Rossi X-ray Timing Explorer Data Research Articles

Is M82 X-1 Really an Intermediate-Mass Black Hole? X-Ray Spectral and Timing Evidence

Ultraluminous X-ray sources (ULXs) with apparent luminosities up to hundreds of times the Eddington luminosity for a neutron star have been discovered in external galaxies. The existence of intermediate-mass black holes has been proposed to explain these sources. We present evidence for an intermediate-mass black hole in the ULX M82 X-1 based on the spectral features and timing (quasi-periodic oscillation [QPO]) properties of the X-radiation from this source. We revisited XMM-Newton and Rossi X-Ray Timing Explorer (RXTE) data for M82 X-1 obtained in 2001 and 1997 for XMM and RXTE, respectively. We show for these observations that the source is either in transition or in a high/soft state with photon spectral indices 2.1 and 2.7, respectively. We confirm the early determination of the QPO frequency nu approx. = 55 mHz in this source by Strohmayer & Mushotzky and identify this as the low-frequency QPO for the source. We apply a new method to determine the black hole mass of M82 X-1. The method uses the index-QPO low-frequency correlation that has been recently established in Galactic black hole candidates GRS 1915+105, XTE JI550-564, 4U 1630-47, and others. Using scaling arguments and the correlation derived from the consideration of Galactic black holes, we conclude that M82 X-1 is an intermediate black hole with a mass of the order of 1000 solar mass,.

Periodicities in the X-Ray Intensity Variations of TV Columbae: An Intermediate Polar

We present results from a temporal analysis of the longest and the most sensitive X-ray observations of TV Columbae—a cataclysmic variable classified as an intermediate polar. The observations were carried out with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array, ROSAT Position Sensitive Proportional Counter, and ASCA. Data were analyzed using a one-dimensional CLEAN and Bayesian algorithms. The presence of a nearly sinusoidal modulation due to the spin of the white dwarf is seen clearly in all the data, confirming the previous reports based on the EXOSAT data. An improved period of 1909.7 ± 2.5 s is derived for the spin from the RXTE data. The binary period of 5.5 hr is detected unambiguously in X-rays for the first time. Several sidebands due to the interaction of these periods are observed in the power spectra, thereby suggesting contributions from both the disk-fed and the stream-fed accretion for TV Col. The accretion disk could perhaps be precessing, as sidebands due to the influence of 4 day period on the orbital period are seen. The presence of a significant power at certain sidebands of the spin frequency indicates that the emission poles are asymmetrically located. The strong power at the orbital sidebands seen in both the RXTE and ROSAT data gives an indication for an absorption site fixed in the orbital frame. Both the spin and the binary modulation are found to be energy dependent. The fact that the spin modulation amplitude in TV Col decreases with energy is confirmed. Hardness ratio variations and the energy-dependent modulation depth during the spin modulation can be explained by partially covered absorbers in the path of the X-ray emission region in the accretion stream. Increased hardness ratio during a broad dip in the intensity at binary phase of 0.75–1.0 confirms the presence of a strong attenuation due to additional absorbers, probably from an impact site of the accretion stream with the disk or magnetosphere.

Preliminary Results on Intermittent Behaviour of Millisecond Pulsar SAX J1808.4-3658

We analyzed RXTE data from the burst of the year 2000 of the X-ray millisecond binary pulsar SAX J 1808.4-3658 with the intent of determining the new orbital parameters.We used the observations of SAX J1808.4-3658 performed by the Rossi X-Ray Timing Explorer (RXTE) when the source was again detected in outburst during the period January-March 2000 (Wijnands et al. 2001). In particular we examined the data from the Proportional Counter Array (РСA) (Jahoda et al. 1996). We first applied barycentric correction to the data using the optical coordinates of the source (Roche et al. 98).

Hard X-Ray Flares Preceding Soft X-Ray Outbursts in Aquila X-1: A Link between Neutron Star and Black Hole State Transitions

We have analyzed Rossi X-Ray Timing Explorer data of the neutron star transient Aquila X-1 obtained during its outbursts in 1999 May/June and 2000 September/October. We find that in the early rise of these outbursts, a hard flare in the energy range above 15 keV preceded the soft X-ray peak. The hard X-ray flux of the hard flares at maximum was more than a factor of 3 stronger than at any other point in the outbursts. The rise of the hard X-ray flare to this maximum was consistent with a monotonically brightening low-/hard-state spectrum. After the peak of the hard flare, a sharp spectral transition occurred with spectral pivoting in the range 8-12 keV. Our timing analysis shows that during the hard flare, the power spectra were composed mainly of band-limited noise and a ~1-20 Hz quasi-periodic oscillation (QPO), which correlate in frequency. Immediately after the hard flare, the power spectra turned into power-law noise. The spectral and timing properties during and after the hard flares are very similar to those in black hole transients during the early rise of an outburst. We suggest that these hard flares and spectral transitions in Aql X-1 are of the same origin as those observed in black hole transients. This leads to the association of the 1-20 Hz QPOs and band-limited noise in Aql X-1 with those in black hole transients. We discuss the impact of this discovery on our understanding of soft X-ray transient outbursts, state transitions, and variability in X-ray binaries.

Orbital and Superorbital Periods of 1E 1740.7−2942 and GRS 1758−258

Five years of Rossi X-Ray Timing Explorer (RXTE) observations of the Galactic black hole candidates 1E 1740.7-2942 and GRS 1758-258 show a periodic modulation with an amplitude of 3%-4% in each source at 12.73 ± 0.05 and 18.45 ± 0.10 days, respectively. We interpret the modulations as orbital, suggesting that the objects have red giant companions. Combining the RXTE data with earlier data (Zhang, Harmon, & Liang) from the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory, we find a long period or quasi-period of about 600 days in 1E 1740.7-2942 and a suggestion of a similar 600 day period in GRS 1758-258. These timescales are longer than any yet found for either precessing systems like Hercules X-1 and SS 433 or binaries like LMC X-3 and Cygnus X-1 with more irregular long periods.

Effects of Resonance in Quasi-periodic Oscillators of Neutron Star Binaries

Using a large quantity of Rossi X-Ray Timing Explorer data presented in the literature, I offer a detailed investigation into the accuracy of the quasi-periodic oscillation (QPO) frequency determination. The QPO phenomenon seen in X-ray binaries is possibly a result of the resonance of the intrinsic (eigen) oscillations and harmonic driving forces of the system. I show that the resonances, in the presence of the damping of oscillations, occur at frequencies that are systematically and randomly shifted with respect to the eigenfrequencies of the system. The shift value strongly depends on the damping rate that is measured by the half-width of the QPO feature. Taking into account this effect, I analyze the QPO data for four Z sources, Scorpius X-I, GX 340+0, GX 5-1, and GX 17+2, and two atoll sources, 4U 1728-34 and 4U 0614+09. The transition-layer model (TLM) predicts the existence of the invariant quantity delta, an inclination angle of the magnetospheric axis with respect to the normal to the disk. I calculate delta and the error bars of delta using the resonance shift, and I find that the inferred delta-values are consistent with constants for these four Z sources, in which horizontal-branch oscillation and kilohertz frequencies have been detected and correctly identified. It is shown that the inferred delta are in the range between 5.5 deg and 6.5 deg. I conclude that the TLM seems to be compatible with the data.

The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed 156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low frequency noise we observed was strongest at intermediate energies, in contrast to the low frequency noise seen in other Z sources. We also detected very low frequency noise, and have calculated complex cross spectra between intensity and hardness. We found that the very low frequency noise is not entirely due to motion along the Z track.

Long‐Term Spectral Variability of Seyfert Galaxies fromRossi X‐Ray Timing ExplorerColor‐Flux Diagrams

We present results from Rossi X-Ray Timing Explorer data obtained during a systematic monitoring program of four Seyfert galaxies (NGC 5548, NGC 5506, MCG -6-30-15, and NGC 4051). We studied the variability of three hardness ratios derived from the light curves in four energy bands (HR1, which describes the continuum variations, and HR2 and HR3, which are sensitive to the iron line and reflection component variations with respect to the continuum, respectively). All the objects show similar spectral variations in all ratios. In order to interpret the results we computed the hardness ratios corresponding to a simple spectral model of a power law plus iron line plus reflection component. In order to derive the model HR2 and HR3 colors, we considered two possibilities: (1) variations with constant line equivalent width and reflection parameter R (the case of a reflecting/reprocessing material that responds with a short delay to the continuum variability) and (2) variations with constant line and reflection flux (the case of a reprocessor that does not respond to the fast, intrinsic variations). The overall, mean observed trends can be explained by spectral slope variations (ΔΓ 0.2-0.3, and 1 for NGC 4051) and a constant-flux Fe line and reflection component, although the existence of a line component that is variable on short timescales cannot be excluded. Finally, we find that the data are not consistent with an increase of R with flux for individual sources, indicating that, as a single source varies, softer spectra do not correspond to larger R values.

RXTEObservations of the Neutron Star Low‐Mass X‐Ray Binary GX 17+2: Correlated X‐Ray Spectral and Timing Behavior

We have analyzed ~600 ks of Rossi X-Ray Timing Explorer data of the neutron star low-mass X-ray binary and Z source GX 17+2. A study was performed of the properties of the noise components and quasi-periodic oscillations (QPOs) as a function of the broadband spectral properties, with the main goal to study the relation between the frequencies of the horizontal branch (HBO) and upper kHz QPOs. It was found that when the upper kHz QPO frequency is below 1030 Hz these frequencies correlate, whereas above 1030 Hz they anticorrelate. GX 17+2 is the first source in which this is observed. We also found that the frequency difference of the high-frequency QPOs was not constant and that the quality factors (Q-values) of the HBO, its second harmonic, and the kHz QPOs are similar and vary almost hand in hand by a factor of more than 3. Observations of the normal branch oscillations during two type I X-ray bursts showed that their absolute amplitude decreased as the flux from the neutron star became stronger. We discuss these and other findings in terms of models that have been proposed for these phenomena. We also compare the behavior of GX 17+2 and other Z sources with that of black hole sources and consider the possibility that the mass accretion rate might not be the driving force behind all spectral and variability changes.

Discovery of X‐Ray Pulsations from the Compact Central Source in the Supernova Remnant 3C 58

We report on high time and spatial resolution observations of the supernova remnant (SNR) 3C 58 using the High Resolution Camera (HRC) on the Chandra X-Ray Observatory. These data show a pointlike central source from which we detect 65.68 ms pulsations at 6.7 σ significance. We interpret these pulsations as corresponding to a young rotation-powered pulsar, PSR J0205+6449, that is associated with and powers SNR 3C 58. Analysis of archival Rossi X-Ray Timing Explorer (RXTE) data from 3 years earlier confirms these pulsations and allows us to determine a spin-down rate of = 1.93 × 10-13 s s-1. Assuming a magnetic dipole model for PSR J0205+6449, we infer a surface magnetic field of 3.6 × 1012 G. The historical age for the pulsar (based on its identification with SN 1181) is 820 yr. Taken with the observed period and spin-down rate, we derive an initial spin period of ~60 ms.

Kilohertz Quasi-periodic Oscillation Frequency Anticorrelated with Megahertz Quasi-periodic Oscillation Flux in 4U 1608-52

We analyzed Rossi X-Ray Timing Explorer data of the low-mass X-ray binary and atoll source 4U 1608-52 obtained on 1996 March 3 in which the source simultaneously showed a strong single kilohertz quasi-periodic oscillation (kHz QPO) around 840 Hz and a 7.5 mHz QPO detected at energies below 5 keV. We find that the frequency of the kHz QPO is approximately anticorrelated with the 2-5 keV X-ray count rate associated with the mHz QPO. The average kHz QPO frequency varies by about 0.6 Hz (0.07%) during a mHz QPO cycle over which the average 2-5 keV count rate varies by about 60 counts s(-1) (4%). This is the opposite of the frequency-count rate correlation observed in the same data on longer timescales and, hence, constitutes the first example of a sign reversal in the frequency-flux correlation related to the origin of the flux. Such a sign reversal is predicted by the radiative disk truncation model for the case where the flux variations originate on the neutron star but are not due to disk accretion rate fluctuations. The results support the nuclear burning interpretation of the mHz QPO and the interpretation of the kHz QPO frequency as an indicator of the orbital frequency at the inner edge of the accretion disk. The varying radiative stresses exerted on the inner disk by the flux due to the quasi-periodic nuclear burning lead to changes in the inner disk radius and, hence, to the observed anticorrelation between kHz QPO frequency and X-ray count rate. We found a time lag of about 10 s of the X-ray count rate relative to the kHz QPO frequency in terms of the anticorrelation between these two quantities, which could be caused by the propagation of the nuclear burning front on the neutron star away from the equatorial region.

Rossi X‐Ray Timing ExplorerSpectrum of A2319

The cluster of galaxies A2319 was observed in 1999 for ~160 ks by the PCA and HEXTE instruments aboard the Rossi X-Ray Timing Explorer (RXTE) satellite. No noticeable variability is seen in the emission measured by either instrument over the ~8 week observation. The quality of the data allows, for the first time, a meaningful search for emission whose spectral properties are distinct from those of the primary thermal emission with (previously measured) mean temperature in the range 8-10 keV. Fitting the RXTE data by a single thermal component, we obtain kT = 8.6 ± 0.1 (all errors are 90% confidence limits), a low iron abundance ZFe ~ 0.16 ± 0.02, and large positive residuals below 6 keV and between 15 to 30 keV. The quality of the fit is drastically improved if a second component is added. A two-temperature model yields kT1 10.1 ± 0.6, kT2 2.8 ± 0.6, and of ZFe ~ 0.23 ± 0.03, which is consistent with previously measured values. An equally good fit is obtained by a combination of a primary thermal and a secondary nonthermal component with kT 8.9 ± 0.6 and a power-law (photon) index α 2.4 ± 0.3. We have repeated the analysis by performing joint fits to both these RXTE measurements and archival ASCA data. At most, 25% of the RXTE secondary component could be present in the ASCA data. Allowing for this difference, very similar results were obtained with only somewhat different values for the temperature (kT = 9.2 ± 0.5) and power-law index (α 2.5 ± 0.3) in the latter model. The deduced value of α is consistent with the measured spectrum of extended radio emission in A2319. Identifying the power-law emission as Compton scattering of the radio-emitting electrons by the cosmic microwave background (CMB), we obtain B ~ 0.1-0.3 μG for the volume-averaged magnetic field, and ~ 4 × 10-14(R/2 Mpc)-3 ergs cm-3 for the mean energy density of the emitting electrons in the central region (radius R) of A2319.

The New X‐Ray Transient SAX J1711.6−3808: Decoupling between Its 3–20 keV Luminosity and Its State Transitions

We present a study of the correlated spectral and timing behavior of the new X-ray transient SAX J1711.6-3808 during its 2001 outburst using data obtained with the Rossi X-Ray Timing Explorer (RXTE). We also investigate the correlations between those source properties and the 3-20 keV X-ray luminosity. The behavior of the source during the observations can be divided into two distinct state types. During the state, the energy spectra are relatively hard and can be described by only a power-law component, and the characteristic frequencies (i.e., the frequency of the 1-7 Hz quasi-periodic oscillations, observed for the first time in this source) in the power spectra are low. However, during the state, the spectra are considerably softer (in addition to the power-law component, a soft component below 8 keV is necessary to fit the spectra), and the frequencies are the highest observed. Remarkably, this distinction into two separate states cannot be extrapolated to also include the 3-20 keV X-ray luminosity. Except for one observation, this luminosity steadily decreased, but the hard state was observed both at the highest and lowest observed luminosities. In contrast, the soft state occurred only at intermediate luminosities. This clearly demonstrates that the state behavior of SAX J1711.6-3808 is decoupled from its X-ray luminosity and that if the X-ray luminosity traces the mass accretion rate in SAX J1711.6-3808, then the state transitions are not good accretion rate indicators, or vice versa. The RXTE data of SAX J1711.6-3808 do not allow us to conclusively determine the exact nature of the compact object in this system. The source resembles both neutron star and black hole systems when they have low luminosities. We discuss our results with respect to the correlated timing and spectral behavior observed in other low-mass X-ray binaries and the implications of our results for the modeling of the outburst light curves of X-ray transients.

Classification of Power Density Spectrum Features and Estimation of the δ-invariant Value for the Z Source GX 340+0

We present a theoretical analysis of Rossi X-Ray Timing Explorer data of Z source GX 340+0 obtained by Jonker et al. In the frameworks of the recently formulated transition layer model, the δ-angle is an angle between the neutron star (NS) magnetospheric axis and the disk (presumably NS rotational) axis. We determine the angle δ = 63 ± 03, which is a combination of the simultaneously observed kilohertz quasi-periodic and horizontal-branch oscillation frequencies. While these three frequencies change by a factor of 3 or more, their δ-combination stays almost constant. GX 340+0 is the fourth source (in addition to 4U 0614+09, Scorpius X-1, and 4U 1702-42) for which δ has been determined. With one (constrained) parameter, at most, we make a complete classification of six observed power spectral features, including the two kilohertz frequencies, the first and second harmonics of the horizontal-branch oscillation frequency, the low-frequency noise component, and the break frequencies. We demonstrate that a new component discovered by Jonker et al. in the GX 340+0 power spectrum is related to the viscous-frequency branch that has in fact been reported earlier in 4U 1728-34 by Ford & van der Klis. Finally, we reclassify several previously misidentified features in the power spectrum.

Normal-Branch Quasi-periodic Oscillations in Scorpius X-1: Viscous Oscillations of a Spherical Shell Near the Neutron Star

We present a comprehensive classification of all observed quasi-periodic oscillations (QPOs) within the framework of the transition layer model using a large set of Rossi X-Ray Timing Explorer data for Scorpius X-1. The model assumes an optically thin material along the observer's line of sight in the horizontal branch and an increasingly optically thick material while in the other two branches that is consistent with X-ray and radio observations and the disk transition layer model of QPOs. We identify the ~6 Hz frequencies in the normal branch as acoustic oscillations of a spherical shell around the neutron star (NS) that is formed after radiation pressure near the Eddington accretion rate destroys the disk. The size of the shell is on the order of one NS radius from the NS. We also estimate the upper limit of Sco X-1's magnetic field to be 0.7 × 106 G at about one NS radius above the NS surface while in the horizontal X-ray branch.

Discovery of a Second High-Frequency Quasi-periodic Oscillation from the Microquasar GRS 1915+105

We report the discovery in archival Rossi X-Ray Timing Explorer data of a ~40 Hz quasi-periodic oscillation (QPO) in the hard X-ray flux from the Galactic microquasar GRS 1915+105. The QPO is detected only in the hard X-ray band above ~13 keV and was discovered in observations in which the previously known 67 Hz QPO is present. The 40 Hz QPO has a typical rms amplitude of ~2% in the 13-27 keV band and a width of ~5.5 Hz (FWHM). We show that the 67 and 40 Hz QPOs are detected in the same observations in 1997 July and November. However, the QPO is not detected in observations from 1996 April, May, and June in which the 67 Hz QPO was first discovered. The frequency of the 67 Hz QPO is significantly higher in the 1997 observations by about 5% compared with the 1996 data. The identification of the 40 Hz QPO makes GRS 1915+105 the second black hole binary to show a pair of simultaneous high-frequency QPOs (the other being GRO J1655-40). The similarities between the properties of the 67 Hz QPO in GRS 1915+105 and the recently discovered 450 Hz QPO in GRO J1655-40 suggest that the pairs of frequencies in these systems may be produced by the same physical mechanism, with the frequency differences between the two being likely due to different black hole masses in the two systems. We discuss the implications of our result for the mass and spin of GRS 1915+105 as well as for models of X-ray variability in black holes and neutron stars.

Angular-Rate Estimation Using Delayed Quaternion Measurements

This paper presents algorithms for estimating the angular-rate vector of satellites using quaternion measurements. Two approaches are compared one that uses differentiated quaternion measurements to yield coarse rate measurements, which are then fed into two different estimators. In the other approach the raw quaternion measurements themselves are fed directly into the two estimators. The two estimators rely on the ability to decompose the non-linear part of the rotas rotational dynamics equation of a body into a product of an angular-rate dependent matrix and the angular-rate vector itself. This non unique decomposition, enables the treatment of the nonlinear spacecraft (SC) dynamics model as a linear one and, thus, the application of a PseudoLinear Kalman Filter (PSELIKA). It also enables the application of a special Kalman filter which is based on the use of the solution of the State Dependent Algebraic Riccati Equation (SDARE) in order to compute the gain matrix and thus eliminates the need to compute recursively the filter covariance matrix. The replacement of the rotational dynamics by a simple Markov model is also examined. In this paper special consideration is given to the problem of delayed quaternion measurements. Two solutions to this problem are suggested and tested. Real Rossi X-Ray Timing Explorer (RXTE) data is used to test these algorithms, and results are presented.

Thermal Comptonization in GRS 1915+105

The Rossi X-ray Timing Explorer data of GRS 1915+105 from several observing periods are modeled with a thermal Comptonization model. Best-fit models indicate that there is a strong correlation between the inner disk temperature and the disk luminosity. The hard Comptonized luminosity does not depend significantly on the total luminosity. The spectral hardness of the Comptonized radiation, the fraction of seed soft photons scattered by the Comptonizing cloud, its Thomson optical depth, and the fraction of the total power dissipated in the optically thin hot plasma, all strongly anticorrelate with the luminosity. We find that the inner disk radius is almost constant and that the hot Comptonizing corona shrinks at high luminosities. We note that the fits using xspec diskbb + power law model underestimate the amplitude of the blackbody component (and therefore the corresponding size of the emitting region) and overestimate the absorption column density and the total, corrected for absorption, luminosity.

Recent results for AGN observed by the Rossi X-ray timing explorer

The Rossi X-ray Timing Explorer (RXTE) has produced many excellent observations of active galaxies, providing so far the best sensitivity over the bandpass of 10 – 20 keV. This paper reports selected RTXE data for X-ray bright Seyfert galaxies in the context of the currently popular models. One is the recent result for two Seyfert 1 galaxies, NGC 5548 and IC 4329a: both show the “canonical” Seyfert 1 X-ray spectra, with an underlying power law, plus iron Kα fluorescence line and Compton reflection. Interestingly, in both cases, the profile of the Fe K line does not extend as far to the red as seen in the famous MCG-6-30-15, and this indicates that the regions where the Fe K lines originate in AGN are diverse. Independently, in both objects we see a strong spectral variability of the primary continua, which soften as the sources brighten. The second result is for the heavily absorbed Seyfert 2 NGC 4945. The RXTE data confirm the strong absorption corresponding to the optical depth to electron scattering of about 2, but also reveal rapid variability of the hard (8 – 30 keV) X-ray emission on a time scale of a day or less. This suggests that for NGC 4945, the putative parsec-size molecular torus cannot be both geometrically and optically thick. It also implies that if the Cosmic X-ray Background is made up of heavily absorbed sources, they are unlikely to have the absorber confined to a geometrically thin disk as inferred for NGC 4945.

Detection of a Compact X-Ray Source in the Supernova Remnant G29.6+0.1: A Variable Anomalous X-Ray Pulsar?

We present follow-up observations of the serendipitously discovered 7 s X-ray pulsar AX J1845-0258, which displays characteristics similar to those observed in the anomalous X-ray pulsars (AXPs). We find a dramatic reduction in its 3-10 keV flux in both new ASCA and Rossi X-Ray Timing Explorer data sets. Within the pulsar's position error locus, we find a faint point source, AX J184453-025640, surrounded by an arc of diffuse X-ray emission. This arc is coincident with the southeast quadrant of the radio shell of the newly discovered supernova remnant G29.6+0.1, recently reported by Gaensler et al. Lack of sufficient flux from the source prevents us from confirming the 7 s pulsed emission observed in the bright state; hence, at present we cannot definitively resolve whether AX J1845-0258 and AX J184453-025640 are one and the same. If they are the same, then the peak-to-peak luminosity changes recorded for AX J1845-0258 may be larger than seen in other AXPs; closer monitoring of this pulsar might lead to a resolution on the mechanism that drives AXPs.