AChandraStudy of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?

Abstract

We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central '2 pc of the larger radio shell. The spectrum shows strong K lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature $2 keV, absorption column $1 Â 10 23 H cm À2 , luminosity $8 Â 10 34 ergs s À1 in the 2-10 keV band, and gas mass $2 1=2 M with a filling factor . The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M . The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich '' mixed morphology '' (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 10 3 cm À3 . The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of $10 40 ergs s À1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

Sgr A East as a possible high energy neutron factory in the Galactic Centre

“…Since the non-thermal radio shell still exists, its present velocity must be faster than the sound velocity in the ambient material. According to the authors of [9] this implies that the temperature of the ionised gas halo surrounding Sgr A East is substantially smaller than 70 K which is consistent with the IR observations. The density of the gas halo is about 10 3 cm −3 and it is almost homogenous barring the molecular cloud M-0.02-0.07 where it becomes as large as 10 5 cm −3 [21].…”

“…The spectacular Chandra [9] X-ray observations, in the energy range 2 − 10 keV, with an angular resolution better than 1", confirmed that Sgr A East is the remnant of a single type II SN with age between 10 4 and 10 5 years. The total luminosity (∼ 8 × 10 34 ergs s −1 in the Chandra energy range) and the spectrum of this source allowed to establish that its progenitor was a main sequence star of mass of 13 − 20 M ⊙ with a metallicity four times larger than that of the Sun.…”

“…The ionising source of the gas halo surrounding the GC was not identified by Chandra. According to the authors of [9] this source may be related with a past activity of the super-massive black-hole in Sgr A* being presently in a quiescent phase. The milliGauss field strength measured by Yusuf-Zadeh et al [3] in the Eastern side of Sgr A East may be explained by the adiabatic compression of the pre-existing field with an intensity B GC ∼ 10 − 100 µG .…”

Interstellar gas within ~10 pc of Sagittarius A∗

“…Some authors consider that the molecular ridge actually belongs to M−0.02−0.07 and coincides with the fraction of the cloud that has been swept up and compressed by the Sgr A East forward shock (e.g., Serabyn et al 1992;Maeda et al 2002). In this scenario, the molecular ridge must adhere to Sgr A East and form the eastern part of the gas-and-dust shell surrounding it.…”

SGR A* and Its Environment: Low-Mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

“…Thermal X-ray emission is concentrated in the interior of the remnant, suggesting that Sgr A East is a mixed morphology SNR interacting with the 50 km s −1 molecular cloud (Maeda et al 2002;Park et al 2005). The thermal X-ray emitting plasma has two components, characterized by temperatures of 1 and 6 keV and corresponding electron density of 4.7 and 0.6 cm −3 , respectively.…”