Magnetic Interaction in Ultra-compact Binary Systems

This article reviews the current works on ultra-compact double-degenerate binaries in the presence of magnetic interaction, in particular, unipolar induction. The orbital dynamics and evolution of compact white-dwarf pairs are discussed in detail. Models and predictions of electron cyclotron masers from unipolar-inductor compact binaries and unipolar-inductor white-dwarf planetary systems are presented. Einstein-Laub effects in compact binaries are briefly discussed.Comment: invited review, accepted for publication in Research in Astronomy and Astrophysics (RAA

Discovery of Two Simultaneous Kilohertz QPOs in the Persistent Flux of GX 349+2

We report the discovery of two simultaneous quasi-periodic oscillations in the persistent flux of GX 349+2 at frequencies 712 +/- 9 and 978 +/- 9 Hz, with rms amplitudes 1.25% +/- 0.34% and 1.34 +/- 0.32%, respectively. During our 152 ks observation with the Rossi X-ray Timing Explorer, GX 342+2 was in either the normal branch or the flaring branch with count rates in the nominal 2-60 keV RXTE-PCA band ranging from a low of 8,000 cps to a high of 15,000 cps. The kHz QPOs were observed only when the source was at the top of the normal branch when the count rate was about 8,200 cps corresponding to a flux of 1.4E-8 ergs/cm**2/s in the 2-10 keV band. With this report, now kHz QPOs have been observed in all the 6 Z sources.Comment: 5 pages, LaTex (aas2pp4), Accepted for publication in ApJ Let

XMM-Newton observations of AM CVn binaries : V396 Hya and SDSS J1240–01

We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions

Coulomb tunneling for fusion reactions in dense matter: Path integral Monte Carlo versus mean field

We compare Path Integral Monte Carlo calculations by Militzer and Pollock (Phys. Rev. B 71, 134303, 2005) of Coulomb tunneling in nuclear reactions in dense matter to semiclassical calculations assuming WKB Coulomb barrier penetration through the radial mean-field potential. We find a very good agreement of two approaches at temperatures higher than ~1/5 of the ion plasma temperature. We obtain a simple parameterization of the mean field potential and of the respective reaction rates. We analyze Gamow-peak energies of reacting ions in various reaction regimes and discuss theoretical uncertainties of nuclear reaction rates taking carbon burning in dense stellar matter as an example.Comment: 13 pages, 7 figures, to appear in Phys. Rev.

KiloHertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636-53

Recent observations of the low-mass X-ray binary 4U 1636-53 with the Rossi X-ray Timing Explorer show, for the first time, a kiloHertz quasi-periodic oscillation (kHz QPO) peak separation that exceeds the neutron star spin frequency as inferred from burst oscillations. This strongly challenges the sonic-point beat frequency model for the kHz QPOs found in low-mass X-ray binaries. We detect two simultaneous kHz QPOs with a frequency separation of 323.3+-4.3 Hz in an average Fourier power spectrum of observations obtained in September 2001 and January 2002. The lower kHz QPO frequency varied between 644 Hz and 769 Hz. In previous observations of this source the peak separation frequency was ~250 Hz when the lower kHz QPO frequency was ~900 Hz. Burst oscillations occur in 4U 1636-53 at ~581 Hz and possibly at half that frequency (290.5 Hz). This is the first source where the peak separation frequency is observed to change from less than (half) the burst oscillation frequency to more than that. This observation contradicts all previously formulated implementations of the sonic-point beat frequency model except those where the disk in 4U 1636-53 switches from prograde to retrograde.Comment: 5 Pages, accepted by MNRAS Letters, 2 Figure

Implizites Lernen und sekundärer Spracherwerb:Modulation durch Ginkgo biloba

Jährlich erleiden bis zu 74.000 Menschen in Deutschland durch einen Schlaganfall Einschränkungen in ihrer Sprachkompetenz. Die Förderung des Wiedererwerbs von Sprache ist daher Ziel wissenschaftlicher Untersuchungen. Pflanzenextrakte auf Basis von Ginkgo biloba sind weit verbreitet und werden zur Gedächtnisförderung eingenommen. Ziel dieser Untersuchung war es, den gedächtnisfördernden Effekt eines Ginkgo biloba Extraktes auf den sekundären Spracherwerb in einer Placebo kontrollierten, randomisierten Doppelblindstudie mit 98 gesunden Probanden zu überprüfen.  Die Probanden im Alter zwischen 18 und 45 Jahren haben über einen Zeitraum von 26 +/- 3 Tagen entweder täglich 240mg EGb 761 oder ein Placebo eingenommen. An den letzten fünf Tagen der Einnahme erlernten die Probanden das Vokabular einer Kunstsprache. Der Vergleich von Placebogruppe und Verumgruppe zeigte hinsichtlich der Lernleistung bei dieser Aufgabe keine signifikanten Unterschiede. Der Ginkgo biloba Extrakt EGb 761 hat in dieser Studie keine Steigerung der Lernleistung bei sekundärem Spracherwerb bewirkt

Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst

The Rossi X-Ray Timing Explorer observed the soft X-Ray transient Aql X-1 during its outburst in February and March 1997. We report the discovery of quasi-periodic oscillations (QPOs) in its persistent flux with frequencies in the range of 740 to 830 Hz, Q-value of over 100, and a fractional RMS amplitude of (6.8 +- 0.6)%, and nearly coherent oscillations (NCOs) during a Type-I burst with a frequency of 549 Hz. The frequency of the QPOs in the persistent flux is correlated with the mass accretion rate on time scale of hours, but not on time scale of days. This is most likely the manifestation in a single source of the kHz QPO puzzle observed among many sources, i.e., on the one hand, individual sources show a correlation between the QPO frequency and the inferred mass accretion rate, on the other hand, the dozen or so sources with luminosities spanning two decades have essentially the same QPO frequencies. We propose that this multi-valued QPO frequency and mass accretion rate correlation indicates the existence of many similar regimes of the accretion disk. These regimes, with a very similar energy spectrum and QPO frequency, are distinguished from each other by the mass accretion rate or the total X-ray flux. The NCOs during the burst can be made almost perfectly coherent by taking into account a large frequency derivative. This strongly suggests that this frequency is related to the neutron star spin frequency. The large frequency derivative is attributable to the expansion or contraction of the neutron star photosphere during the burst.Comment: 6 pages, LaTex (aas2pp4), Accepted for publication in ApJ Let

Carbon Flashes in the Heavy Element Ocean on Accreting Neutron Stars

We show that burning of a small mass fraction of carbon in a neutron star ocean is thermally unstable at low accumulated masses when the ocean contains heavy ashes from the hydrogen burning rapid proton (rp) process. The key to early unstable ignition is the low thermal conductivity of a heavy element ocean. The instability requires accretion rates in excess of one-tenth the Eddington limit when the carbon mass fraction is 0.1 or less. The unstable flashes release 10^{42} to 10^{43} ergs over hours to days, and are likely the cause of the recently discovered large Type I X-ray bursts (so-called ``superbursts'') from six Galactic low mass X-ray binaries. In addition to explaining the energetics, recurrence times, and durations of the superbursts, these mixed carbon/heavy element flashes have an accretion rate dependence of unstable burning similar to that observed. Though the instability is present at accretion rates near Eddington, there is less contrast with the accretion luminosity there, explaining why most detections are made at accretion rates between 0.1 and 0.3 Eddington. Future comparisons of time dependent calculations with observations will provide new insights into the rp process.Comment: Submitted to Astrophysical Journal Letters (6 pages, 3 figures

A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105

(abridged) In this paper we extend the idea suggested previously by Petri (2005a,b) that the high frequency quasi-periodic oscillations observed in low-mass X-ray binaries may be explained as a resonant oscillation of the accretion disk with a rotating asymmetric background (gravitational or magnetic) field imposed by the compact object. Here, we apply this general idea to black hole binaries. It is assumed that a test particle experiences a similar parametric resonance mechanism such as the one described in paper I and II but now the resonance is induced by the interaction between a spiral density wave in the accretion disk, excited close to the innermost stable circular orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry to deduce the characteristic frequencies of this test particle. The response of the test particle is maximal when the frequency ratio of the two strongest resonances is equal to 3:2 as observed in black hole candidates. Finally, applying our model to the microquasar GRS 1915+105, we reproduce the correct value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz features in the power spectrum time analysis is predicted. Moreover, based only on the two HF-QPO frequencies, our model is able to constrain the mass $M_{\rm BH}$ and angular momentum $a_{\rm BH}$ of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc

CHANDRA Observations of V407 Vul: Confirmation of the Spin-up

V407 Vul is a candidate double-degenerate binary with a putative 1.756 mHz (9.5 min) orbital frequency. In a previous timing study using archival ROSAT and ASCA data we reported evidence for an increase of this frequency at a rate consistent with expectations for gravitational radiation from a detached ultracompact binary system. Here we report the results of new CHANDRA timing observations which confirm the previous indications of spin-up of the X-ray frequency, and provide much tighter constraints on the frequency derivative. We obtained with CHANDRA a total of 90 ksec of exposure in two epochs separated in time by 11.5 months. The total time span of the archival ROSAT, ASCA and new CHANDRA data is now 10.5 years. This more than doubles the interval spanned by the ROSAT and ASCA data alone, providing much greater sensitivity to a frequency derivative. With the addition of the CHANDRA data an increasing frequency is unavoidable, with a value df/dt = 7.0 x 10-18 Hz/s. Although a long-term spin-up trend is confirmed, there is excess variance in the phase timing residuals, perhaps indicative of shorter timescale torque fluctuations or phase instability associated with the source of the X-ray flux. Power spectral searches for periods longward of the 9.5 minute period do not find any significant modulations, however, the sensitivity of searches in this frequency range are somewhat compromised by the dithering of the Chandra attitude. The observed spin-up is of a magnitude consistent with that expected from gravitational radiation decay, however, the factor of 3 variations in flux combined with the timing noise could conceivably result from accretion-induced spin-up of a white dwarf. Continued monitoring to explore correlations of torque with X-ray flux could provide a further test of this hypothesis.Comment: AASTeX, 21 pages, 10 figures. Accepted for publication in the Astrophysical Journa