A. B. Giles

The optical counterpart of SAX J1808.4 — 3658, the transient bursting millisecond X-ray pulsar

A set of CCD images have been obtained during the decline of the X-ray transient SAX J1808.4 - 3658 during 1998 April--June. The optical counterpart has been confirmed by several pieces of evidence. The optical flux shows a modulation on several nights that is consistent with the established X-ray binary orbit period of 2 h. This optical variability is roughly in antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7 on April 18 declined rapidly after April 22. From May 2 onwards the magnitude was more constant at around V=18.45 but by June 27 it was below our sensitivity limit. The optical decline precedes the rapid second phase of the X-ray decrease by 3 ± 1 d. The source has been identified on a 1974 UK Schmidt plate at an estimated magnitude of ∼ 20. The nature of the optical companion is discussed.

Burst Oscillation Periods from 4U 1636–53: A Constraint on the Binary Doppler Modulation

The burst oscillations seen during type I X-ray bursts from low-mass X-ray binaries typically evolve in period toward an asymptotic limit that likely reflects the spin of the underlying neutron star. If the underlying period is stable enough, measurement of it at different orbital phases may allow a detection of the Doppler modulation caused by the motion of the neutron star with respect to the center of mass of the binary system. Testing this hypothesis requires enough X-ray bursts and an accurate optical ephemeris to determine the binary phases at which they occurred. We present here a study of the distribution of asymptotic burst oscillation periods for a sample of 26 bursts from 4U 1636-53 observed with the Rossi X-Ray Timing Explorer. The burst sample includes both archival and proprietary data and spans more than 4.5 yr. We also present new optical light curves of V801 Arae, the optical counterpart of 4U 1636-53, obtained during 1998-2001. We use these optical data to refine the binary period measured by Augusteijn et al. to 3.7931206(152) hr. We show that a subset of ~70% of the bursts form a tightly clustered distribution of asymptotic periods consistent with a period stability of ~1 × 10-4. The tightness of this distribution, made up of bursts spanning more than 4 yr in time, suggests that the underlying period is highly stable, with a time to change the period of ~3 × 104 yr. This is comparable to similar numbers derived for X-ray pulsars. We investigate the period and orbital phase data for our burst sample and show that it is consistent with binary motion of the neutron star, with vns sin i < 38 and 50 km s-1 at 90% and 99% confidence, respectively. We use this limit as well as previous radial velocity data to constrain the binary geometry and component masses in 4U 1636-53. Our results suggest that unless the neutron star is significantly more massive than 1.4 M☉, the secondary is unlikely to have a mass as large as 0.36 M☉, the mass estimated assuming that it is a main-sequence star that fills its Roche lobe. We show that a factor of ~3 increase in the number of bursts with asymptotic period measurements should allow a detection of the neutron star velocity.

PLUTO'S ATMOSPHERE FROM THE 2015 JUNE 29 GROUND-BASED STELLAR OCCULTATION AT THE TIME OF THE NEW HORIZONS FLYBY *

We present results from a multi-chord Pluto stellar occultation observed on 2015 June 29 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Plutos atmosphere is still expanding, with a significant pressure increase of 5 ± 2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Plutos recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness whose base lies at about 4 km above Plutos surface, and where an average thermal gradient of about 5 K km−1 prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Plutos limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we obtain a possible range of 11.9–13.7 μbar for the surface pressure.

Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4−3114

Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4-3114 and A 0535 + 26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4-3114, which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth of photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX 1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with a standard deviation ranging between 2 degrees and 6 degrees depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and from the individual eclipses demonstrate that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate (M)over dot.

Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State

The Rossi X-ray Timing Explorer spacecraft observed the X-ray pulsar GX 1+4 for a period of 34 hr on 1996 July 19/20. The source faded from an intensity of ~20 mcrab to a minimum of ≤0.7 mcrab and then partially recovered toward the end of the observation. This extended minimum lasted ~40,000 s. Phase-folded light curves at a barycentric rotation period of 124.36568±0.00020 s show that near the center of the extended minimum the source stopped pulsing in the traditional sense but retained a weak dip feature at the rotation period. Away from the extended minimum, the dips are progressively narrower at higher energies and may be interpreted as obscurations or eclipses of the hot spot by the accretion column. The pulse profile changed from leading-edge bright before the extended minimum to trailing-edge bright after it. Data from the Burst and Transient Source Experiment show that a torque reversal occurred less than 10 days after our observation. Our data indicate that the observed rotation departs from a constant period with a /P value of ~-1.5% per year at a 4.5 σ significance. We infer that we may have serendipitously obtained data, with high sensitivity and temporal resolution about the time of an accretion disk spin reversal. We also observed a rapid flare that had some precursor activity, close to the center of the extended minimum.

The optical counterpart of XTE J0929-314: the third transient millisecond X-ray pulsar

A blue and variable optical counterpart of the X-ray transient XTE J0929-314 was identified on 2002 May 1. We conducted frequent BVRI broad-band photometry on this object using the Mt Canopus 1-m telescope during May and June until it had faded to below 21st magnitude. Nearly continuous I-band CCD photometry on 2002 May 2-4 revealed a ∼10 per cent sinusoidal modulation at the binary period lasting ∼6 cycles during the latter half of May 2. The phase indicates that the modulation may be due to a combination of emission by a hotspot on the disc and X-ray heating of the secondary. The emission generally trended bluer with B - I decreasing by 0.6 mag during the observations, but there were anomalous changes in colour during the first few days after optical identification when the I-band flux decreased slightly while fluxes in other bands increased. Spectral analysis of the BVRI broad-band photometry shows evidence of a variable excess in the R and I bands. We suggest that this may be due to synchrotron emission in matter flowing out of the system, and note that similar processes may have been responsible for anomalous V- and I-band measurements in 1998 of the persistent millisecond X-ray pulsar SAX J1808.4-3658.

A transient I‐band excess in the optical spectrum of the accreting millisecond pulsar SAX J1808.4–3658

The optical counterpart of the transient, millisecond X-ray pulsar SAX J1808.4-3658 was observed in four colours (BVRI) for five weeks during the 2005 June-July outburst. The optical fluxes declined by � 2 magnitudes during the first 16 days and then commenced quasi-periodic secondary outbursts, with time-scales of several days, similar to those seen in 2000 and 2002. The broadband spectra derived from these measurements were generally consistent with emission from an X-ray heated accretion disc. During the first 16 days decline in intensity the spectrum became redder. We suggest that the primary outburst was initiated by a viscosity change driven instability in the inner disc and note the contrast with another accreting millisecond pulsar, XTE J0929–314, for which the spectrum becomes bluer during the decline. On the night of 2005 June 5 (HJD 2453527) the I band flux was � 0.45 magnitudes brighter than on the preceding or following nights whereas the BV & R bands showed no obvious enhancement. A Type I X-ray burst was detected by the RXTE spacecraft during this I band integration. It seems unlikely that reprocessed radiation from the burst was sufficient to explain the observed increase. We suggest that amajor part of the I band excess was due to synchrotron emission triggered by the X-ray burst. Several other significant short duration changes in V-I were detected. One occurred at about HJD 2453546 in the early phase of the first secondary outburst and may be due to a mass transfer instability or to another synchrotron emission event.

Balloon altitude studies of southern hemisphere hard X-ray sources by the Imperial College-University of Tasmania-INPE collaboration

Abstract Hard X-ray balloon altitude measurements with a 1600 cm 2 phoswich array are described. Data from observations on Sco X-1, GX1+4, GX5−1, Nova Oph. 1977, SMC X-1, SS433, IC 4329A and MR 2251-178 are presented. The role of Comptonisation in X-ray production for Sco X-1 and GX1+4 is discussed.

X-ray astronomy observations with a xenon filled fluorescence gated detector

Abstract The University of Tasmania xenon filled multiwire proportional counter provides post-flight analysis of data in both normal and fluorescence gated modes. Fluorescence gating leads to improved energy resolution above the K-edge in xenon (35-keV). Theoretical and laboratory studies suggest that sensitivity for detection of weak sources may also be improved in background-limited situations. This has not previously been demonstrated in astronomical observations. We observed the X-ray pulsar GX 1+4 during a balloon flight from Alice Springs, Australia and analysed the data in both modes. Preliminary results suggest that the sensitivity was little changed by the use of fluorescence gating, but that energy resolution was improved. An anomaly exists in the background at ∼60-keV. Mathematical analysis and Monte Carlo simulation show that fluorescence gating cannot be used in a small energy range about 60-keV.

Real time science displays for the PCA experiment on the Rossi X-ray Timing Explorer

Abstract The Rossi X-ray Timing Explorer (RXTE) spacecraft was launched on 30th December 1995 and contains a large Proportional Counter Array (PCA) experiment developed at GSFC. Telemetry from RXTE is returned via the NASA Tracking and Data Relay Satellite System (TDRSS) which, apart from specific gaps in coverage, provides a steady stream of nearly continuous real time data packets. The PCA has an area of 7000 cm2 and produces high count rates for many x-ray sources. This provides the opportunity for some serious interpretation and decision making in real time. The display programs developed by the PCA team fall into 4 classes: Housekeeping, General performance (Instrumental and calibration bias), Spectral (Science bias) and Temporal (Science bias). These displays are used by the duty scientist and experiment controllers to monitor the observation in progress to try and ensure that the observation is proceeding as planned and that modifications to the observing modes are not required. Guest Observers (GOs) can be present in the Science Operations Facility (SOF) for their observations. The PCA team can also monitor their experiment using these programs. The SOF are planning to make some or all of these display tools available to GOs at their home institutions for the specific duration of their observations. This paper briefly describes the available display options.