Supernova Remnant Sgr Research Articles

X-ray spectra of Sagittarius A East and diffuse X-ray background near the Galactic center

AbstractThis paper reports the analysis procedure and results of simultaneous spectral fits of the Suzaku archive data for Sagittarius (Sgr) A East and the nearby Galactic center X-ray emission (GCXE). The results are that the mixed-morphology supernova remnant Sgr A East has a recombining plasma (RP) with Cr and Mn Heα lines, and a power-law component (PL) with an Fe i Kα line. The nearby GCXE has a ∼1.5-times larger surface brightness than the mean GCXE far from Sgr A East, although the spectral shape is almost identical. Based on these results, we interpret that the origins of the RP and the PL with the Fe i Kα line are past big flares of Sgr A⋆.

GREEN BANK TELESCOPE OBSERVATIONS OF THE NH3(3, 3) AND (6, 6) TRANSITIONS TOWARD SAGITTARIUS A MOLECULAR CLOUDS

Ammonia (3, 3) and (6, 6) transitions have been observed using the Green Bank Telescope toward the Sgr A region. The gas is mainly concentrated in 50 km s{sup -1} and 20 km s{sup -1} clouds located in a plane inclined to the galactic plane. These 'main' clouds appear to be virialized and influenced by the expansion of the supernova remnant Sgr A East. The observed emission shows very complicated features in the morphology and velocity structure. Gaussian multi-component fittings of the observed spectra revealed that various 'streaming' gas components exist all over the observed region. These components include those previously known as 'streamers' and 'ridges', but most of these components appear not to be directly connected to the major gas condensations (the 50 km s{sup -1} and 20 km s{sup -1} clouds). They are apparently located out of the galactic plane, and they may have a different origin than the major gas condensations. Some of the streaming components are expected to be sources that feed the circumnuclear disk of our Galactic center directly and episodically. They may also evolve differently than major gas condensations under the influence of the activities of the Galactic center.

Localizing the VHE γ-ray source at the Galactic Centre

The inner 10 pc of our galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) gamma-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. measurement, at least three objects are capable of accelerating particles to very high energies and beyond, and of providing the observed gamma-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between those objects were of the order of the error circle radius of the emission centroid (34", dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during gamma-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20" to 6" per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted gamma-ray count map. A spatial comparison of the best-fit position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13", coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE gamma-ray emission.

Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk

We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic Center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km/s molecular cloud (M-0.02-0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km/s cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in the CND. We argue that the high velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently.

A Catalog of Diffuse X‐Ray–emitting Features within 20 pc of Sagittarius A*: Twenty Pulsar Wind Nebulae?

We present a catalog of 34 diffuse features identified in X-ray images of the Galactic center taken with the Chandra X-Ray Observatory. Several of the features have been discussed in the literature previously, including seven that are associated with a complex of molecular clouds that exhibits fluorescent line emission, four that are superimposed on the supernova remnant Sgr A East, two that are coincident with radio features that are thought to be the shell of another supernova remnant, and one that is thought to be a pulsar wind nebula only a few arcseconds in projection from Sgr A*. However, this leaves 20 features that have not been reported previously. On the basis of the weakness of the iron emission in their spectra, we propose that most of them are nonthermal. One long, narrow feature points toward Sgr A*, and so we propose that this feature is a jet of synchrotron-emitting particles ejected from the supermassive black hole. For the others, we show that their sizes (0.1-2 pc in length for D = 8 kpc), X-ray luminosities (between 1032 and 1034 erg s−1; 2-8 keV), and spectra (power laws with Γ = 1-3) are consistent with those of pulsar wind nebulae. On the basis of the star formation rate at the Galactic center, we expect that ~20 pulsars have formed in the last 300 kyr and could be producing pulsar wind nebulae. Only 1 of the 19 candidate pulsar wind nebulae is securely detected in an archival radio image of the Galactic center; the remainder have upper limits corresponding to LR≲ 1031 erg s−1. These radio limits do not strongly constrain their natures, which underscores the need for further multiwavelength studies of this unprecedented sample of Galactic X-ray-emitting structures.

A RIAF Interpretation for the Past Higher Activity of the Galactic Center Black Hole and the 511 keV Annihilation Emission

Abstract There are several lines of evidence to show that the supermassive black hole at the Galactic center had higher activities in the past than directly observed at present. It is shown here that these lines of evidence can quantitatively and consistently be explained if the mean accretion rate during the past $\sim 10^7 \,\mathrm{yr}$ has been $\sim 10^{3\hbox{--}4}$ times higher than the current rate, based on the picture of radiatively inefficient accretion flow (RIAF) and associated outflow that has been successfully applied to Sgr $\mathrm{A}^{*}$. We argue that this increased rate and its duration are theoretically reasonable in the Galactic center environment, while the accretion rate suddenly dropped about 300 yr ago, most likely because of the shell passage of the supernova remnant Sgr A East. The chance probability of witnessing Sgr $\mathrm{A}^{*}$ in such a low state is not extremely small ($\sim 0.5\%$). The outflow energetics is sufficient to keep the hot ($\sim 8 \,\mathrm{keV}$) diffuse gas observed in the Galactic center region. It is then shown that a significant amount of positrons should have been created around the event horizon during the higher activity phase, and injected into interstellar medium by the outflow. The predicted positron production rate and propagation distance are close to those required to explain the observed 511 keV annihilation line emission from the Galactic bulge, giving a natural explanation for the large bulge-to-disk ratio of the emission. The expected injection energy into interstellar medium is $\sim \mathrm{MeV}$, which is also favorable for an explanation of the 511 keV line emission.

OH 1720 MHz Masers in the Galactic Center: Sagittarius A East and the Circumnuclear Disk

We present new and archival Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic Center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. However, here we also present some previously detected positive velocity masers together with some newly detected negative velocity masers. In combination, those masers appear to bracket Sgr A* in position and velocity. The location and velocities of those masers are consistent with their being in the circumnuclear disk (CND).

Exploring High‐Velocity NH3(6, 6) Emission at the Center of Our Galaxy

Using the NH\3 (6,6) transition, which samples dense ($\sim 10^{5}$) molecular gas with an energy above ground of 412 K, we find hot gas at high velocities (--142 to --210 km s$^{-1}$) associated with the central 2 pc of the Galactic center. This material may be either infalling gas due to shocks or tidal stripping, or possibly gas swept from the nuclear region. We identify two high-velocity features, which we call the Southern Runner and the Cap, and correlate these features with others detected in various molecular observations of the Galactic center. The characteristic linewidths of the Southern Runner and Cap, 10 -- 15 \kms, are similar to those of other hot Galactic center clouds. The estimated H$_{2}$ masses of these clouds are 4$\times 10^{3}$ M$\sol$ and 2$\times 10^{3}$ M$\sol$, consistent with the masses of the western streamer and northern ridge, NH\3 (6,6) emission features detected within the central 10 pc at lower velocities. Three possible explanations for this emission are discussed assuming that they lie at the Galactic center, including sweeping by the supernova remnant Sgr A East, infall and/or shock from the circumnuclear disk (CND), and stripping from the central rotating low-velocity NH\3 (6,6) cloud.

A Persistent High‐Energy Flux from the Heart of the Milky Way:INTEGRAL’s View of the Galactic Center

Highly sensitive imaging observations of the Galactic center (GC) at high energies with an angular resolution of order 10' is a very recent development in the field of high-energy astrophysics. The IBIS/ISGRI imager on the INTEGRAL observatory detected for the first time a hard X-ray source, IGR J17456-2901, located within 1' of Sagittarius A* (Sgr A*) over the energy range 20-100 keV. Here we present the results of a detailed analysis of approximately 7 × 106 s of observations of the GC obtained since the launch of INTEGRAL in 2002 October. Two years and an effective exposure of 4.7 × 106 s have allowed us to obtain more stringent positional constraints on this high-energy source and to construct its spectrum in the range 20-400 keV. Furthermore, by combining the ISGRI spectrum with the total X-ray spectrum corresponding to the same physical region around Sgr A* from XMM-Newton data collected during part of the γ-ray observations, we constructed and present the first accurate wideband high-energy spectrum for the central arcminutes of the Galaxy. Our complete and updated analysis of the emission properties of the INTEGRAL source shows that it is faint but persistent with no variability above 3 σ, contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the soft and hard X-ray emission from this region, suggests that the source is most likely not pointlike but rather that it is a compact yet diffuse nonthermal emission region. The centroid of IGR J17456-2901 is estimated to be R.A. = 17h45m425, decl. = -28°59'28'' (J2000.0), offset by 1' from the radio position of Sgr A* and with a positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L = (5.37 ± 0.21) × 1035 ergs s-1. A 3 σ upper limit on the flux at the electron-positron annihilation energy of 511 keV from the direction of Sgr A* is set at 1.9 × 10-4 photons cm-2 s-1. Very recently, the HESS collaboration presented the detection of a source of ~TeV γ-rays also located within an arcminute of Sgr A*. We present arguments in favor of an interpretation that the photons detected by INTEGRAL and HESS arise from the same compact region of diffuse emission near the central black hole and that the supernova remnant Sgr A East could play an important role as a contributor of very high energy γ-rays to the overall spectrum from this region. There is also evidence for hard emission from a region located between the central black hole and the radio arc near l ~ 01 along the Galactic plane and known to contain giant molecular clouds.

A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East

We present imaging and spectral studies of the supernova remnant (SNR) Sagittarius (Sgr) A East from deep observations with Chandra. The spatially resolved spectral analysis of Sgr A East reveals the presence of a two-temperature thermal plasma (kT ~ 1 and 5 keV) near the center of the SNR. The central region is dominated by emission from highly ionized Fe-rich ejecta. We estimate a conservative upper limit to the total Fe ejecta mass of the SNR, MFe < 0.27 M☉. Comparisons with standard SN nucleosynthesis models suggest that this Fe mass limit is consistent with a Type II SN explosion for the origin of Sgr A East. On the other hand, the soft X-ray emission extending toward the north of the SNR can be described by a single-temperature (kT ~ 1.3 keV) thermal plasma with normal chemical composition. This portion of the SNR is thus X-ray emission from the heated interstellar medium rather than the metal-rich stellar ejecta. We point out that a hard pointlike source CXOGC J174545.5-285829 (the so-called cannonball) at the northern edge of the SNR shows X-ray characteristics unusual among other Galactic center sources. The morphological, spectral, and temporal characteristics of this source suggest identification as a high-velocity neutron star. Based on the suggested Type II origin for the SNR Sgr A East and the proximity between the two, we propose that CXOGC J174545.5-285829 is a high-velocity neutron star candidate, born from the core-collapse SN that also created the SNR Sgr A East.

A Chandra View of Diffuse X-Ray Emission in the Central 20 Parsecs of the Galaxy

Over the last three years, the Galactic center region has been monitored with a series of Chandra/ACIS observations. Besides the target object Sgr A*, the surrounding diffuse X-ray emission has been detected within the 17′ × 17′ field of view. As of 2002 June, combining all 12 GTO and GO observations, the total effective exposure reaches ∼ 590 ks, which reveals the detail structure of the faint filamentary diffuse X-ray emission with significant photon statistics. We here present preliminary results from the imaging/spectral analyses of these data. The “true-color” X-ray image and the equivalent width (EW) images for the detected elemental species of the Galactic center region indicate that the diffuse X-ray features have complex spatiospectral structures. We detect strong enhancements of He-like Fe within the 1.3′ diameter circular region in the immediate east of Sgr A*, which is most likely emission from the highly ionized Fe associated with the supernova remnant Sgr A East. Enhancements of the low ionization state Fe and highly ionized S and Si EWs in the northeast of Sgr A* may be interpreted as irradiation of photons from external X-ray sources and/or emission from bombardments of high energy particles such as unresolved SN ejecta.

Centenary of S Andromedae (SN 1885a)

AbstractObservations by Hartwig, Wolf, Dunér and Bigourdan are used to show that SN 1885a reached MV(max) = −18.8 ± 0.5 on 17 or 18 August 1885. The lightcurve of S And is found to have declined more rapidly after maximum than that of any other well-observed supernova. The existence of S And and the relatively young age of the supernova remnant Sgr A East appear to indicate that the supernova frequencies near the nucleus of M31 and the Galactic centre are respectively ∽102and ∽103times higher than expected. It is spectulated that S And may belong to a population of supernovae (“type 0”) that only occur near the nuclei of giant galaxies. Beyond the Local Group such objects might perhaps have been missed because of their fast decline and close proximity to luminous nuclei.

The supernovae near the nuclei of M31 and the Galaxy

view Abstract Citations (4) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The supernovae near the nuclei of M 31 and the galaxy. van den Bergh, S. Abstract The fact that the supernova remnant Sgr A East is quite small suggests that it is fairly young (not greater than approximately 6800 yr). The a priori probability that a supernova should have occurred so recently within about 2.5 pc of the galactic nucleus is calculated to be not greater than approximately 0.1. The a priori probability that a supernova such as S And would have occurred within 16 arcsec of the nucleus of M31 is found to be approximately 0.005. Such low probabilities suggest the remote possibility that the occurrence of these supernovae might, in some unspecified way, be related to their proximity to galactic nuclei. Publication: The Astrophysical Journal Pub Date: February 1983 DOI: 10.1086/160717 Bibcode: 1983ApJ...265..719V Keywords: Andromeda Galaxy; Galactic Nuclei; Milky Way Galaxy; Supernova Remnants; Abundance; Binary Stars; Galactic Structure; Metallic Stars; Nebulae; Stellar Evolution; Astrophysics full text sources ADS | data products SIMBAD (2) NED (1)