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Register a new userPossible detection of gamma rays from Epsilon Eridani
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We use the Fermi-LAT gamma-ray observatory to search for gamma-ray emission from four nearby, debris disk-hosting main sequence stars: $tau$ Ceti, $epsilon$ Eridani, Fomalhaut, and Vega. For three stars ($tau$ Ceti, Fomalhaut, and Vega), we establish upper limits that are consistent with theoretical expectations. For $epsilon$ Eridani, we find a possible spatially coincident source with a soft energy spectrum of $dN/dE sim E^{-3.6}$. However, at this stage we are unable to rule out that this emission is due to a more extended feature in the diffuse background. In the interpretation that the emission is due to $epsilon$ Eridani, the $> 100$ MeV gamma-ray luminosity is $sim 10^{27}$ erg/s $simeq 3times 10^{-7}$ L$_odot$, which is $sim 10^{10}$ times the gamma-ray luminosity from the disk of the quiet Sun. We find $lesssim 2 sigma$ evidence of source variability over a $sim 7$ year timescale. In the interpretation that the gamma-ray emission from $epsilon$ Eridani itself, we consider two possible models: 1) cosmic-ray collisions with solid bodies in the debris disk which extends out $sim$60 AU from the host star, and 2) emission from the stellar activity. For the former model, assuming a total disk mass consistent with infrared measurements, we find that the size distribution of bodies is steeper than expected for a collisional cascade state. If confirmed as being associated with $epsilon$ Eridani, this would be the first indication of gamma-ray emission from the vicinity of a main sequence star other than the Sun.
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The nearby star $rm epsilon Eridani$ has been a frequent target of radio surveys for stellar emission and extraterrestial intelligence. Using deep $rm 2-4 GHz$ observations with the Very Large Array, we have uncovered a $29 mu {rm Jy}$ compact, steady continuum radio source coincident with $rm epsilon Eridani$ to within 0.06 arcseconds ($lesssim 2sigma$; 0.2 au at the distance of the star). Combining our data with previous high frequency continuum detections of $rm epsilon Eridani$, our observations reveal a spectral turnover at $rm 6 GHz$. We ascribe the $rm 2-6 GHz$ emission to optically thick, thermal gyroresonance radiation from the stellar corona, with thermal free-free opacity likely becoming relevant at frequencies below $rm 1 GHz$. The steep spectral index ($alpha simeq 2$) of the $rm 2-6 GHz$ spectrum strongly disfavors its interpretation as stellar wind-associated thermal bremsstrahlung ($alpha simeq 0.6$). Attributing the entire observed $rm 2-4 GHz$ flux density to thermal free-free wind emission, we thus, derive a stringent upper limit of $3 times 10^{-11} M_{odot} {rm yr}^{-1}$ on the mass loss rate from $rm epsilon Eridani$. Finally, we report the non-detection of flares in our data above a $5sigma$ threshold of $rm 95 mu Jy$. Together with the optical non-detection of the most recent stellar maximum expected in 2019, our observations postulate a likely evolution of the internal dynamo of $rm epsilon Eridani$.
A recently observed bump in the cosmic ray (CR) spectrum from 0.3--30 TV is likely caused by a stellar bow shock that reaccelerates emph{preexisting} CRs, which further propagate to the Sun along the magnetic field lines. Along their way, these particles generate an Iroshnikov-Kraichnan (I-K) turbulence that controls their propagation and sustains the bump. {it Ad hoc} fitting of the bump shape requires six adjustable parameters. Our model requires none, merely depending on emph{three physical unknowns that we constrain using the fit.} These are the shock Mach number, $M$, its size, $l_{perp}$, and the distance to it, $zeta_{text{obs}}$. Altogether, they define the bump rigidity $R_{0}$. With $M$$approx$1.5--1.6 and $R_{0}$$approx$4.4 TV, the model fits the data with $approx$$0.08%$ accuracy. The fit critically requires the I-K spectrum predicted by the model and rules out the alternatives. These fits attributes make an accidental agreement highly unlikely. In turn, $R_{0}$ and $M$ derived from the fit impose the distance-size %($zeta_{{rm obs}}$$-$$l_{perp}$) relation on the shock: $zeta_{{rm obs}}$(pc)$sim$$10^{2}sqrt{l_{perp}(text{pc})}$. For sufficiently large bow shocks, $l_{perp}$$=$$10^{-3}$$-$$10^{-2}$ pc, we find the distance of $zeta_{{rm obs}}$$=$3--10 pc. Three promising stars in this range are: Scholzs Star at 6.8 pc, Epsilon Indi at 3.6 pc, and Epsilon Eridani at 3.2 pc. Based on their current positions and velocities, we propose that Epsilon Indi and Epsilon Eridani can produce the observed spectral bump. Moreover, Epsilon Eridanis position is only $sim$$6.7^{circ}$ off of the magnetic field direction in the solar neighborhood, which also changes the CR arrival direction distribution. Given the proximity of these stars, the bump appearance may change in a relatively short time.
As part of a wider search for radio emission from nearby systems known or suspected to contain extrasolar planets $epsilon$ Eridani was observed by the Jansky Very Large Array (VLA) in the 2-4 GHz and 4-8 GHz frequency bands. In addition, as part of a separate survey of thermal emission from solar-like stars, $epsilon$ Eri was observed in the 8-12 GHz and the 12-18 GHz bands of the VLA. Quasi-steady continuum radio emission from $epsilon$ Eri was detected in the three high-frequency bands at levels ranging from 67-83 $mu$Jy. No significant variability is seen in the quasi-steady emission. The emission in the 2-4 GHz emission, however, is shown to be the result of a circularly polarized (up to 50%) radio pulse or flare of a few minutes duration that occurred at the beginning of the observation. We consider the astrometric position of the radio source in each frequency band relative to the expected position of the K2V star and the purported planet. The quasi-steady radio emission at frequencies $ge !8$ GHz is consistent with a stellar origin. The quality of the 4-8 GHz astrometry provides no meaningful constraint on the origin of the emission. The location of the 2-4 GHz radio pulse is $>2.5sigma$ from the star yet, based on the ephemeris of Benedict et al. (2006), it is not consistent with the expected location of the planet either. If the radio pulse has a planetary origin, then either the planetary ephemeris is incorrect or the emission originates from another planet.
Recent theoretical predictions of the lowest very high energy (VHE) luminosity of SN 1006 are only a factor 5 below the previously published H.E.S.S. upper limit, thus motivating further in-depth observations of this source. Deep observations at VHE energies (above 100 GeV) were carried out with the High Energy Stereoscopic System (H.E.S.S.) of Cherenkov Telescopes from 2003 to 2008. More than 100 hours of data have been collected and subjected to an improved analysis procedure. Observations resulted in the detection of VHE gamma-rays from SN 1006. The measured gamma-ray spectrum is compatible with a power-law, the flux is of the order of 1% of that detected from the Crab Nebula, and is thus consistent with the previously established H.E.S.S. upper limit. The source exhibits a bipolar morphology, which is strongly correlated with non-thermal X-rays. Because the thickness of the VHE-shell is compatible with emission from a thin rim, particle acceleration in shock waves is likely to be the origin of the gamma-ray signal. The measured flux level can be accounted for by inverse Compton emission, but a mixed scenario that includes leptonic and hadronic components and takes into account the ambient matter density inferred from observations also leads to a satisfactory description of the multi-wavelength spectrum.
Detection of gamma-rays emitted by radioactive isotopes synthesized in stellar explosions can give important insights into the processes that power transients such as supernovae, as well as providing a detailed census of the abundance of different isotope species relevant to the chemical evolution of the Universe. Observations of nearby supernovae have yielded observational proof that $^{57}$Co powered the late-time evolution of SN1987As lightcurve, and conclusive evidence that $^{56}$Ni and its daughter nuclei power the light curves of Type Ia supernovae. In this paper we describe the prospects for detecting nuclear decay lines associated with the decay of $^{48}$V, the daughter nucleus of $^{48}$Cr, which is expected to be synthesised in large quantities - $M_{mathrm{Cr}}sim1.9times10^{-2},mathrm{M_odot}$ - in transients initiated by explosive helium burning ($alpha$-capture) of a thick helium shell. We calculate emergent gamma-ray line fluxes for a simulated explosion model of a thermonuclear explosion of carbon-oxygen white dwarf core of mass $0.45,M_{odot}$ surrounded by a thick helium layer of mass $0.21,M_{odot}$. We present observational limits on the presence of $^{48}$V in nearby SNe Ia 2014J using the textit{INTEGRAL} space telescope, excluding a $^{48}$Cr production on the surface of more than $0.1,mathrm{M_{odot}}$. We find that the future gamma-ray mission AMEGO will have an approximately 5 per cent chance of observing $^{48}$V gamma-rays from such events during the currently-planned operational lifetime, based on our birthrate predictions of faint thermonuclear transients. We describe the conditions for a $3sigma$ detection by the gamma-ray telescopes textit{INTEGRAL}/SPI, COSI and AMEGO.
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