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Register a new userThe TeV Cosmic Ray Bump: a Message from Epsilon Indi or Epsilon Eridani Star?
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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.
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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.
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.
In 2015 we started the XMM-Newton monitoring of the young solar-like star Epsilon Eridani (440 Myr), one of the youngest solar-like stars with a known chromospheric CaII cycle. By analyzing the most recent Mount Wilson S-index CaII data of this star, we found that the chromospheric cycle lasts 2.92 +/- 0.02 yr, in agreement with past results. From the long-term X-ray lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric CaII cycle. The average X-ray luminosity results to be 2 x 10^28 erg/s, with an amplitude that is only a factor 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of Epsilon Eridani in terms of solar magnetic structures: active regions, cores of active regions and flares covering the stellar surface at varying filling fractions. Combinations of these magnetic structures can describe the observed X-ray emission measure of Epsilon Eridani only if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of Epsilon Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of Epsilon Eridani. Our analysis revealed also that the X-ray cycle of Epsilon Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of Epsilon Eridani is thus explained by the high percentage of magnetic structures on its surface.
We have identified a new early T dwarf only 3.6pc from the Sun, as a common proper motion companion (separation 1459AU) to the K5V star Epsilon Indi (HD209100). As such, Epsilon Indi B is one of the highest proper motion sources outside the solar system (~4.7 arcsec/yr), part of one of the twenty nearest stellar systems, and the nearest brown dwarf to the Sun. Optical photometry obtained from the SuperCOSMOS Sky Survey was combined with approximate infrared photometry from the 2MASS Quicklook survey data release, yielding colours for the source typical of early T dwarfs. Follow up infrared spectroscopy using the ESO NTT and SOFI confirmed its spectral type to be T2.5+/-0.5. With Ks=11.2, Epsilon Indi B is 1.7 magnitudes brighter than any previously known T dwarf and 4 magnitudes brighter than the typical object in its class, making it highly amenable to detailed study. Also, as a companion to a bright nearby star, it has a precisely known distance (3.626pc) and relatively well-known age (0.8-2Gyr), allowing us to estimate its luminosity as logL/Lsun=-4.67, its effective temperature as 1260K, and its mass as ~40-60Mjup. Epsilon Indi B represents an important addition to the census of the Solar neighbourhood and, equally importantly, a new benchmark object in our understanding of substellar objects.
We present observations of Epsilon Eridani from the Submillimeter Array (SMA) at 1.3 millimeters and from the Australia Telescope Compact Array (ATCA) at 7 millimeters that reach an angular resolution of ~4 (13 AU). These first millimeter interferometer observations of Epsilon Eridani, which hosts the closest debris disk to the Sun, reveal two distinct emission components: (1) the well-known outer dust belt, which, although patchy, is clearly resolved in the radial direction, and (2) an unresolved source coincident with the position of the star. We use direct model-fitting of the millimeter visibilities to constrain the basic properties of these two components. A simple Gaussian shape for the outer belt fit to the SMA data results in a radial location of $64.4^{+2.4}_{-3.0}$ AU and FWHM of $20.2^{+6.0}_{-8.2}$ AU (fractional width $Delta R/R = 0.3$. Similar results are obtained taking a power law radial emission profile for the belt, though the power law index cannot be usefully constrained. Within the noise obtained (0.2 mJy/beam), these data are consistent with an axisymmetric belt model and show no significant azimuthal structure that might be introduced by unseen planets in the system. These data also limit any stellocentric offset of the belt to $<9$ AU, which disfavors the presence of giant planets on highly eccentric ($>0.1$) and wide (10s of AU) orbits. The flux density of the unresolved central component exceeds predictions for the stellar photosphere at these long wavelengths, by a marginally significant amount at 1.3 millimeters but by a factor of a few at 7 millimeters (with brightness temperature $13000 pm 1600$ K for a source size of the optical stellar radius). We attribute this excess emission to ionized plasma from a stellar corona or chromosphere.
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