Photon propagation in a cold axion background with and without magnetic field

A cold relic axion condensate resulting from vacuum misalignment in the early universe oscillates with a frequency m, where m is the axion mass. We determine the properties of photons propagating in a simplified version of such a background where the sinusoidal variation is replaced by a square wave profile. We prove that previous results that indicated that charged particles moving fast in such a background radiate, originally derived assuming that all momenta involved were much larger than m, hold for long wavelengths too. We also analyze in detail how the introduction of a magnetic field changes the properties of photon propagation in such a medium. We briefly comment on possible astrophysical implications of these results.Comment: 17 pages, 4 figures, revised version includes an extended discussion on physical implication

Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei

Stripped-envelope stars (SESs) form in binary systems after losing mass through Roche-lobe overflow. They bear astrophysical significance as sources of UV and ionizing radiation in older stellar populations and, if sufficiently massive, as stripped supernova progenitors. Binary evolutionary models predict them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type companions are known. This could be the result of observational biases hindering detection, or an incorrect understanding of binary evolution. We reanalyze the well-studied post-interaction binary phi Persei. Recently, new data improved the orbital solution of the system, which contains a ~1.2 Msun SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid of evolutionary models using a Bayesian approach and find initial masses of the progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star. The system must have evolved through near-conservative mass transfer. These findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is in excellent agreement with the age of the alpha Persei cluster. We note that neither star was initially massive enough to produce a core-collapse supernova, but mass exchange pushed the Be star above the mass threshold. We find that the subdwarf is overluminous for its mass by almost an order of magnitude, compared to the expectations for a helium core burning star. We can only reconcile this if the subdwarf is in a late phase of helium shell burning, which lasts only 2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our findings can be interpreted as a strong indication that a substantial population of SESs indeed exists, but has so far evaded detection because of observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&

Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state

(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the SEI method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. The Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity $v$ increases with radius $r$ according to $v=v_\infty(1-r_\star/r)^\beta$, with$\beta\approx0.75$ and $v_\infty\approx1420$ km s$^{-1}$.The microturbulent velocity was $\approx160$ km s$^{-1}$. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate of L$_{X,38}/\dot M_{-6} \approx 0.33$, measured at $\dot M_{-6}$ = 4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L$_x$ matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap

H-alpha Emission Variability in the gamma-ray Binary LS I +61 303

LS I +61 303 is an exceptionally rare example of a high mass X-ray binary (HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of "gamma-ray binaries". Here we present H-alpha spectra that show strong variability during the 26.5 day orbital period and over decadal time scales. We detect evidence of a spiral density wave in the Be circumstellar disk over part of the orbit. The H-alpha line profile also exhibits a dramatic emission burst shortly before apastron, observed as a redshifted shoulder in the line profile, as the compact source moves almost directly away from the observer. We investigate several possible origins for this red shoulder, including an accretion disk, mass transfer stream, and a compact pulsar wind nebula that forms via a shock between the Be star's wind and the relativistic pulsar wind.Comment: Accepted to Ap

Effects of Metallicity on the Rotation Rates of Massive Stars

Recent theoretical predictions for low metallicity massive stars predict that these stars should have drastically reduced equatorial winds (mass loss) while on the main sequence, and as such should retain most of their angular momentum. Observations of both the Be/(B+Be) ratio and the blue-to-red supergiant ratio appear to have a metallicity dependence that may be caused by high rotational velocities. We have analyzed 39 archival Hubble Space Telescope Imaging Spectrograph (STIS), high resolution, ultraviolet spectra of O-type stars in the Magellanic Clouds to determine their projected rotational velocities V sin i. Our methodology is based on a previous study of the projected rotational velocities of Galactic O-type stars using International Ultraviolet Explorer (IUE) Short Wavelength Prime (SWP) Camera high dispersion spectra, which resulted in a catalog of V sin i values for 177 O stars. Here we present complementary V sin i values for 21 Large Magellanic Cloud and 22 Small Magellanic Cloud O-type stars based on STIS and IUE UV spectroscopy. The distribution of V sin i values for O type stars in the Magellanic Clouds is compared to that of Galactic O type stars. Despite the theoretical predictions and indirect observational evidence for high rotation, the O type stars in the Magellanic Clouds do not appear to rotate faster than their Galactic counterparts.Comment: accepted by ApJ, to appear 20 December 2004 editio