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Register a new userDiscovery of a small diameter young supernova remnant G354.4+0.0
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We report discovery of a shell like structure G354.4+0.0 of size 1.6 that shows morphology of a shell supernova remnant. Part of the structure show polarized emission in NRAO VLA sky survey (NVSS) map. Based on 330 MHz, 1.4 GHz Giant Metrewave Radio Telescope (GMRT) observations and existing observations at higher frequencies, we conclude the partial shell structure showing synchrotron emission is embedded in an extended HII region of size ~4. The spectrum of the diffuse HII region turns over between 1.4 GHz and 330 MHz. HI absorption spectrum shows it to be located more than 5 kpc away from Sun. Based on morphology, non-thermal polarized emission and size, this object is one of the youngest supernova remnants discovered in the Galaxy with an estimated age of about 100-500 years.
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Sensitive radio continuum surveys of the Galactic plane are ideal for discovering new supernova remnants (SNRs). From the Sino-German {lambda}6 cm polarisation survey of the Galactic plane, an extended shell-like structure has been found at l = 21.8 degree, b = -3.0 degree, which has a size of about 1 degree. New observations were made with the Effelsberg 100-m radio telescope at {lambda}11 cm to estimate the source spectrum together with the Urumqi {lambda}6 cm and the Effelsberg {lambda}21 cm data. The spectral index of G21.8-3.0 was found to be {alpha} = -0.72 {pm} 0.16. Polarised emission was mostly detected in the eastern half of G21.8-3.0 at both {lambda}6 cm and {lambda}11 cm. These properties, together with the H{alpha} filament along its northern periphery and the lack of infrared emission, indicate that the emission is non-thermal as is usual in shell-type SNRs.
Phosphorus ($^{31}$P), which is essential for life, is thought to be synthesized in massive stars and dispersed into interstellar space when these stars explode as supernovae (SNe). Here we report on near-infrared spectroscopic observations of the young SN remnant Cassiopeia A, which show that the abundance ratio of phosphorus to the major nucleosynthetic product iron ($^{56}$Fe) in SN material is up to 100 times the average ratio of the Milky Way, confirming that phosphorus is produced in SNe. The observed range is compatible with predictions from SN nucleosynthetic models but not with the scenario in which the chemical elements in the inner SN layers are completely mixed by hydrodynamic instabilities during the explosion.
We report on the first detection of GeV high-energy gamma-ray emission from a young supernova remnant with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope. These observations reveal a source with no discernible spatial extension detected at a significance level of 12.2$sigma$ above 500 MeV at a location that is consistent with the position of the remnant of the supernova explosion that occurred around 1680 in the Cassiopeia constellation - Cassiopeia A. The gamma-ray flux and spectral shape of the source are consistent with a scenario in which the gamma-ray emission originates from relativistic particles accelerated in the shell of this remnant. The total content of cosmic rays (electrons and protons) accelerated in Cas A can be estimated as $W_{mathrm{CR}} approx (1-4) times 10^{49}$ erg thanks to the well-known density in the remnant assuming that the observed gamma-ray originates in the SNR shell(s). The magnetic field in the radio-emitting plasma can be robustly constrained as B $gt 0.1$ mG, providing new evidence of the magnetic field amplification at the forward shock and the strong field in the shocked ejecta.
We report the discovery of a new Small Magellanic Cloud Pulsar Wind Nebula (PWN) at the edge of the Supernova Remnant (SNR)-DEM S5. The pulsar powered object has a cometary morphology similar to the Galactic PWN analogs PSR B1951+32 and the mouse. It is travelling supersonically through the interstellar medium. We estimate the Pulsar kick velocity to be in the range of 700-2000 km/s for an age between 28-10 kyr. The radio spectral index for this SNR PWN pulsar system is flat (-0.29 $pm$ 0.01) consistent with other similar objects. We infer that the putative pulsar has a radio spectral index of -1.8, which is typical for Galactic pulsars. We searched for dispersion measures (DMs) up to 1000 cm/pc^3 but found no convincing candidates with a S/N greater than 8. We produce a polarisation map for this PWN at 5500 MHz and find a mean fractional polarisation of P $sim 23$ percent. The X-ray power-law spectrum (Gamma $sim 2$) is indicative of non-thermal synchrotron emission as is expected from PWN-pulsar system. Finally, we detect DEM S5 in Infrared (IR) bands. Our IR photometric measurements strongly indicate the presence of shocked gas which is expected for SNRs. However, it is unusual to detect such IR emission in a SNR with a supersonic bow-shock PWN. We also find a low-velocity HI cloud of $sim 107$ km/s which is possibly interacting with DEM S5. SNR DEM S5 is the first confirmed detection of a pulsar-powered bow shock nebula found outside the Galaxy.
Context. While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio emission, we have serendipitously discovered extended radio emission close to the Galactic plane. The radio morphology suggests the presence of a previously unknown Galactic supernova remnant. An unclassified {gamma}-ray source detected by EGRET (3EG J1744-3934) is present in the same location and may stem from the interaction between high-speed particles escaping the remnant and the surrounding interstellar medium. Aims. Our aim is to confirm the presence of a previously unknown supernova remnant and to determine a possible association with the {gamma}-ray emission 3EG J1744-3934. Methods. We have conducted optical and radio follow-ups of the target using the Dark Energy Camera (DECam) on the Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO) and the Giant Meterwave Radio Telescope (GMRT). We then combined these data with archival radio and {gamma}-ray observations. Results. While we detected the extended emission in four different radio bands (325, 1400, 2417, and 4850 MHz), no optical counterpart has been identified. Given its morphology and brightness, it is likely that the radio emission is caused by an old supernova remnant no longer visible in the optical band. Although an unclassified EGRET source is co-located with the supernova remnant, Fermi-LAT data do not show a significant {gamma}-ray excess that is correlated with the radio emission. However, in the radial distribution of the {gamma}-ray events, a spatially extended feature is related with SNR at a confidence level $sim 1.5$ {sigma}. Conclusions. We classify the newly discovered extended emission in the radio band as the old remnant of a previously unknown Galactic supernova: SNR G351.0-5.4.
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