Hoinga: a supernova remnant discovered in the SRG/eROSITA All-Sky Survey eRASS1

Supernova remnants (SNRs) are observable for about (6−15) × 104 yr before they fade into the Galactic interstellar medium. With a Galactic supernova rate of approximately two per century, we can expect to have of the order of 1200 SNRs in our Galaxy. However, only about 300 of them are known to date, with the majority having been discovered in Galactic plane radio surveys. Given that these SNRs represent the brightest tail of the distribution and are mostly located close to the plane, they are not representative of the complete sample. The launch of the Russian-German observatory SRG/eROSITA in July 2019 brought a promising new opportunity to explore the Universe. Here we report findings from the search for new SNRs in the eROSITA all-sky survey data which led to the detection of one of the largest SNRs discovered at wavelengths other than the radio: G249.5+24.5. This source is located at a relatively high Galactic latitude, where SNRs are not usually expected to be found. The remnant, ‘Hoinga’, has a diameter of about 4. °4 and shows a circular shaped morphology with diffuse X-ray emission filling almost the entire remnant. Spectral analysis of the remnant emission reveals that an APEC spectrum from collisionally ionised diffuse gas and a plane-parallel shock plasma model with non-equilibrium ionisation are both able to provide an adequate description of the data, suggesting a gas temperature of the order of kT = 0.1−0.02+0.02 keV and an absorbing column density of NH = 3.6−0.6+0.7 × 1020 cm−2. Various X-ray point sources are found to be located within the remnant boundary but none seem to be associated with the remnant itself. Subsequent searches for a radio counterpart of the Hoinga remnant identified its radio emission in archival data from the Continuum HI Parkes All-Sky Survey and the 408-MHz ‘Haslam’ all-sky survey. The radio spectral index α = −0.69 ± 0.08 obtained from these data definitely confirms the SNR nature of Hoinga. We also analysed INTEGRAL SPI data for fingerprints of 44Ti emission, which is an ideal candidate with which to study nucleosynthesis imprinting in young SNRs. Although no 44Ti emission from Hoinga was detected, we were able to set a 3σ upper flux limit of 9.2 × 10−5 ph cm−2 s−1. From its size and X-ray and radio spectral properties we conclude that Hoinga is a middle-aged Vela-like SNR located at a distance of about twice that of the Vela SNR, i.e. at ~500 pc.

Unexpected circular radio objects at high Galactic latitude

We have found a class of circular radio objects in the Evolutionary Map of the Universe Pilot Survey, using the Australian Square Kilometre Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs, about one arcmin diameter, that are unlike other objects previously reported in the literature. We explore several possible mechanisms that might cause these objects, but none seems to be a compelling explanation.

Galactic diffuse gamma-ray emission at TeV energy

Context. Measuring the diffuse Galactic γ-ray flux in the TeV range is difficult for ground-based γ-ray telescopes because of the residual cosmic-ray background, which is higher than the γ-ray flux by several orders of magnitude. Its detection is also challenging for space-based telescopes because of low signal statistics. Aims. We characterise the diffuse TeV flux from the Galaxy using decade-long exposures of the Fermi Large Area Telescope. Methods. Considering that the level of diffuse Galactic emission in the TeV band approaches the level of residual cosmic-ray background, we estimated the level of residual cosmic-ray background in the SOURCEVETO event selection and verified that the TeV diffuse Galactic emission flux is well above the residual cosmic-ray background up to high Galactic latitude regions. Results. We study spectral and imaging properties of the diffuse TeV signal from the Galactic plane. We find much stronger emission from the inner Galactic plane than in previous HESS telescope estimates (lower bound). We also find a significant difference in the measurement of the Galactic longitude and latitude profiles of the signal measured by Fermi and HESS. These discrepancies are presumably explained by the fact that regions of background estimate in HESS have non-negligible γ-ray flux. Comparing Fermi measurements with those of ARGO-YBJ, we find better agreement, with the notable exception of the Cygnus region, where we find much higher flux (by a factor 1.5). We also measure the TeV diffuse emission spectrum up to high Galactic latitude and show that the spectra of different regions of the sky have spectral slopes consistent with Γ = 2.34 ± 0.04, which is harder than the slope of the locally observed spectrum of cosmic rays with energies 10–100 TeV, which produce TeV diffuse emission on their way through the interstellar medium. We discuss the possible origin of the hard slope of the TeV diffuse emission. Conclusions. Fermi/LAT provides reliable measurements of the diffuse Galactic emission spectrum in the TeV range, which are almost background free at low Galactic latitudes. The diffuse flux becomes comparable to the residual cosmic-ray background at Galactic latitudes |b| > 50°. Its measurement in these regions might suffer from systematic uncertainty stemming from the uncertainty of our phenomenological model of the residual cosmic-ray background in the Pass 8 Fermi/LAT data.

G4.8+6.2, a possible kilonova remnant?

ABSTRACT Kilonova explosions typically release ∼1050–51 erg in kinetic energy, which is sufficient to constitute a kilonova remnant (KNR); however, it has not yet been confirmed. In this work, we investigate the probable association between G4.8+6.2 and the guest star of AD 1163, which is recorded by the Korean ancient astronomers. Although the evidence available is insufficient to draw a definite conclusion, it is at least theoretically self-consistent that the guest star of AD 1163 was a historical kilonova associated with G4.8+6.2, considering the possible short visible timescale of AD 1163, the relatively high Galactic latitude of G4.8+6.2, and that G4.8+6.2 is spatially coincident with the guest star of AD 1163. Further observation of G4.8+6.2 is needed to test our hypothesis. If our interpretation is correct, our results indicate that young KNRs should have a large diameter and low surface brightness, unlike other young supernova remnants.

Revised and updated catalogue of the First Byurakan Survey of Late-Type Stars: second edition

ABSTRACT A total of 18 lists of the First Byurakan Survey of Late-Type Stars (FBS LTS) were published between 1990 and 2016. The stars were found on FBS low-dispersion spectroscopic plates. A systematic search and selection were carried out on a surface of ∼16 000 deg2 on almost the whole area of the FBS. As a result, the ‘Revised and Updated Catalogue of the First Byurakan Survey of Late-Type Stars’ was generated (FBS LTS v1; 1045 objects). We present the second version of the catalogue of FBS LTS with new data (FBS LTS v2), comprising 1471 objects. It is a homogeneous and complete data base for high-Galactic-latitude, late-type stars, including M and C types. Since 2007, all FBS low-resolution spectral plates have been digitized. All digitized FBS (DFBS) spectral plates have been analysed with FITSView and SAO Image ds9, and numerous relatively faint LTSs have been discovered. We have performed cross-correlations with DFBS, USNO-B1.0, 2MASS, AllWISE, IRAS PSC/FSC, AKARI, ROSAT BSC/FSC, GCVS, SDSS and added updated SIMBAD data. For numerous new detected objects, we present accurate DSS2 positions, approximate spectral subtypes refined from the DFBS low-dispersion spectra, luminosity classes estimated from 2MASS colours, and available proper motions for 1471 FBS LTSs. The Revised and Updated Catalogue v2 lists a large number of completely new objects, which promises to extend very significantly the census of M giants, faint N-type asymptotic giant branch carbon stars, CH-type carbon giants at high Galactic latitudes, and M dwarfs in the Solar vicinity up to 16.0–17.0 mag in the visual. Some important data from Gaia DR2 and supplementary spectra from the Byurakan Astrophysical Observatory 2.6-m and LAMOST telescopes are presented.