HESS J1825-137: A pulsar wind nebula associated with PSR B1823-13?

HESS J1825-137 was detected with a significance of 8.1 $\sigma$ in the Galactic Plane survey conducted with the H.E.S.S. instrument in 2004. Both HESS J1825-137 and the X-ray pulsar wind nebula G18.0--0.7 (associated with the Vela-like pulsar PSR B1823-13) are offset south of the pulsar, which may be the result of the SNR expanding into an inhomogeneous medium. The TeV size ($\sim 35$ pc, for a distance of 4 kpc) is $\sim 6$ times larger than the X-ray size, which may be the result of propagation effects as a result of the longer lifetime of TeV emitting electrons, compared to the relatively short lifetime of keV synchrotron emitting electrons. The TeV photon spectral index of $\sim 2.4$ can also be related to the extended PWN X-ray synchrotron photon index of $\sim 2.3$, if this spectrum is dominated by synchrotron cooling. The anomalously large size of the pulsar wind nebula can be explained if the pulsar was born with a relatively large initial spindown power and braking index $n\sim 2$, provided that the SNR expanded into the hot ISM with relatively low density ($\sim 0.003$ cm$^{-3}$).Comment: 4 pages, 4 figures, to appear in the Proc. of the 29th International Cosmic Ray Conference, OG Sessio

Probing the Nature of the Vela X Cocoon

Vela X is a pulsar wind nebula (PWN) associated with the active pulsar B0833-45 and contained within the Vela supernova remnant (SNR). A collimated X-ray filament ("cocoon") extends south-southwest from the pulsar to the center of Vela X. VLA observations uncovered radio emission coincident with the eastern edge of the cocoon and H.E.S.S. has detected TeV $\gamma$-ray emission from this region as well. Using XMM-\textit{Newton} archival data, covering the southern portion of this feature, we analyze the X-ray properties of the cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model with a powerlaw component. Our analysis of the thermal emission shows enhanced abundances of O, Ne, and Mg within the cocoon, indicating the presence of ejecta-rich material from the propagation of the SNR reverse shock, consistent with Vela X being a disrupted PWN. We investigate the physical processes that excite the electrons in the PWN to emit in the radio, X-ray and $\gamma$-ray bands. The radio and non-thermal X-ray emission can be explained by synchrotron emission. We model the $\gamma$-ray emission by Inverse Compton scattering of electrons off of cosmic microwave background (CMB) photons. We use a 3-component broken power law to model the synchrotron emission, finding an intrinsic break in the electron spectrum at $\sim5 \times 10^{6}$ keV and a cooling break at $\sim$ 5.5 $\times 10^{10}$ keV. This cooling break along with a magnetic field strength of 5 $\times 10^{-6}$ G indicate that the synchrotron break occurs at $\sim$1 keV.Comment: accepted for publication to ApJ

Periodic Modulations in an X-ray Flare from Sagittarius A*

We present the highly significant detection of a quasi-periodic flux modulation with a period of 22.2 min seen in the X-ray data of the Sgr A* flare of 2004 August 31. This flaring event, which lasted a total of about three hours, was detected simultaneously by EPIC on XMM-Newton and the NICMOS near-infrared camera on the HST. Given the inherent difficulty in, and the lack of readily available methods for quantifying the probability of a periodic signal detected over only several cycles in a data set where red noise can be important, we developed a general method for quantifying the likelihood that such a modulation is indeed intrinsic to the source and does not arise from background fluctuations. We here describe this Monte Carlo based method, and discuss the results obtained by its application to a other XMM-Newton data sets. Under the simplest hypothesis that we witnessed a transient event that evolved, peaked and decayed near the marginally stable orbit of the supermassive black hole, this result implies that for a mass of 3.5 x 10^{6} Msun, the central object must have an angular momentum corresponding to a spin parameter of a=0.22.Comment: 4 pages, 6 figures, submitted to ApJ