Monthly Notices of the Royal Astronomical Society Letters

Panaitescu, A.

doi: 10.1111/j.1745-3933.2005.00134.xpmid: N/A

We use the available radio, optical and X-ray measurements for the afterglows of the short bursts 050709 and 050724 to constrain the blast-wave energy, its collimation and the density of the circumburst medium. For GRB 050709 (the duration of which was 0.07 s), we identify two kinds of models: (i) a high-density solution, where the ejecta are collimated in a jet of half-angle θjet > 6° and interact with a medium of particle density 10−4 < n < 0.1 cm−3; and (ii) a low-density solution with θjet > 2° and n < 10−5 cm−3. These density ranges are compatible with those expected in the vicinity of the host galaxy and in the intergalactic medium, lending support to the hypothesis that the progenitor of GRB 050709 is a neutron star–neutron star or neutron star–black hole merger. For GRB 050724 (which lasted 1.5 s), we obtain 0.1 < n < 103 cm−3 and θjet > 8°. The range of allowed densities shows that this burst occurred in the interstellar medium. The dynamical parameters of the high-density model for the GRB afterglow 050709 are similar to those for 050724. If these parameters are representative for short-GRB outflows, then these jets are less collimated and have a lower kinetic energy than those of long bursts, which suggests that GRB jets are not magnetically collimated and are powered by the gravitational energy of the torus. Evidently, the analysis of more short-GRB afterglows is required for a more robust conclusion.

Horner, J.; Wyn Evans, N.

doi: 10.1111/j.1745-3933.2006.00131.xpmid: N/A

Large-scale simulations of Centaurs have yielded vast numbers of data, the analysis of which allows interesting but uncommon scenarios to be studied. One such rare phenomenon is the temporary capture of Centaurs as Trojans of the giant planets. Such captures are generally short (10–100 kyr), but occur with sufficient frequency (∼40 objects larger than 1 km in diameter every Myr) that they may well contribute to the present-day populations. Uranus and Neptune seem to have great difficulty capturing Centaurs into the 1 : 1 resonance, while Jupiter captures some, and Saturn the most (∼80 per cent). We conjecture that such temporary capture from the Centaur population may be the dominant delivery route into the Saturnian Trojans. Photometric studies of the Jovian Trojans may reveal outliers with Centaur-like as opposed to asteroidal characteristics, and these would be prime candidates for captured Centaurs.

McIvor, T.; Jardine, M.; Holzwarth, V.

doi: 10.1111/j.1745-3933.2005.00098.xpmid: N/A

Recent observations have shown what is believed to be planet-induced chromospheric activity on stars with hot Jupiters. We present a model of the magnetic interaction between a planet and a star with a dipolar magnetic field. Reconnection between the fields of the star and planet accelerates electrons along the field lines that connect the star and planet. By determining the locations at which these field lines connect to the stellar surface, we can model the surface pattern of the chromospheric response to the planet–star interaction. We find that the inclination of the rotation axis of the star determines the magnitude of the emission, while a misalignment of the magnetic and rotation axes produces a phase shift between the peak of the observed chromospheric emission and the phase where the planet is directly in front of the star. This phase shift increases as the extent of the stellar corona increases. This model reproduces the cyclic signature of chromospheric enhancement seen in stars with hot Jupiters. It can reproduce the 65° phase lag reported for HD 179949 if the closed corona of the star extends out to the orbital radius of the planet at 8.5R⋆. It cannot, however, reproduce the phase lag of 169° reported for υ And.

Salvaterra, Ruben; Ferrara, Andrea

doi: 10.1111/j.1745-3933.2005.00129.xpmid: N/A

The observed near-infrared background excess over light from known galaxies is commonly ascribed to redshifted radiation from early, very massive, Population III (Pop III) stars. We show here that this interpretation must be discarded as it largely overpredicts the number of J-dropouts and Lyα emitters in ultradeep field searches. Independently of the detailed physics of Lyα line emission, J-dropouts limit the background excess fraction due to Pop III sources to be (at best) ⩽1/24. As alternative explanations can either be rejected (e.g. miniquasars, decaying neutrinos) or appear unlikely (zodiacal light), but the reality of the excess is supported by the interpretation of the angular fluctuations, the origin of this component remains very puzzling. We briefly discuss possible hints to solve the problem.

Dehnen, Walter; King, Andrew

doi: 10.1111/j.1745-3933.2005.00132.xpmid: N/A

Low-mass X-ray binaries (LMXBs), which occur in old stellar populations, have velocities exceeding those of their parent distribution by at least 20 km s−1. This makes them ideal probes for dark matter, in particular in dwarf spheroidals (dSph), where the LMXBs should penetrate well outside the visible galaxy. We argue that the most likely explanation of the observation of LMXBs in the Sculptor dSph by Maccarone et al. is the presence of a dark matter halo of ≳109 M⊙, corresponding to a total-mass to light ratio of ≳600 (M/LV)⊙. In this case there should be an extended halo of LMXBs which may be observable.

Fender, R. P.; Muxlow, T. W. B.; Garrett, M. A.; Kouveliotou, C.; Gaensler, B. M.; Garrington, S. T.; Paragi, Z.; Tudose, V.; Miller-Jones, J. C. A.; Spencer, R. E.; Wijers, R. A. M.; Taylor, G. B.

doi: 10.1111/j.1745-3933.2006.00123.xpmid: N/A

On 2004 December 27, the magnetar SGR 1806-20 underwent an enormous outburst resulting in the formation of an expanding, moving, and fading radio source. We report observations of this radio source with the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array. The observations confirm the elongation and expansion already reported based on observations at lower angular resolutions, but suggest that at early epochs the structure is not consistent with the very simplest models such as a smooth flux distribution. In particular, there appears to be significant structure on small angular scales, with ∼10 per cent of the radio flux arising on angular scales ⩽100 milliarcsec. This structure may correspond to localized sites of particle acceleration during the early phases of expansion and interaction with the ambient medium.

Kumar, P.; McMahon, E.; Barthelmy, S. D.; Burrows, D.; Gehrels, N.; Goad, M.; Nousek, J.; Tagliaferri, G.

doi: 10.1111/j.1745-3933.2006.00138.xpmid: N/A

Data from the Swift satellite have enabled us for the first time to provide a complete picture of the gamma-ray (γ-ray) burst emission mechanism and its relationship with the early afterglow emissions. We show that γ-ray photons for two bursts, 050126 and 050219A, for which we have carried out detailed analysis were produced as a result of the synchrotron self-Compton process in the material ejected in the explosion when it was heated to a mildly relativistic temperature at a distance from the centre of explosion of order the deceleration radius. Both of these bursts exhibit rapidly declining early X-ray afterglow light curves; this emission is from the same source that produced the γ-ray burst. The technique that we exploit to determine this is very general and makes no assumption about any particular model for γ-ray generation except that the basic radiation mechanism is some combination of synchrotron and inverse Compton processes in a relativistic outflow. For GRB 050219A we can rule out the possibility that energy from the explosion is carried outward by magnetic fields, and that the dissipation of this field produced the γ-ray burst.

Gonzalez, Guillermo

doi: 10.1111/j.1745-3933.2005.00136.xpmid: N/A

Results from detailed spectroscopic analyses of stars hosting massive planets are employed to search for trends between abundances and condensation temperatures. The elements C, S, Na, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni and Zn are included in the analysis of 64 stars with planets and 33 comparison stars. No significant trends are evident in the data. This null result suggests that accretion of rocky material on to the photospheres of stars with planets is not the primary explanation for their high metallicities. However, the differences between the solar photospheric and meteoritic abundances do display a weak but significant trend with condensation temperature. This suggests that the metallicity of the Sun's envelope has been enriched relative to its interior by about 0.07 dex.

Rodríguez-Ardila, A.; Mazzalay, X.

doi: 10.1111/j.1745-3933.2006.00139.xpmid: N/A

We report 0.8–4.5 μm SpeX spectroscopy of the narrow-line Seyfert 1 galaxy Mrk 1239. The spectrum is outstanding because the nuclear continuum emission in the near-infrared (NIR) is dominated by a strong bump of emission peaking at 2.2 μm, with a strength not reported before in an active galactic nucleus. A comparison of the Mrk 1239 spectrum to that of Ark 564 allowed us to conclude that the continuum is strongly reddened by E(B−V) = 0.54. The excess of emission, confirmed by aperture photometry and additional NIR spectroscopy, follows a simple blackbody curve at T∼ 1200 K. This suggests that we may be observing direct evidence of dust heated to near to the sublimation temperature, likely produced by the putative torus of the unification model. Although other alternatives are also plausible, the lack of star formation, the strong polarization and low extinction derived for the emission lines support the scenario where the hot dust is located between the narrow line region and the broad line region.

Mitra, A.

doi: 10.1111/j.1745-3933.2006.00141.xpmid: N/A

By using elementary astrophysical concepts, we show that for any self-luminous astrophysical object the ratio of radiation energy density inside the body (ρr) and the baryonic energy density (ρ0) may be crudely approximated, in the Newtonian limit, as ρr/ρ0∝GM/Rc2, where G is constant of gravitation, c is the speed of light, M is gravitational mass and R is the radius of the body. The key idea is that radiation quanta must move out in a diffusive manner rather than stream freely inside the body of the star. When one would move to the extreme general relativistic case, i.e. if the surface gravitational redshift z≫ 1, it is found that ρr/ρ0∝ (1 +z). Earlier treatments of gravitational collapse, in contrast, generally assumed ρr/ρ0≪ 1. Thus, actually, during continued general relativistic gravitational collapse to the black hole state (z→∞), the collapsing matter may essentially become an extremely hot fireball with ρr/ρ0≫ 1, a la the very early Universe, even though the observed luminosity of the body as seen by a faraway observer L∞∝ (1 +z)−1→ 0 as z→∞, and the collapse might appear as ‘adiabatic’.