Counter Array Research Articles

The Discovery of an Outburst and Pulsed X-Ray Flux from SMC X-2 Using the [ITAL]Rossi X-Ray Timing Explorer[/ITAL

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of SMC X-2 show that the source experienced an outburst in 2000 January-April reaching a peak luminosity of greater than ~1038 ergs s-1. RXTE Proportional Counter Array observations during this outburst reveal the presence of pulsations with a 2.37 s period. However, optical photometry of the optical counterpart shows that the source was still significantly fainter than it was more than a half a year after the outburst in the 1970s when SMC X-2 was discovered.

RXTEInvestigation into Extended Hard X‐Ray Emission from Elliptical Galaxies

We present results of the first RXTE investigation of hard X-ray emission from normal elliptical galaxies. Joint spectral analysis of RXTE Proportional Counter Array and ASCA Gas Imaging Spectrometer data for NGC 4472 and NGC 4649 reveals the presence of ~5 × 10-12 ergs cm-2 s-1 of 2-10 keV emission emanating from a region outside of the optical galaxy and of a spectral hardness intermediate between the galactic hot interstellar gas and X-ray binary components. A large fraction of this emission originates in the Virgo intracluster medium (ICM), and we present constraints on the ICM temperature and metallicity in the vicinity of these two galaxies. However, the diffuse flux is significantly greater than expected based on the soft X-ray intensity measured with ROSAT, allowing for the possible presence of an additional diffuse nonthermal component. We also derive new constraints on the integrated X-ray binary emission from these two galaxies and on the presence of a nuclear flat-spectrum point source.

High-Energy X-Ray Timing Experiment Detections of Hard X-Ray Tails in Scorpius X-1

We report the detection of a nonthermal hard X-ray component from Sco X-1 based on the analysis of 20-220 keV spectra obtained with the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in five of 16 observations analyzed. Using Proportional Counter Array data, we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power-law index hardens with increasing , as indicated from the position on the Z diagram. We find that the derived nonthermal luminosities are ~10% of that derived for the brightest of the atoll sources.

Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748−288

We report evidence for an Fe K-alpha fluorescence line feature and disk reflection in the very high, high-, and low-state X-ray spectra of the Galactic microquasar XTE J1748 - 288 during its 1998 June outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-Ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fitted to the data. In the very high state the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the high state the line profile is less redshifted and increasingly prominent. The low-state line profile is very strong (approx. 0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between approx. 20R(sub g) and - approx. 100R(sub g) in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the very high and high states, and reflection from a relatively neutral disk is preferred in the low state. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748 - 288.

A Broadband X‐Ray Study of Supernova Remnant 3C 397

We present a broadband imaging and spectral study of the radio-bright supernova remnant (SNR) 3C 397 with ROSAT, ASCA, and RXTE. A bright X-ray spot seen in the HRI image hints at the presence of a pulsar-powered component and gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imaging shows that the remnant is highly asymmetric, with its X-ray emission peaking at the western lobe. The hard-band images obtained with the ASCA Gas Imaging Spectrometer show that much of the hard X-ray emission arises from the western lobe, associated with the SNR shell, with little hard X-ray emission associated with the central hot spot. The spectrum from 3C 397 is heavily absorbed and dominated by thermal emission with emission lines evident from Mg, Si, S, Ar and Fe. Single-component models fail to describe the X-ray spectrum, and at least two components are required: a soft component characterized by a low temperature and a large ionization timescale, and a hard component required to account for the Fe-K emission line and characterized by a much lower ionization timescale. We use a set of nonequilibrium ionization (NEI) models (Borkowski et al., in preparation), and find that the fitted parameters are robust. The temperatures from the soft and hard components are ~0.2 keV and ~1.6 keV respectively. The corresponding ionization timescales n0t (n0 being the preshock hydrogen density) are ~6 × 1012 cm-3 s and ~6 × 1010 cm-3 s, respectively. The large n0t of the soft component suggests it is approaching ionization equilibrium; thus it can be fit equally well with a collisional equilibrium ionization model. The spectrum obtained with the Proportional Counter Array (PCA) of RXTE is contaminated by emission from the Galactic ridge, with only ~15% of the count rate originating from 3C 397 in the 5-15 keV range. The PCA spectrum allowed us to confirm the thermal nature of the hard X-ray emission. A third component originating from a pulsar-driven component is possible, but the contamination of the source signal by the Galactic ridge did not allow us to determine its parameters or find pulsations from any hidden pulsar. We discuss the X-ray spectrum in the light of two scenarios: a young ejecta-dominated remnant of a core-collapse SN, and a middle-aged SNR expanding in a dense ISM. In the first scenario, the hot component arises from the SNR shell, and the soft component from an ejecta-dominated component. 3C 397 would be a young SNR (a few thousand years old), but intermediate in dynamical age between the young historical shells (like Tycho or Kepler), and those that are well into the Sedov phase of evolution (like Vela). In the second scenario, the soft component represents the blast wave propagating in a dense medium, and the hard component is associated with hot gas encountering a fast shock, or arising from thermal conduction. In this latter scenario, the SNR would be ~twice as old, and transitioning into the radiative phase. The current picture we present in this paper is marginally consistent with this scenario, but it cannot be excluded. A spatially resolved spectroscopic study is needed to resolve the soft and hard components and differentiate between the two scenarios. Future Chandra and XMM data will also address the nature of the mysterious central (radio-quiet) X-ray spot.

Discovery of the Orbit of the Transient X-Ray Pulsar SAX J2103.5+4545

Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out a pulse-timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An outburst was detected by the All-Sky Monitor (ASM) in 1999 October 25 and reached a peak X-ray brightness of 27 mcrab on October 28. Between November 19 and December 27, the RXTE Proportional Counter Array (PCA) carried out pointed observations that provided us with pulse arrival times. These yield an eccentric orbit (e = 0.4 ± 0.2) with an orbital period of 12.68 ± 0.25 days and a light-travel time across the projected semimajor axis of 72 ± 6 s. The pulse period was measured to be 358.62171 ± 0.00088 s, and the spin-up rate was (2.50 ± 0.15) × 10-13 Hz s-1. The ASM data for the 1997 February-September outburst, in which F. Hulleman, J. J. M. in 't Zand, & J. Heise discovered SAX J2103.5+4545 using BeppoSAX, are modulated at timescales close to the orbital period. Folded light curves of the 1997 ASM data and the 1999 PCA data are similar and show that the intensity increases at periastron passages.

ASCAObservation of the New Transient X‐Ray Pulsar XTE J0111.2−7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the proportional counter array on board the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6 × 10-10 ergs cm-2 s-1 in the 0.7-10.0 keV band, which corresponds to the X-ray luminosity of 1.8 × 1038 ergs s-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 ± 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with an average value of ~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law-type model. The soft excess is eliminated if the spectrum is fitted to an "inversely broken power-law" model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 ± 14 eV. Pulse phase-resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.

RXTE observation of NGC 6240: a search for the obscured active nucleus

ABSTRA C T The wide-band energy spectrum of NGC 6240 over the range 0.5‐200 keV is investigated using the RXTE and ASCA data. The RXTE data provide the spectrum beyond the ASCA range (0.5‐10 keV) with significant detection of signals up to 20 keV and upper limits above 20 keV. The spectrum above 10 keV is found to be very flat. A strong iron K emission line discovered in the previous ASCA observation is also confirmed with the RXTE Proportional Counter Array (PCA). These results provide further evidence for the dominance of a reflection component, i.e. emission from cool material illuminated by an active galactic nucleus (AGN). By fitting the spectra obtained with RXTE and ASCA simultaneously, we satisfactorily modelled the AGN spectrum with a Compton reflection component and probably a transmitted AGN component penetrating through a thick absorber. The X-ray luminosity of the AGN is estimated to be in the range 4 10 43 to 6 10 44 erg s 21 in the range 2‐10 keV, which categorizes NGC 6240 among the most luminous Seyfert nuclei. The ratio of the 2‐10 keV X-ray luminosity to the infrared luminosity, LX(2‐10 keV)/LIR ,i s 0.01‐0.1, which implies a quite substantial, if not dominant, contribution of AGN to the infrared luminosity.

RXTEObservations of the X‐Ray Binary 2S 0114+650

Observations from the proportional counter array experiment on board the Rossi X-Ray Timing Explorer (RXTE) of the temporal and spectral properties of the high-mass X-ray binary 2S 0114+650 are reported. We find highly variable X-ray emission modulated at a period of 2.73 hr. The temporal results indicate that 2S 0114+650 is the slowest neutron star rotator presently known. Spectral analysis reveals parameters typical of a high-mass X-ray binary system having an absorbed power law with an index of about 1.3 with a high-energy cutoff at approximately 8 keV. The introduction of a fluorescent iron line at 6.4 keV allowed better spectral fits, particularly during low states of emission. We have also determined that the intensity of 2S 0114+650 is strongly attenuated during part of the orbit, which makes it a member of the small class of eclipsing massive X-ray binaries.

Timing Noise in SGR 1806-20.

We have phase-connected a sequence of Rossi X-Ray Timing Explorer Proportional Counter Array observations of SGR 1806-20 covering 178 days. We find that a simple secular spin-down model does not adequately fit the data. The period derivative varies gradually during the observations between 8.1x10-11 and 11.7x10-11 s s(-1) (at its highest, approximately 40% larger than the long-term trend), while the average burst rate as seen with the Burst and Transient Source Experiment drops throughout the time interval. The phase residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in radio pulsars (Delta8=4.8; frequency derivative average power approximately 7x10-20 cycles(2) s(-3)). Combining these results with the noise levels measured for some anomalous X-ray pulsars, we find that all magnetar candidates have Delta(8) values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find that the timing noise in SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52, and 4U 1626-67), but the spin-down of SGR 1806-20 has thus far maintained coherence over 6 yr. Alternatively, an orbital model with a period Porb=733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion, then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.

Statistical Properties of SGR 1806-20 Bursts.

We present statistics of SGR 1806-20 bursts, combining 290 events detected with the Rossi X-Ray Timing Explorer/Proportional Counter Array, 111 events detected with the Burst and Transient Source Experiment, and 134 events detected with the International Cometary Explorer. We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.76, and 1.67, respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is no correlation between the burst intensity and either the waiting times until the next burst or the time elapsed since the previous burst. In all these statistical properties, SGR 1806-20 bursts resemble a self-organized critical system, similar to earthquakes and solar flares. Our results thus support the hypothesis that the energy source for soft gamma repeater bursts is crustquakes due to the evolving, strong magnetic field of the neutron star, rather than any accretion or nuclear power.

Simultaneous [ITAL]EXTREME ULTRAVIOLET EXPLORER[/ITAL][ITAL]Extreme Ultraviolet Explorer[/ITAL] and [ITAL]ROSSI X-RAY TIMING EXPLORER[/ITAL][ITAL]Rossi X-Ray Timing Explorer[/ITAL] Observations of AM Herculis

We present coordinated Extreme Ultraviolet Explorer (EUVE) and Rossi X-Ray Timing Explorer (RXTE) target of opportunity observations of the 1998 August high state of AM Herculis. The EUVE light curve sampled phases ~0.0–0.3 and ~0.5–0.8 and showed AM Her to be uncharacteristically bright during these phases but lacking the expected counts at phase ≈0.5, possibly because of absorption. The RXTE proportional counter array (PCA) 2–20 keV and HEXTE 15–60 keV light curves clearly show the 3.09 hr orbital modulation. No correlations between the soft and hard bands predicted in the simple one-pole accretion model were detected in the ≈1600 s of true, simultaneous EUVE and RXTE data. Quasi-periodic oscillations (QPOs) of 1–3 s found in optical data for several magnetic cataclysmic variables were not detected in PCA high time resolution data of AM Her with a 90% confidence upper limit for the rms amplitude of 3%. No significant QPOs were detected in the 10s–100s of seconds range with EUVE or RXTE. The EUVE spectrum was well fitted with a 19 eV blackbody. The phase-averaged PCA spectrum was well fitted with a 17.6 keV thermal bremsstrahlung (TB) continuum plus a broad iron line centered at 6.5 keV and absorption edge at 9.2 keV. HEXTE detections extended to 100 keV and preferred a highly absorbed TB continuum with temperature of 37 keV. The thermal bremsstrahlung plus emission-line (Eline = 6.7 keV) and edge model fit to PCA spectra as a function of phase could be described as a single TB temperature of 18.9 ± 0.2 keV with a variable column density of 7.1–53.5 × 1021 cm-2 and did not require more complex continuum models. The ratio of EUV to X-ray luminosity showed no soft excess and was more in accord with the simple one-pole model than previous observations.

The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114.

We present the results of Rossi X-Ray Timing Explorer (RXTE) observations of the Be star X-ray binary system RX J0812.4-3114. A light curve obtained with the RXTE All-Sky Monitor (ASM) shows that the source is currently in an active state with outbursts occurring at approximately 80 day intervals. The source underwent a transition from an inactive state to this regular outburst state early in 1998. An observation of RX J0812.4-3114 was obtained with the RXTE Proportional Counter Array (PCA) close to the time of a predicted maximum in 1999 March, and strong pulsations were detected at a period of 31.88 s. This confirms the result of an earlier PCA observation by Reig & Roche that was also obtained serendipitously near the predicted maximum flux of the 80 day period and also near the start of the current active state. We interpret the periodicity in the ASM light curve as indicating the orbital period of RX J0812.4-3114 with outbursts occurring around periastron passage.

The Rapid X-Ray Variability of V4641 Sagittarii (SAX J1819.3-2525 = XTE J1819-254).

We report on the rapid X-ray variability of the variable star and X-ray transient V4641 Sagittarii (SAX J1819.3-2525 = XTE J1819-254) as observed on 1999 September 15 by the proportional counter array (PCA) on board the Rossi X-Ray Timing Explorer. During the first approximately 900 s of the first PCA observation, V4641 Sgr showed very strong X-ray fluctuations by a factor of 4 on timescales of seconds to about 500 on timescales of minutes. The spectrum of the source during this flaring episode became harder when the count rate decreased. After this flaring episode, V4641 Sgr entered a quiescent state in which it remained for the rest of this and subsequent PCA observations. The X-ray spectrum was considerably softer in this quiescent state than during the flaring episode. The intrinsic X-ray luminosity (during both the flaring episode and the quiescent state) and the rapid X-ray variability do not strongly constrain the nature of the compact object (neutron star or black hole) in the system, although a black hole seems to be more likely. The very short duration of the bright X-ray phase of V4641 Sgr and its likely close proximity suggest that many similar objects could be present in our Galaxy, most of which are not noticed when they are in outburst because of the short duration of these outbursts. A considerable number of the black holes present in our Galaxy might be contained in systems similar to V4641 Sgr.

Coordinated Optical and X‐Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

ABSTRACTWe obtained coordinated broadband optical photometry and hard X‐ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X‐ray data were obtained using the Proportional Counter Array detectors of the Rossi X‐Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter‐American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV∼17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV>17.7. VV Pup and V834 Cen had average 2–10 keV X‐ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm−2 s−1, respectively. EF Eri was not detected in X‐rays. The X‐ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best‐fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm−2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2–10 keV flux for VV Pup quoted above was based on the best‐fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5–30 keV.We detected ∼1 Hz optical quasi‐periodic oscillations (QPOs) with amplitude ∼2.2% from VV Pup and ∼1% from V834 Cen. We did not detect optical QPOs over the range 0.2–1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X‐ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ∼14% at the 90% confidence level over 0.2–1.2 Hz.

Statistical Properties of SGR 1900+14 Bursts.

We study the statistics of soft gamma repeater (SGR) bursts using a database of 187 events detected with BATSE and 837 events detected with the Rossi X-Ray Timing Explorer Proportional Counter Array; all events are from SGR 1900+14 during its 1998-1999 active phase. We find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter law for earthquakes and gives evidence for self-organized criticality in SGRs. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a lognormal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus, our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star and plausibly magnetic.

X‐Ray and Optical Observations of BL Hydri

We acquired hard X-ray and unfiltered optical photometric data of the AM Herculis system BL Hydri. The X-ray data were obtained using the Proportional Counter Array detector of the Rossi X-Ray Timing Explorer satellite on 1997 October 17. The optical data were obtained using the 1.5 m telescope of the Cerro Tololo Inter-American Observatory on 1997 September 24. At the time of our observations, BL Hyi was in an unusual high-luminosity state with average visual magnitude mV ≈ 14.4 and orbital phase-averaged 2-10 keV X-ray flux ≈1.5 × 10-11 ergs cm-2 s-1, roughly 50% larger than had been seen previously. The X-ray light curves suggested that two accretion poles were active. The primary X-ray emission region was extended, covering ~45° in longitude on the white dwarf, while the secondary X-ray emission region was consistent with a point source with flux ~33% that of the primary region. The X-ray spectra were well fitted by absorbed bremsstrahlung with Gaussian emission-line models; and absorbed power law with Gaussian emission-line models. For the thermal models, the X-ray spectra were consistent with kTX ≈ 11 keV and an emission line at E ≈ 6.7-6.8 keV with equivalent width EW = 0.86-1.2 keV. The absorbed power-law models had slope α ≈ 2.1 and an emission line at E = 6.7-6.8 keV with EW = 1.1-1.5 keV. The optical data showed, 2.7% ± 0.2% rms quasi-periodic oscillations (QPOs) over the frequency range 0.2-0.8 Hz modulated strongly on the orbital period suggesting that the QPOs were connected with the dominant accretion hot spot. We did not find corresponding QPOs in the X-ray emission for an upper limit of 20% rms. There were no detections of other short-period QPOs or coherent features in either the optical or the X-ray data. The X-ray and optical data were consistent with a radiative shock model. Based primarily on the X-ray continuum and line spectrum, we infer that the mass of the white dwarf in the BL Hyi system is 0.3-0.7 M☉, depending on the relative cyclotron and hard X-ray luminosities.

Scattering and Iron Fluorescence Revealed during Absorption Dips in Circinus X‐1

Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor light curves of Circinus X-1 show that intensity dips below the ~1 crab baseline occur near phase zero of the 16.55 day cycle of the source. RXTE Proportional Counter Array observations carried out between 1996 September 20 and 22 provided 60% observing efficiency for 48 hr around phase zero. These observations showed significant dipping activity during much of those 2 days. The dramatic spectral evolution associated with the dips is well fitted by variable and at times heavy absorption (NH > 1024 cm-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is about 10% of that of the unabsorbed bright component. A prominent iron emission line at 6.4-6.6 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, with equivalent width increasing from as low as 44 eV outside dips to more than 400 eV during dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial covering model could explain spectra obtained in ASCA observations of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2 × 1023 cm-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an iron Kα fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through such a scattering cloud. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.

Variable Spin-Down in the Soft Gamma Repeater SGR 1900+14 and Correlations with Burst Activity

We have analyzed Rossi X-ray Timing Explorer Proportional Counter Array observations of the pulsed emission from SGR 1900+14 during September 1996, June - October 1998, and early 1999. Using these measurements and results reported elsewhere, we construct a period history of this source for 2.5 years. We find significant deviations from a steady spin-down trend during quiescence and the burst active interval. Burst and Transient Source Experiment observations of the burst emission are presented and correlations between the burst activity and spin-down rate of SGR 1900+14 are discussed. We find an 80 day interval during the summer of 1998 when the average spin-down rate is larger than the rate elsewhere by a factor ~ 2.3. This enhanced spin-down may be the result of a discontinuous spin-down event or ``braking glitch'' at the time of the giant flare on 27 August 1998. Furthermore, we find a large discrepancy between the pulsar period and average spin-down rate in X-rays as compared to radio observations for December 1998 and January 1999.

Spin-down of Pulsations in the Cooling Tail of an X-Ray Burst from 4U 1636−53

We report the discovery with the proportional counter array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) of a decrease in the frequency of X-ray brightness oscillations in the cooling tail of an X-ray burst from 4U 1636-53. This is the first direct evidence for a spin down of the pulsations seen during thermonuclear bursts. We find that the spin down episode is correlated with the appearance in this burst of an extended tail of emission. We present both time resolved energy and variability spectra during this burst and compare them with results from a second burst which shows neither a spin down episode nor an extended tail. A spectral evolution study of the ``spin down'' burst reveals a secondary signature of weak radius expansion, not seen in other bursts, and correlated with the spin down episode, which may indicate a secondary thermonuclear energy release. We interpret the spin down episode in the context of an angular momentum conserving shell, which is reexpanded and therefore spun down by an additional thermonuclear energy release which could also explain the extended X-ray tail.