The Evolution of the Accretion Disk around 4U 1820-30 During a Superburst

Accretion from a disk onto a collapsed, relativistic star -- a neutron star or black hole -- is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not yet possible to directly observe the evolution or dynamics of the inner parts of the accretion disk where general relativistic effects are dominant. Here, we use the bright X-ray emission from a superburst on the surface of the neutron star 4U 1820-30 as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk to fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. The time-resolved spectral fitting shows that the inner region of the disk is disrupted by the burst, possibly being heated into a thicker, more tenuous flow, before recovering its previous form in ~1000 s. This marks the first instance that the evolution of the inner regions of an accretion disk has been observed in real-time.Comment: 5 pages, 2 figures, accepted by ApJ Letter

Interpretation of 35 Hz QPO in the Atoll Source 4U 1702-42 as a Low Branch of the Keplerian Oscillations Under the Influence of the Coriolis Force

The recent model of quasi-periodic oscillations in neutron star binaries (Osherovich and Titarchuk 1999, Titarchuk and Osherovich 1999) has suggested the existence of two branches of QPOs due to the influence of Coriolis force on the linear Keplerian oscillator: one branch with frequencies from 400 to 1200 Hz and another branch with frequencies an order of magnitude lower. The frequencies of the high branch nu_h hold a hybrid frequency relation with the Keplerian frequency nu_K: nu_h^2=nu_K^2+ [Omega/pi]^2, where Omega is the rotational frequency of the star's magnetosphere. The frequency of the low branch is nu_L=(Omega/pi)(nu_K/nu_h) sin(delta), where delta is a small angle between vector Omega and the vector normal to the plane of Keplerian oscillations. The observations of the source 4U 1702-42 (Markwardt et al 1999) have shown that the centroid of the 35 Hz QPO tracks the frequency of the kilohertz oscillations. We interpret the 35 Hz oscillations as nu_L and find delta=3.9^o +/- 0.2^o. Our results make 4U 1702-42 the second source (after Sco X-1) for which the theoretically derived lower branch is identified (within our model) and delta is calculated. The inferred angle delta stays approximately the same over the significant range of nu_K (650 - 900 Hz), as expected from the model. Based on our model we present a classification of QPO frequencies in the source 4U 1702-42 observed above and below nu_L.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter

R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?

During a Type-I burst, the turbulent deflagation front may excite waves in the neutron star ocean and upper atmosphere with frequencies, $\omega \sim 1$ Hz. These waves may be observed as highly coherent flux oscillations during the burst. The frequencies of these waves changes as the upper layers of the neutron star cool which accounts for the small variation in the observed QPO frequencies. In principle several modes could be excited but the fundamental buoyant $r-$mode exhibits significantly larger variability for a given excitation than all of the other modes. An analysis of modes in the burning layers themselves and the underlying ocean shows that it is unlikely these modes can account for the observed burst oscillations. On the other hand, photospheric modes which reside in a cooler portion of the neutron star atmosphere may provide an excellent explanation for the observed oscillations.Comment: 18 pages, 1 figure, substantial changes and additions to reflect version to appear in Ap

Differences between the Two Anomalous X-Ray Pulsars: Variations in the Spin Down Rate of 1E 1048.1-5937 and An Extended Interval of Quiet Spin Down in 1E 2259+586

We analysed the RXTE archival data of 1E 1048.1-5937 covering a time span of more than one year. The spin down rate of this source decreases by 30 percent during the observation. We could not resolve the X-ray flux variations because of contamination by Eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of accretion powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a constant spin down with a very low upper limit on timing noise. We used the RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic X-ray luminosity times series is also stable, with an rms fractional variation of less than 15 percent. The source could have been in a quiet phase of accretion with a constant X-ray luminosity and spin down rate.Comment: MNRAS in pres

Discovery of Fast X-ray Oscillations During the 1998 Giant Flare from SGR 1900+14

We report the discovery of complex high frequency variability during the August 27, 1998 giant flare from SGR 1900+14 using the Rossi X-ray Timing Explorer (RXTE). We detect an approx. equals 84 Hz oscillation (QPO) during a 1 s interval beginning approximately 1 min after the initial hard spike. The amplitude is energy dependent, reaching a maximum of 26% (rms) for photons above 30 keV, and is not detected below 11 keV, with a 90% confidence upper limit of 14% (rms). Remarkably, additional QPOs are detected in the average power spectrum of data segments centered on the rotational phase at which the 84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are strongly detected, while a third feature at 28 Hz is found with lower significance. These QPOs are not detected at other rotational phases. The phenomenology seen in the SGR 1900+14 flare is similar to that of QPOs recently reported by Israel et al. (2005) from the December 27, 2004 flare from SGR 1806-20, suggesting they may have a common origin, perhaps torsional vibrations of the neutron star crust. Indeed, an association of the four frequencies (in increasing order) found in SGR 1900+14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. We discuss our findings in the context of this model and show that if the stars have similar masses then the magnetic field in SGR 1806-20 must be about twice as large as in SGR 1900+14, broadly consistent with estimates from pulse timing

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

On the Statistical Analysis of X-ray Polarization Measurements

In many polarimetry applications, including observations in the X-ray band, the measurement of a polarization signal can be reduced to the detection and quantification of a deviation from uniformity of a distribution of measured angles of the form alpha plus beta cosine (exp 2)(phi - phi(sub 0) (0 (is) less than phi is less than pi). We explore the statistics of such polarization measurements using both Monte Carlo simulations as well as analytic calculations based on the appropriate probability distributions. We derive relations for the number of counts required to reach a given detection level (parameterized by beta the "number of sigma's" of the measurement) appropriate for measuring the modulation amplitude alpha by itself (single interesting parameter case) or jointly with the position angle phi (two interesting parameters case). We show that for the former case when the intrinsic amplitude is equal to the well known minimum detectable polarization (MDP) it is, on average, detected at the 3sigma level. For the latter case, when one requires a joint measurement at the same confidence level, then more counts are needed, by a factor of approximately equal to 2.2, than that required to achieve the MDP level. We find that the position angle uncertainty at 1sigma confidence is well described by the relation sigma(sub pi) equals 28.5(degrees) divided by beta

Discovery of the Neutron Star Spin Frequency in EXO 0748-676

We report the results of a search for burst oscillations during thermonuclear X-ray bursts from the low mass X-ray binary (LMXB) EXO 0748-676. With the proportional counter array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) we detected a 45 Hz oscillation in the average power spectrum of 38 thermonuclear X-ray bursts from this source. We computed power spectra with 1 Hz frequency resolution for both the rising and decaying portions of 38 X-ray bursts from the public RXTE archive. We averaged the 1 Hz power spectra and detected a significant signal at 45 Hz in the decaying phases of the bursts. The signal is detected at a significance level of 4 x 10^-8. No similar signal was detected in the rising intervals. A fit to the oscillation peak at 0.25 Hz resolution gives a frequency of 44.7 +/- 0.06 Hz and an oscillation quality factor of Q = 80 +/- 18. The average signal amplitude is 3% (rms). The detection of 45 Hz burst oscillations from EXO 0748-676 provides compelling evidence that this is the neutron star spin frequency in this system. We use the inferred spin frequency to model the widths of absorption lines from the neutron star surface and show that the widths of the absorption lines from EXO 0748-676 recently reported by Cottam et al. are consistent with a 45 Hz spin frequency as long as the neutron star radius is in the range from about 9.5-15 km. With a known spin frequency, precise modelling of the line profiles from EXO 0748-676 holds great promise for constraining the dense matter equation of state.Comment: 13 pages, 5 figures, to be published in ApJ

Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence

We analyze the time dependence of fluid variables in general relativistic, magnetohydrodynamic simulations of accretion flows onto a black hole with dimensionless spin parameter a/M=0.9. We consider both the case where the angular momentum of the accretion material is aligned with the black hole spin axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted flow). In comparison to the untilted simulation, the tilted simulation exhibits a clear excess of inertial variability, that is, variability at frequencies below the local radial epicyclic frequency. We further study the radial structure of this inertial-like power by focusing on a radially extended band at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables. The three dimensional density structure at this frequency suggests that the power is a composite oscillation whose dominant components are an over dense clump corotating with the background flow, a low order inertial wave, and a low order inertial-acoustic wave. Our results provide preliminary confirmation of earlier suggestions that disk tilt can be an important excitation mechanism for inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap