Energy-dependent Pulse Profiles Research Articles

Optical and X-ray studies of Be/X-ray binary 1A 0535+262 during its 2020 giant outburst

ABSTRACT We report results obtained from the optical and X-ray studies of the Be/X-ray binary 1A 0535+262/HD 245770 during the 2020 October giant X-ray outburst, using the 1.2-m telescope at Mount Abu Infrared observatory and AstroSat, respectively. The peak flux of the outburst was recorded to be ∼11 Crab in the 15–50 keV range, the highest ever observed from the pulsar. We performed optical observations in the 6000–7200 Å band before, during, and after the outburst to investigate the evolution of the circumstellar disc of the Be star between 2020 February and 2022 February. Our optical spectra exhibit prominent emission lines at 6563 Å (H i), 6678 Å (He i), and 7065 Å (He i). We found a significantly variable H α line in the spectra. The single-peaked line profile appeared asymmetric with broad red- and blue-wings in the data before and during the outburst. The post-outburst observations, however, resulted in a double-peaked profile with asymmetry in the blue-wing. Our observations before the outburst confirmed a larger Be disc that decreased in size as the outburst progressed. Furthermore, the observed variabilities in the H α line profile and parameters suggest the presence of a highly misaligned, precessing, and warped Be disc. AstroSat observation of the pulsar detected pulsations at ∼103.55 s in the light curve up to 110 keV. We found strongly energy-dependent pulse profiles with increasing contribution of the pulsing component in hard X-rays. The broad-band spectral fitting in the 0.7–90.0 keV range confirmed the presence of the known cyclotron resonance scattering feature at ∼46.3 keV.

Measuring the Nonaxially Symmetric Surface Temperature Distribution of the Central Compact Object in Puppis A

Abstract The surface temperature distributions of central compact objects (CCOs) are powerful probes of their crustal magnetic field strengths and geometries. Here we model the surface temperature distribution of RX J0822−4300, the CCO in the Puppis A supernova remnant, using 471 ks of XMM-Newton data. We compute the energy-dependent pulse profiles in 16 energy bands, fully including the general relativistic effects of gravitational redshift and light bending, to accurately model the two heated surface regions of different temperatures and areas, in addition to constraining the viewing geometry. This results in precise measurements of the two temperatures: kT warm = ( 1 + z ) × 0.222 − 0.019 + 0.018 keV and kT hot = (1 + z) × 0.411 ± 0.011 keV. The two heated surface regions are likely located very close to the rotational poles, with the most probable position of the hotter component ≈ 6° from the rotational pole. For the first time, we are able to measure a deviation from a pure antipodal hot-spot geometry, with a longitudinal offset δ γ = 11 .° 7 − 2 .° 5 + 2 .° 6 . The discovery of this asymmetry, along with the factor of ≈2 temperature difference between the two emitting regions, may indicate that RX J0822−4300 was born with a strong, tangled crustal magnetic field.

The X-ray evolution and geometry of the 2018 outburst of XTE J1810−197

ABSTRACT After 15 yr, in late 2018, the magnetar XTE J1810−197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM–Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of 2 change in its spin-down rate from ∼7.2 × 10−12 to ∼1.5 × 10−11 s s−1 after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810−197 is well modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810−197 has likely evolved relative to that found for the 2003 event.

Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat

ABSTRACT We present the results obtained from the analysis of high-mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in July 2015 and 2017, respectively. X-ray pulsations at ≈604 s are clearly detected in our study. Based on the long-term spin-frequency evolution, the source is found to spun-up in the last 17 yr. We observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft X-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 keV. This behaviour ensured a positive correlation between the energy and pulse fraction. The pulse profile morphology and its energy evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. The broad-band energy spectrum of the pulsar is approximated by an absorbed high-energy exponential cut-off power-law model with iron emission lines. In contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the X-ray spectra. We performed phase-resolved spectroscopy using data from the NuSTAR observation. Our results showed a clear signature of absorbing material at certain pulse phases of the pulsar. These findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. We also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4U 1909+07 depending on the global spin-up rate.

Temporal and spectral X-ray properties of magnetar SGR 1900+14 derived from observations with NuSTAR and XMM-Newton

Abstract X-ray observations play a crucial role in understanding the emission mechanism and relevant physical phenomena of magnetars. We report on X-ray observations made in 2016 of a young magnetar, SGR 1900+14, which is famous for a giant flare in 1998 August. Simultaneous observations were conducted with XMM-Newton and NuSTAR on 2016 October 20 with 23 and 123 ks exposures, respectively. The NuSTAR hard X-ray coverage enabled us to detect the source up to 70 keV. The 1–10 keV and 15–60 keV fluxes were $3.11(3)\times 10^{-12} \, {\rm erg \, s^{-1} \, cm^{-2}}$ and $6.8(3)\times 10^{-12} \, {\rm erg \, s^{-1} \, cm^{-2}}$, respectively. The 1–70 keV spectra were fitted well by a blackbody plus power-law model with a surface temperature of $kT=0.52(2) \, {\rm keV}$, a photon index of the hard power-law of Γ = 1.21(6), and a column density of $N_{\,\rm H}=1.96(11)\times 10^{22} \, {\rm cm^{-2}}$. Compared with previous observations with Suzaku in 2006 and 2009, the 1–10 keV flux showed a decrease by 25%–40%, while the spectral shape did not show any significant change with differences of kT and NH being within 10% of each other. Through timing analysis, we found that the rotation period of SGR 1900+14 on 2016 October 20 was $5.22669(3) \, {\rm s}$. The long-term evolution of the rotation period shows a monotonic decrease in the spin-down rate $\dot{P}$ lasting for more than 15 yr. We also found characteristic behavior of the hard-tail power-law component of SGR 1900+14. The energy-dependent pulse profiles vary in morphology with a boundary of 10 keV. The phase-resolved spectra show the differences between photon indices (Γ = 1.02–1.44) as a function of the pulse phase. Furthermore, the photon index is positively correlated with the X-ray flux of the hard power-law component, which could not be resolved by the previous hard X-ray observations.

Deep Chandra Survey of the Small Magellanic Cloud. II. Timing Analysis of X-Ray Pulsars

Abstract We report the timing analysis results of X-ray pulsars from a recent deep Chandra survey of the Small Magellanic Cloud (SMC). We analyzed a total exposure of 1.4 Ms from 31 observations over a 1.2 deg2 region in the SMC under a Chandra X-ray Visionary Program. Using the Lomb–Scargle and epoch-folding techniques, we detected periodic modulations from 20 pulsars and a new candidate pulsar. The survey also covered 11 other pulsars with no clear sign of periodic modulation. The 0.5–8 keV X-ray luminosity (L X ) of the pulsars ranges from 1034 to 1037 erg s−1 at 60 kpc. All of the Chandra sources with L X ≳ 4 × 1035 erg s−1 exhibit X-ray pulsations. The X-ray spectra of the SMC pulsars (and high-mass X-ray binaries) are in general harder than those of the SMC field population. All but SXP 8.02 can be fitted by an absorbed power-law model with a photon index of Γ ≲ 1.5. The X-ray spectrum of the known magnetar SXP 8.02 is better fitted with a two-temperature blackbody model. Newly measured pulsation periods of SXP 51.0, SXP 214, and SXP 701, are significantly different from the previous XMM-Newton and RXTE measurements. This survey provides a rich data set for energy-dependent pulse profile modeling. Six pulsars show an almost eclipse-like dip in the pulse profile. Phase-resolved spectral analysis reveals diverse spectral variations during pulsation cycles: e.g., for an absorbed power-law model, some exhibit an (anti)-correlation between absorption and X-ray flux, while others show more intrinsic spectral variation (i.e., changes in photon indices).

A multi-model approach to X-ray pulsars

The emission characteristics of X-ray pulsars are governed by magnetospheric accretion within the Alfven radius, leading to a direct coupling of accretion column properties and interactions at the magnetosphere. The complexity of the physical processes governing the formation of radiation within the accreted, strongly magnetized plasma has led to several sophisticated theoretical modelling efforts over the last decade, dedicated to either the formation of the broad band continuum, the formation of cyclotron resonance scattering features (CRSFs) or the formation of pulse profiles. While these individual approaches are powerful in themselves, they quickly reach their limits when aiming at a quantitative comparison to observational data. Too many fundamental parameters, describing the formation of the accretion columns and the systems’ overall geometry are unconstrained and different models are often based on different fundamental assumptions, while everything is intertwined in the observed, highly phase-dependent spectra and energy-dependent pulse profiles. To name just one example: the (phase variable) line width of the CRSFs is highly dependent on the plasma temperature, the existence of B-field gradients (geometry) and observation angle, parameters which, in turn, drive the continuum radiation and are driven by the overall two-pole geometry for the light bending model respectively. This renders a parallel assessment of all available spectral and timing information by a compatible across-models-approach indispensable. In a collaboration of theoreticians and observers, we have been working on a model unification project over the last years, bringing together theoretical calculations of the Comptonized continuum, Monte Carlo simulations and Radiation Transfer calculations of CRSFs as well as a General Relativity (GR) light bending model for ray tracing of the incident emission pattern from both magnetic poles. The ultimate goal is to implement a unified fitting model for phase-resolved spectral and timing data analysis. We present the current status of this project.

A DOUBLE-PEAKED OUTBURST OF A 0535+26 OBSERVED WITH INTEGRAL , RXTE , AND SUZAKU

The Be/X-ray binary A 0535+26 showed a normal (type I) outburst in August 2009. It is the fourth in a series of normal outbursts associated with the periastron, but is unusual by presenting a double-peaked light curve. The two peaks reached a flux of ~450 mCrab in the 15-50 keV range. We present results of the timing and spectral analysis of INTEGRAL, RXTE, and Suzaku observations of the outburst. The energy dependent pulse profiles and their evolution during the outburst are studied. No significant differences with respect to other normal outbursts are observed. The centroid energy of the fundamental cyclotron line shows no significant variation during the outburst. A spectral hardening with increasing luminosity is observed. We conclude that the source is accreting in the sub-critical regime. We discuss possible explanations for the double-peaked outburst.

TIMING AND SPECTRAL PROPERTIES OF Be/X-RAY PULSAR EXO 2030+375 DURING A TYPE I OUTBURST

We present results from a study of broadband timing and spectral properties of EXO 2030+375 using a Suzaku observation. Pulsations with a period of 41.41 s and strong energy dependent pulse profiles were clearly detected up to ~100 keV. Narrow dips are seen in the profiles up to ~70 keV. Presence of prominent dips at several phases in the profiles up to such high energy ranges were not seen before. At higher energies, these dips gradually disappeared and the profile appeared single-peaked. The 1.0-200.0 keV broad-band spectrum is found to be well described by a partial covering high energy cut-off power-law model. Several low energy emission lines are also detected in the pulsar spectrum. We fitted the spectrum using neutral as well as partially ionized absorbers along with above continuum model yielding similar parameter values. The partial covering with partially ionized absorber resulted into marginally better fit. The spectral fitting did not require any cyclotron feature in the best fit model. To investigate the changes in spectral parameters at dips, we carried out pulse-phase-resolved spectroscopy. During the dips, the value of additional column density was estimated to be high compared to other pulse phases. While using partially ionized absorber, the value of ionization parameter is also higher at the dips. This may be the reason for the presence of dips up to higher energies. No other spectral parameters show any systematic variation with pulse phases of the pulsar.

AN ACCRETION MODEL FOR THE ANOMALOUS X-RAY PULSAR 4U 0142+61

We propose that the quiescent emission of AXPs/SGRs is powered by accretion from a fallback disk, requiring magnetic dipole fields in the range 10^{12}-10^{13} G, and that the luminous hard tails of their X-ray spectra are produced by bulk-motion Comptonization in the radiative shock near the bottom of the accretion column. This radiation escapes as a fan beam, which is partly absorbed by the polar cap photosphere, heating it up to relatively high temperatures. The scattered component and the thermal emission from the polar cap form a polar beam. We test our model on the well-studied AXP 4U 0142+61, whose energy-dependent pulse profiles show double peaks, which we ascribe to the fan and polar beams. The temperature of the photosphere (kT~0.4 keV) is explained by the heating effect. The scattered part forms a hard component in the polar beam. We suggest that the observed high temperatures of the polar caps of AXPs/SGRs, compared with other young neutron stars, are due to the heating by the fan beam. Using beaming functions for the fan beam and the polar beam and taking gravitational bending into account, we fit the energy-dependent pulse profiles and obtain the inclination angle and the angle between the spin axis and the magnetic dipole axis, as well as the height of the radiative shock above the stellar surface. We do not explain the high luminosity bursts, which may be produced by the classical magnetar mechanism operating in super-strong multipole fields.

4U 0115 + 63: phase lags and cyclotron resonant scattering

High mass X-ray binaries are among the brightest objects of our Galaxy in the high energy domain (0.1-100 keV). Despite our relatively good knowledge of their basic emission mechanisms, the complex problem of understanding their time and energy dependent X-ray emission is not completely solved. <P> In this paper, we study the energy dependent pulse profiles of the high mass X-ray binary pulsar 4U 0115+63 to investigate how they are affected by cyclotron resonant scattering. <P> We analyze archival BeppoSAX and RXTE observations performed during the giant outburst of the source which occurred in 1999. We exploit a cross correlation technique to compare the pulse profiles in different energy ranges and develop a relativistic ray-tracing model to interpret our findings. We also study the phase dependency of the cyclotron absorption features by performing phase resolved spectroscopy. <P> The pulse profiles of 4U 0115+63 displayed clear "phase-lags" at energies close to those of the cyclotron absorption features that characterize the X-ray emission of the source. We reproduce qualitatively this phenomenon by assuming an energy dependent beaming of the emission from the column surface and verify that our model is also compatible with the results of phase resolved spectral analysis. <P> We showed that cyclotron resonant scattering affects the pulse profile formation mechanisms in a complex way, which necessitates both improvements in the modeling and the study of other sources to be better understood.

Analyzing X-ray pulsar profiles: geometry and beam pattern of A 0535+26

We applied a decomposition method to the energy dependent pulse profiles of the accreting binary pulsar A 0535+26, in order to identify the contribution of the two magnetic poles of the neutron star and to obtain constraints on the geometry of the system and on the beam pattern. We analyzed pulse profiles obtained from RXTE observations in the X-ray regime. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles and that the emission regions have axisymmetric beam patterns. Constraints on the geometry of the pulsar and a possible solution of the beam pattern are given. We interpreted the reconstructed beam pattern in terms of a geometrical model of a hollow column plus a halo of scattered radiation on the neutron star surface, which includes relativistic light deflection.

4U 1909+07: a Hidden Pearl

We present a detailed spectral and timing analysis of the High Mass X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U1909+07 is a persistent accreting X-ray pulsar with a period of approximately 605 s. The period changes erratically consistent with a random walk expected for a wind accreting system. INTEGRAL detects the source with an average of 2.4 cps (corresponding to 15mCrab), but sometimes exhibits flaring activity up to 50 cps (i.e. 300mCrab). The strongly energy dependent pulse profile shows a double peaked structure at low energies and only a single narrow peak at energies above 20 keV. The phase averaged spectrum is well described by a powerlaw modified at higher energies by an exponential cutoff and photoelectric absorption at low energies. In addition at 6.4 keV a strong iron fluorescence line and at lower energies a blackbody component are present. We performed phase resolved spectroscopy to study the pulse phase dependence of the spectral parameters: while most spectral parameters are constant within uncertainties, the blackbody normalization and the cutoff folding energy vary strongly with phase.

MODELING THE SURFACE X-RAY EMISSION AND VIEWING GEOMETRY OF PSR J0821–4300 IN PUPPIS A

We show that a pair of thermal, antipodal hot-spots on the neutron star surface is able to fully account for the pulsar's double blackbody spectrum and energy-dependent pulse profile, including the observed 180 degree phase reversal at approximately 1.2 keV. By comparing the observed pulse modulation and phase to the model predictions, we strongly constrain the hot-spot pole (xi) and the line-of-sight (psi) angles with respect to the spin axis. For a nominal radius of R = 12 km and distance D = 2.2 kpc, we find (xi,psi) = (86d,6d), with 1-sigma error ellipse of (2d,1d); this solution is degenerate in the two angles. The best-fit spectral model for this geometry requires that the temperatures of the two emission spots differ by a factor of 2 and their areas by a factor of ~ 20. Including a cosine-beamed pattern for the emitted intensity modifies the result, decreasing the angles to (84d,3d); however this model is not statistically distinguishable from the isotropic emission case. We also present a new upper limit on the period derivative of Pdot < 3.5E-16 (2-sigma), which limits the global dipole magnetic field to B_s < 2.0E11 G, confirming PSR J0821-4300 as an "anti-magnetar." We discuss the results in the context of observations and theories of nonuniform surface temperature on isolated NSs of both weak and strong magnetic field. To explain the nonuniform temperature of PSR J0821-4300 may require a crustal field that is much stronger than the external, global dipole field.

High‐Altitude Emission from Pulsar Slot Gaps: The Crab Pulsar

We present results of a 3D model of optical to gamma-ray emission from the slot gap accelerator of a rotation-powered pulsar. Primary electrons accelerating to high-altitudes in the unscreened electric field of the slot gap reach radiation-reaction limited Lorentz factors of 2 x 10^7, while electron-positron pairs from lower-altitude cascades flow along field lines interior to the slot gap. The curvature, synchrotron and inverse Compton radiation of both primary electrons and pairs produce a broad spectrum of emission from infra-red to GeV energies. Both primaries and pairs undergo cyclotron resonant absorption of radio photons, allowing them to maintain significant pitch angles. Synchrotron radiation from pairs with a power-law energy spectrum with Lorentz factors 10^2 - 10^5, dominate the spectrum up to 10 MeV. Synchrotron and curvature radiation of primaries dominates from 10 MeV up to a few GeV. We examine the energy-dependent pulse profiles and phase-resolved spectra for parameters of the Crab pulsar as a function of magnetic inclination and viewing angle, comparing to broad-band data. In most cases, the pulse profiles are dominated by caustics on trailing field lines. We also explore the relation of the high-energy and the radio profiles, as well as the possibility of caustic formation in the radio cone emission. We find that the Crab pulsar profiles and spectrum can be reasonably well reproduced by a model with viewing angle 45 degrees and inclination angle 100 or 80 degrees. This model predicts that the slot gap emission below 200 MeV will exhibit correlations in time and phase with the radio emission.

The pre-outburst flare of the A 0535+26 August/September 2005 outburst

Aims. We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a “pre-outburst flare” which took place ∼5 d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst. Methods. We analyse RXTE observations that monitored A 0535+26 during the outburst. We complete spectral and timing analyses of the data. We study the evolution of the pulse period, present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves. Results. We present three main results: a constant period P = 103.3960(5) s is measured until periastron passage, followed by a spin-up with a decreasing period derivative of u P = (−1.69 ± 0.04) × 10 −8 ss −1 at MJD 53 618, and P remains constant again at the end of the main outburst. The spin-up provides evidence for the existence of an accretion disk during the normal outburst. We measure a CRSF energy of Ecyc ∼ 50 keV during the pre-outburst flare, and Ecyc ∼ 46 keV during the main outburst. The pulse shape, which varies significantly during both pre-outburst flare and main outburst, evolves strongly with photon energy.

The Spectral Evolution of Transient Anomalous X‐Ray Pulsar XTE J1810−197

We present a multiepoch spectral study of the transient anomalous X-ray pulsar XTE J1810-197 obtained with the X-Ray Multi-Mirror Mission (XMM-Newton). Four observations taken over the course of a year reveal strong spectral evolution as the source fades from outburst. The origin of this is traced to the individual decay rates of the pulsar's spectral components. A two-temperature fit at each epoch requires that the temperatures remain nearly constant at kT1 = 0.25 keV and kT2 = 0.67 keV, while the luminosities of these components decrease exponentially with τ1 = 900 and τ2 = 300 days, respectively. The integrated outburst energy is E1 = 1.3 × 1042d ergs and E2 = 3.9 × 1042d ergs for the two spectral components, respectively. One possible interpretation of the XMM-Newton observations is that the slowly decaying cooler component is the radiation from a deep heating event that affected a large fraction of the crust, while the hotter component is powered by external surface heating at the footpoints of twisted magnetic field lines by magnetospheric currents that are decaying more rapidly. The energy-dependent pulse profile of XTE J1810-197 is well modeled at all epochs by the sum of a broad pulse that dominates in the soft X-rays and a narrower one at higher energies. These profiles peak at the same phase, suggesting a concentric emission geometry on the neutron star surface. The spectral and pulse evolution together argue against the presence of a significant power-law contribution to the X-ray spectrum below 8 keV. The extrapolated flux is projected to return to the historic quiescent level, characterized by an even cooler blackbody spectrum, by the year 2007.

Orbital Decay in LMC X‐4

We report on the results of observations of the binary X-ray pulsar LMC X-4 with the Rossi X-Ray Timing Explorer. Our analysis of the Doppler delays of the 13.5 s X-ray pulsations yields the most accurate determination of the LMC X-4 orbital parameters available to date. The epoch of orbital phase zero for the 1.4 day orbit is determined with an uncertainty of ~20 s and is combined with five earlier determinations of the epoch of phase zero to obtain the rate of change in the orbital period: orb/Porb = (-9.8 ± 0.7) × 10-7 yr-1. This is the first high significance measurement of the rate of change of the orbital period in LMC X-4. We present data on one of three strong X-ray flares as well as energy-dependent pulse profiles for both nonflaring and flaring time intervals. The pulse profiles during the nonflaring time intervals are typically strikingly different from the flare profiles but at other times can be similar. Possible origins of the flaring behavior are discussed. We reconsider the orbital decay of LMC X-4 in the context of tidal evolution. We find that while the orbital decay is most likely driven by tidal interactions, the asynchronism between the orbit and the rotation of the companion star is probably maintained by the evolutionary expansion of the companion star, just as is thought to be the case for Cen X-3 and SMC X-1. For LMC X-4, we find that the evidence favors the companion star being in a late stage of its evolution on the main sequence. The orbital decay timescale of about 1,000,000 yr implies that the Roche lobe will move catastrophically deep into the atmosphere of the companion within a few times 104 yr. This short X-ray lifetime must be similar to the lifetimes of SMC X-1 and Cen X-3 which have decay timescales of 300,000 and 550,000 yr, respectively, and may be typical of all massive X-ray binaries in Roche lobe or near Roche lobe contact.

Analyzing X‐Ray Pulsar Profiles: Geometry and Beam Pattern of Hercules X‐1

We report on our analysis of a large sample of energy-dependent pulse profiles of the X-ray binary pulsar Hercules X-1. We find that all data are compatible with the assumption of a slightly distorted magnetic dipole field as sole cause of the asymmetry of the observed pulse profiles. Further, the analysis provides evidence that the emission from both poles is equal. We determine an angle Θm < 20° between the rotation axis and the local magnetic axis. One pole has an offset δ < 5° from the antipodal position of the other pole. The beam pattern shows structures that can be interpreted as pencil- and fan-beam configurations. Since no assumptions on the polar emission are made, the results can be compared with various emission models. A comparison of results obtained from pulse profiles of different phases of the 35 day cycle indicates different attenuation of the radiation from the poles being responsible for the change of the pulse shape during the main-on state. These results also suggest the resolution of an ambiguity within a previous analysis of pulse profiles of Cen X-3, leading to a unique result for the beam pattern of this pulsar as well. The analysis of pulse profiles of the short-on state indicates that a large fraction of the radiation cannot be attributed to the direct emission from the poles. We give a consistent explanation of both the evolution of the pulse profile and the spectral changes with the 35 day cycle in terms of a warped precessing accretion disk.

4U 1626-67 - The binary with the smallest known mass function

The pulsing X-ray source 4U 1626-67 is an accreting neutron star in a binary system with a very low mass companion. The source was observed by Exosat continuously for 23 hr on March 30-31, 1986 UT. It is found that, if the orbital inclination angle equals 90 deg, the optical companion star has a mass of less than about 0.002 solar mass; however, it is found that a companion star mass of greater than about 0.06 solar mass is required if gravitational radiation is responsible for driving the mass transfer in this system. Also presented are results on flaring activity in the system on time scales of about 1000 s, the energy-dependent pulse profiles, and the pulse period history over the past decade.