Correlation between X-ray flux and rotational acceleration in Vela X-1

The results of a search for correlations between X-ray flux and angular acceleration for the accreting binary pulsar Vela X-1 are presented. Results are based on data obtained with the Hakucho satellite during the interval 1982 to 1984. In undertaking this correlation analysis, it was necessary to modify the usual statistical method to deal with conditions imposed by generally unavoidable satellite observing constraints, most notably a mismatch in sampling between the two variables. The results are suggestive of a correlation between flux and the absolute value of the angular acceleration, at a significance level of 96 percent. The implications of the methods and results for future observations and analysis are discussed

Neutron Star Masses and Radii as Inferred from kilo-Hertz QPOs

Kilo-Hertz (kHz) Quasi-periodic oscillations (QPOs) have been discovered in the X-ray fluxes of 8 low-mass X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer (RXTE). The characteristics of these QPOs are remarkably similar from one source to another. In particular, the highest observed QPO frequencies for 6 of the 8 sources fall in a very narrow range: 1,066 to 1,171 Hz. This is the more remarkable when one considers that these sources are thought to have very different luminosities and magnetic fields, and produce very different count rates in the RXTE detectors. Therefore it is highly unlikely that this near constancy of the highest observed frequencies is due to some unknown selection effect or instrumental bias. In this letter we propose that the highest observed QPO frequency can be taken as the orbital frequency of the marginally stable orbit. This leads to the conclusions that the neutron stars in these LMXBs are inside their marginally stable orbits and have masses in the vicinity of 2.0 solar masses. This mass is consistent with the hypothesis that these neutron stars were born with about 1.4 solar masses and have been accreting matter at a fraction of the Eddington limit for 100 million years.Comment: 7 pages, uses aas2pp4.sty, Accepted by ApJ

Confirmation of Two Cyclotron Lines in Vela X-1

We present pulse phase-resolved X-ray spectra of the high mass X-ray binary Vela X-1 using the Rossi X-ray Timing Explorer. We observed Vela X-1 in 1998 and 2000 with a total observation time of ~90 ksec. We find an absorption feature at 23.3 +1.3 -0.6 kev in the main pulse, that we interpret as the fundamental cyclotron resonant scattering feature (CRSF). The feature is deepest in the rise of the main pulse where it has a width of 7.6 +4.4 -2.2 kev and an optical depth of 0.33 +0.06 -0.13. This CRSF is also clearly detected in the secondary pulse, but it is far less significant or undetected during the pulse minima. We conclude that the well known CRSF at 50.9 +0.6 -0.7 kev, which is clearly visible even in phase-averaged spectra, is the first harmonic and not the fundamental. Thus we infer a magnetic field strength of B=2.6 x 10^12 G.Comment: 12 pages, LaTeX, 15 Figures, accepted by A&

General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars

Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray spectra and pulsations by means of radiation processes that occur on the surfaces of neutron stars. For any such model, general relativistic deflection of light severely suppresses the amplitude of the observed pulsations. We calculate the expected pulsation amplitudes of AXPs according to various models and compare the results with observations. We show that the high (<= 70%) pulse amplitudes observed in some AXPs can be accounted for only if the surface emission is localized (spot radius <40 degrees) and strongly beamed (cos^n[theta'] with n>2, where theta' is the angle to the normal). These constraints are incompatible with those cooling and magnetar models in which the observed X-rays originate as thermal emission from the neutron-star surface. Accretion models, on the other hand, are compatible with observations for a wide range of parameters. Finally, definitive conclusions cannot be reached on magnetospheric models, since their localization and beaming properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa

Locking of the Rotation of Disk-Accreting Magnetized Stars

We investigate the rotational equilibrium state of a disk accreting magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In this ``locked'' state, the spin-up torque balances the spin-down torque so that the net average torque on the star is zero. We investigated two types of initial conditions, one with a relatively weak stellar magnetic field and a high coronal density, and the other with a stronger stellar field and a lower coronal density. We observed that for both initial conditions the rotation of the star is locked to the rotation of the disk. In the second case, the radial field lines carry significant angular momentum out of the star. However, this did not appreciably change the condition for locking of the rotation of the star. We find that in the equilibrium state the corotation radius $r_{co}$ is related to the magnetospheric radius $r_A$ as $r_{co}/r_A\approx 1.2-1.3$ for case (1) and $r_{co}/r_A\approx 1.4-1.5$ for case (2). We estimated periods of rotation in the equilibrium state for classical T Tauri stars, dwarf novae and X-ray millisecond pulsars.Comment: 10 pages, 9 figures. Accepted by ApJ, will appear in vol. 634, 2005 December

An X-Ray Dip in the X-Ray Transient 4U 1630-47

An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer observation of the recurrent x-ray transient 4U 1630-47. During the dip, the 2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point of the dip, the x-ray spectrum is considerably softer than at non-dip times. We find that the 4U 1630-47 dip is best explained by absorption of the inner part of an accretion disk, while the outer part of the disk is unaffected. The spectral evolution during the dip is adequately described by the variation of a single parameter, the column density obscuring the inner disk.Comment: 13 pages, 4 figures, Accepted for publication in Ap

Ionization States and Plasma Structures of Mixed-morphology SNRs Observed with ASCA

We present the results of a systematic study using ASCA of the ionization state for six ``mixed-morphology'' supernova emnants (MMSNRs): IC 443, W49B, W28, W44, 3C391, and Kes 27. MMSNRs show centrally filled thermal X-ray emission, which contrasts to shell-like radio morphology, a set of haracteristics at odds with the standard model of SNR evolution (e.g., the Sedov model). We have therefore studied the evolution of the MMSNRs from the ionization conditions inferred from the X-ray spectra, independent of X-ray morphology. We find highly ionized plasmas approaching ionization equilibrium in all the mmsnrs. The degree of ionization is systematically higher than the plasma usually seen in shell-like SNRs. Radial temperature gradients are also observed in five remnants, with cooler plasma toward the limb. In IC 443 and W49B, we find a plasma structure consistent with shell-like SNRs, suggesting that at least some MMSNRs have experienced similar evolution to shell-like SNRs. In addition to the results above, we have discovered an ``overionized'' ionization state in W49B, in addition to that previously found in IC 443. Thermal conduction can cause the hot interior plasma to become overionized by reducing the temperature and density gradients, leading to an interior density increase and temperature decrease. Therefore, we suggest that the ``center-filled'' X-ray morphology develops as the result of thermal conduction, and should arise in all SNRs. This is consistent with the results that MMSNRs are near collisional ionization equilibrium since the conduction timescale is roughly similar to the ionization timescale. Hence, we conclude that MMSNRs are those that have evolved over$\sim10^4$ yr. We call this phase as the ``conduction phase.''Comment: 34 pages, 20 figures, 9 tables, accepted for publication in The Astrophysical Journa

Timing, glitches and braking index of PSR B0540-69

We report a pulse-time history of PSR B0540$-$69 based on the analysis of an extended Data set including ASCA, BeppoSAX and RXTE observations spanning a time interval of about 8 years. This interval includes also the epoch of the glitch episode reported by Zhang et al. (2001). Our analysis shows the presence of a relevant timing noise and does not give a clear evidence of the glitch occurrence. We performed an accurate evaluation of the main timing parameters, $\nu$, $\dot{\nu}$ and $\ddot{\nu}$ and derived a mean braking index of $n=2.125\pm0.001$ quite different from the lower value found by Zhang et al. (2001), but in rather good agreement with other several values reported in the literature.Comment: 9 pages 5 figures, accepted by A&A, main journa