arXiv subjects

F. Yusef-Zadeh

Publications and source records attributed to F. Yusef-Zadeh.

129 records · Page 8Linked to original sources

arXiv1998

G359.87+0.18: A Young SNR Candidate Near the Galactic Center?

Sub-arcsecond radio continuum observations of the Galactic center region at $λ$6 and 2cm reveal a 0.5$^{\prime\prime}$ diameter source with a shell-like morphology. This source is linearly polarized at a level of 16% at $λ$6cm and has a steep nonthermal spectrum with spectral index 1.6 between $λ$6 and 2 cm. The distance to this source is not known but the large rotation measure value of 3000 rad m$^{-2}$ suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic.

astro-ph↗

arXiv1997

Anomalous Motion of Ionized Gas in the Sickle (G0.18--0.04) Near the Galactic Center

We present VLA measurements of H92alpha radio recombination line emission from an unusual HII region G0.18--0.04, the ``Sickle'', with spatial and spectral resolutions of 27" times 24".9 and 14 km/s, respectively. These observations detected two new kinematic components of ionized gas whose velocities differ greatly from the +25 km/s molecular cloud surrounding the Sickle. One component is highly red-shifted with peak velocity of about +150 km/s and the other is a blue-shifted velocity feature peaking near -35 km/s. Neither of these high-velocity features have molecular counterparts. The blue-shifted feature is forbidden in the sense of Galactic rotation and coincides with the prominent nonthermal filaments crossing the Sickle, thus suggesting that they are physically associated with each other. The results presented here are interpreted in terms of ionized gas being accelerated away from the surface of the cloud associated with the Sickle region, either by the magnetic field associated with the nonthermal filaments or by the stellar winds from the hot helium stars near G0.18--0.04.

astro-ph↗

arXiv1997

The Origin of the High-Energy Activity at the Galactic Center

Recent X-ray and gamma-ray observations of the Galactic center region by the ASCA and CGRO/EGRET instruments show evidence of 2--10 keV and $>1$ GeV continuum emission as well as 6.7 and 6.4 keV line emission from the inner $0.2^\circ$ of the Galactic center. This region is also known to host a bright nonthermal radio continuum source Sgr A East and a dense molecular cloud M--0.02--0.07 known as the 50 km s$^{-1}$ cloud. The oval-shaped nonthermal Sgr A East is physically interacting with M--0.02--0.07 at the Galactic center. A comparison between the distribution of ionized, synchrotron and neutral gas suggests a self-consistent interpretation of the high-energy activity at the Galactic center. Our preliminary analysis of the data suggest a shock model of cosmic ray acceleration at the site of the interaction to explain the enhanced GeV $γ$-ray emission. We also address a number of issues related to the spatial correlation of the diffuse radio and X-ray emitting gas as well as to the origin of the fluorescent 6.4 and 6.7 keV emission at the Galactic center.

astro-ph↗