accelerated electrons. Because of the Doppler boosting of the produced radiation, one should expect a variable gamma-ray signal from the system. In particular, this effect may naturally explain the reported tendency of a decrease of TeV gamma-ray flux close to the periastron. The modeling of the interaction of pulsar and stellar winds allows self-consistent calculations of adiabatic losses. Our results show that adiabatic losses dominate over the radiative losses. These results have direct impact on the orbital variability of radio, X-ray and gamma-ray signals detected from the binary pulsar PSR 1259-63/SS2883.Comment: 4 pages, 4 figures; based on poster presentation at "High Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for publication in International Journal of Modern Physics