Showing papers by "Deepto Chakrabarty published in 1998"

Neutron Star Mass Measurements. I. Radio Pulsars

Abstract: There are now about fifty known radio pulsars in binary systems, including at least five in double neutron star binaries. In some cases, the stellar masses can be directly determined from measurements of relativistic orbital effects. In others, only an indirect or statistical estimate of the masses is possible. We review the general problem of mass measurement in radio pulsar binaries, and critically discuss all current estimates of the masses of radio pulsars and their companions. We find that significant constraints exist on the masses of twenty-one radio pulsars, and on five neutron star companions of radio pulsars. All the measurements are consistent with a remarkably narrow underlying gaussian mass distribution, m = 1.35 +- 0.04 solar masses. There is no evidence that extensive mass accretion (delta m >~ 0.1 solar mass) has occurred in these systems. We also show that the observed inclinations of millisecond pulsar binaries are consistent with a random distribution, and thus find no evidence for either alignment or counteralignment of millisecond pulsar magnetic fields.

The two-hour orbit of a binary millisecond X-ray pulsar

Abstract: Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. In contrast, millisecond radio pulsars spin very rapidly even though many are billions of years old1. The most compelling explanation is that they have been ‘spun up’ by the transfer of angular momentum during the accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting matter from a companion with mass less than one solar mass2.) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected low-mass X-ray binary system appears to confirm this link3. Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind the low-mass companion star. This system seems closely related to the ‘black-widow’ millisecond radio pulsars, which are evaporating their companions through irradiation4,5,6,7,8. It may appear as an eclipsing radio pulsar during periods of X-ray quiescence.

High-Speed Optical Photometry of the Ultracompact X-Ray Binary 4U 1626–67

Abstract: Rapid UBVRI photometry of the ultracompact low-mass X-ray binary (LMXB) pulsar 4U 1626-67/KZ TrA has detected 130.4 mHz (7.67 s) optical pulsations in all five bands. The optical pulsations, at the same frequency as the X-ray pulsations caused by rotation of the highly magnetized accreting neutron star primary, can be understood as a reprocessing of the pulsed X-ray emission in the accretion disk. The optical pulsed fraction is roughly 6%, independent of wavelength, indicating that the optical emission is dominated by X-ray reprocessing. A weaker (1.5%) sideband, downshifted 0.395(15) mHz from the main optical pulsation, is also present. This is consistent with a previously reported sideband downshifted 0.4011(21) mHz from the main pulsation, corroborating the 42 minute binary period proposed by Middleditch et al. A 0.048 Hz optical quasi-periodic oscillation (QPO), corresponding to a previously reported X-ray feature, was also detected in some of the observations, with a fractional rms amplitude of 3%-5%. This is the first measurement of an optical QPO in an X-ray binary pulsar. I discuss constraints on the nature of the mass donor and show that mass transfer via a radiatively driven wind is inconsistent with the optical data. I also review the basic theory of X-ray-heated accretion disks and show that such models provide a good fit to the optical photometry. If the effective X-ray albedo of LMXB accretion disks is as high as recently reported (ηd 0.9), then the optical data imply a distance of ~8 kpc and an X-ray luminosity of ≈ 1037 ergs s-1.

The Outbursts and Orbit of the Accreting Pulsar GS 1843-02 = 2S 1845-024

Abstract: We present observations of a series of 10 outbursts of pulsed hard X-ray flux from the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. These outbursts occurred regularly every 242 days, coincident with the ephemeris of the periodic transient GRO J1849-03 (Zhang et al. 1996), which has recently been identified with the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Our pulsed detection provides the first clear identification of GS 1843-02 with 2S 1845-024. We present a pulse timing analysis which shows that the 2S 1845-024 outbursts occur near the periastron passage of the neutron star's highly eccentric (e = 0.88+-0.01) 242.18+-0.01 day period binary orbit about a high mass (M > 7 solar masses) companion. The orbit and transient outburst pattern strongly suggest the pulsar is in a binary system with a Be star. Our observations show a long-term spin-up trend, with most of the spin-up occurring during the outbursts. From the measured spin-up rates and inferred luminosities we conclude that an accretion disk is present during the outbursts.

Warped disks as a possible origin of torque reversals in accretion-powered pulsars

Abstract: Enigmatic transitions between spin-up and spin-down have been observed in several X-ray pulsars accreting matter via an accretion disk In these transitions, the torque changes sign but remains at nearly the same magnitude It has been noted previously that alternating prograde and retrograde disk flows would explain many features of the torque reversals, although it has been unclear how a stable retrograde disk could be formed We suggest that the reversals may be related to the disk at times being warped to such an extent that the inner region becomes tilted by more than 907 This region would thus become retrograde, leading to a negative torque Accretion disk models can show such behavior, if account is taken of a warping instability due to irradiation The resulting “flipovers” of the inner parts of the disk can reproduce most characteristics of the observations, although it remains unclear what sets the timescale on which the phenomenon occurs If this model were correct, it would have a number of ramifications, for instance, that in the spin-down state, the X-ray source would mostly be observed through the accretion disk Subject headings: accretion, accretion disks — binaries: close — pulsars: individual (Centaurus X-3, GX 114, OAO 16572415, 4U 1626267) — stars: neutron — X-rays: stars

Discovery of the 198 Second X-Ray Pulsar GRO J2058+42

Abstract: GRO J2058+42, a transient 198 s X-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mcrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 to 196 s during the 46 day outburst. The pulse shape evolved over the course of the outburst and exhibited energy-dependent variations. BATSE observed five additional weak outbursts from GRO J2058 + 42, each with a 2 week duration and a peak-pulsed flux of about 15 mcrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-Ray Timing Explorer (RXTE) proportional counter array (PCA) localized the source to within a 4 s radius error circle (90% confidence) centered on R.A. = 20h 59m.0, decl. = 41 deg 43 s (J2000). Additional shorter outbursts with peak-pulsed fluxes of about 8 mcrab were detected by BATSE halfway between the first four 15 mcrab outbursts. The RXTE All-Sky Monitor detected all eight weak outbursts with approximately equal durations and intensities. GRO J2058 + 42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron, No optical counterpart has been identified to date, and no X-ray source was present in the error circle in archival ROSAT observations.

Infrared Spectroscopy of GX 1+4/V2116 Ophiuchi: Evidence for a Fast Red Giant Wind?

Abstract: We present infrared spectroscopy of the low-mass X-ray binary GX 1+4/V2116 Ophiuchi. This symbiotic binary consists of a 2 minute accretion-powered pulsar and an M5 III red giant. A strong He I 1.083 μm emission line with a pronounced P Cygni profile was observed. From the blue edge of this feature, we infer an outflow velocity of 250±50 km s−1. This is an order of magnitude faster than a typical red giant wind, and we suggest that radiation from the accretion disk or the neutron star may contribute to the acceleration of the outflow. We infer a wind mass-loss rate of ~10-6 M☉ yr−1. Accretion from such a strong stellar wind provides a plausible alternative to Roche lobe overflow for supplying the accretion disk that powers the X-ray source. The H I Paβ and He I 1.083 μm emission lines show no evidence for the dramatic variability previously reported in some optical lines and no evidence for pulsations at the 2 minute pulsar period.

Infrared Spectroscopy of GX 1+4/V2116 Oph: Evidence for a Fast Red Giant Wind?

Abstract: We present infrared spectroscopy of the low-mass X-ray binary GX 1+4/V2116 Oph. This symbiotic binary consists of a 2-min accretion-powered pulsar and an M5 III red giant. A strong He I 1.083 micron emission line with a pronounced P Cygni profile was observed. From the blue edge of this feature, we infer an outflow velocity of 250(50) km/s. This is an order of magnitude faster than a typical red giant wind, and we suggest that radiation from the accretion disk or the neutron star may contribute to the acceleration of the outflow. We infer a wind mass loss rate of around 10^-6 Msun/yr. Accretion from such a strong stellar wind provides a plausible alternative to Roche lobe overflow for supplying the accretion disk which powers the X-ray source. The H I Paschen beta and He I 1.083 micron lines showed no evidence for the dramatic changes previously reported in some optical lines, and no evidence for pulsations at the 2-min pulsar period.

Discovery of the 198 s X-ray Pulsar GRO J2058+42

Abstract: GRO J2058+42, a transient 198 second x-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO), during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mCrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 s to 196 s during the 46-day outburst. The pulse shape evolved over the course of the outburst and exhibited energy dependent variations. BATSE observed five additional weak outbursts from GRO J2058+42, each with two week duration and peak pulsed flux of about 15 mCrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) localized the source to within a 4\arcmin radius error circle (90 % confidence) centered on R.A. = 20h 59m.0, Decl. = 41 deg 43 arcmin (J2000). Additional shorter outbursts with peak pulsed fluxes of about 8 mCrab were detected by BATSE halfway between the first four 15 mCrab outbursts. The RXTE All-Sky Monitor detected all 8 weak outbursts with approximately equal durations and intensities. GRO J2058+42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron. No optical counterpart has been identified to date and no x-ray source was present in the error circle in archival ROSAT observations.

Sidebands due to Quasi-periodic Oscillations in 4U 1626–67

Abstract: The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic oscillations (QPOs) and red noise variability, as well as coherent pulsations at the 0.130 Hz neutron star spin frequency. Power density spectra of observations made with the Rossi X-Ray Timing Explorer show significant sidebands separated from the pulsar spin frequency (and its harmonics) by the QPO frequency. These show that the instantaneous amplitude of the coherent pulsations is modulated by the amplitude of the QPOs. This phenomenon is expected in models such as the magnetospheric beat frequency model, where the QPOs originate near the polar caps of the neutron star. In the 4-8 keV energy range, however, the lower frequency sidebands are significantly stronger than their higher frequency complements; this is inconsistent with the magnetospheric beat frequency model. We suggest that the 0.048 Hz QPOs are produced instead by a structure orbiting the neutron star at the QPO frequency. This structure crosses the line of sight once per orbit and attenuates the pulsar beam, producing the symmetric (amplitude modulation) sidebands. It also reprocesses the pulsar beam at the beat frequencies between the neutron star spin frequency and the QPOs, producing the excess variability observed in the lower frequency sidebands. Quite independently, we find no evidence that the red noise variability modulates the amplitude of the coherent pulsations. This is also in contrast to the expectations of the magnetospheric beat frequency model and differs from the behavior in some high-mass X-ray binary pulsars.

Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67

Abstract: The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic oscillations (QPOs) and red noise variability as well as coherent pulsations at the 0.130 Hz neutron star spin frequency. Power density spectra of observations made with the Rossi X-ray Timing Explorer show significant sidebands separated from the pulsar spin frequency (and its harmonics) by the QPO frequency. These show that the instantaneous amplitude of the coherent pulsations is modulated by the amplitude of the QPOs. This phenomenon is expected in models such as the magnetospheric beat frequency model where the QPOs originate near the polar caps of the neutron star. In the 4--8 keV energy range, however, the lower-frequency sidebands are significantly stronger than their higher-frequency complements; this is inconsistent with the magnetospheric beat frequency model. We suggest that the 0.048 Hz QPOs are instead produced by a structure orbiting the neutron star at the QPO frequency. This structure crosses the line of sight once per orbit and attenuates the pulsar beam, producing the symmetric (amplitude modulation) sidebands. It also reprocesses the pulsar beam at the beat frequencies between the neutron star spin frequency and the QPOs, producing the excess variability observed in the lower-frequency sidebands. Quite independently, we find no evidence that the red noise variability modulates the amplitude of the coherent pulsations. This is also in contrast to the expectations of the magnetospheric beat frequency model and differs from the behavior in some high-mass X-ray binary pulsars.

A Search for Optical Pulsations from Two Young Southern Pulsars

Abstract: We report on high-speed optical photometry of the radio positions of two young rotation-powered pulsars. No pulsations were detected from the optical counterpart proposed by Caraveo et al. for PSR B1509-58, with a 2 σ upper limit on the pulsed fraction of less than 12%, significantly lower than that measured in the five known optical pulsars. Given its low pulsed fraction, high optical luminosity, and significant (8%) chance coincidence probability, we suggest that this candidate is not associated with the pulsar. We also find that the still unidentified optical counterpart of PSR B1706-44 has R18 and lies within 3'' of an R=16.6 star.

A Search for Optical Pulsations from Two Young Southern Pulsars

Abstract: We report on high-speed optical photometry of the radio positions of two young rotation-powered pulsars. No pulsations were detected from the optical counterpart proposed by Caraveo et al. (1994) for PSR B1509-58, with a 2 sigma upper limit on the pulsed fraction of 12 percent, significantly lower than that measured in the five known optical pulsars. Given its low pulsed fraction, high optical luminosity, and significant (8 percent) chance coincidence probability, we suggest that this candidate is not associated with the pulsar. We also find that the still-unidentified optical counterpart of PSR B1706-44 has R>18 and lies within 3 arcsec of an R=16.6 star.

The Disk-Magnetosphere Interaction in the Accretion-Powered Millisecond Pulsar SAX J1808.4-3658

Abstract: The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer provides the first direct probe of the interaction of an accretion disk with the magnetic field of a weakly magnetic (B<10^10 G) neutron star. We demonstrate that the presence of coherent pulsations from a weakly magnetic neutron star over a wide range of accretion rates places strong constraints on models of the disk-magnetosphere interaction. We argue that the simple Mdot^(3/7) scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4-3658 for most proposed equations of state for stable neutron stars. We show that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we obtain a robust estimate of mu~(1-10)x10^26 G cm^3 for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of ~10^8-10^9 G at the stellar equator. We therefore infer that after the end of its accretion phase, this source will become a normal millisecond radio pulsar. Finally, we compare the physical properties of this pulsar to those of the non-pulsing, weakly magnetic neutron stars in low-mass X-ray binaries and argue that the absence of coherent pulsations from the latter does not necessarily imply that these neutron stars have significantly different magnetic field strengths from SAX J1808.4-3658.