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M. J. Coe

Publications and source records attributed to M. J. Coe.

At least 91 records · Page 5Linked to original sources
arXiv2006

A 0535+26: Back in business

In May/June 2005, after 10 years of inactivity, the Be/X-ray binary system A 0535+26 underwent a major X-ray outburst. In this paper data are presented from 10 years of optical, IR and X-ray monitoring showing the behaviour of the system during the quiescent epoch and the lead up to the new outburst. The results show the system going through a period when the Be star in the system had a minimal circumstellar disk and then a dramatic disk recovery leading, presumably, to the latest flare up of X-ray emission. The data are interpreted in terms of the state of the disk and its interaction with the neutron star companion.

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arXiv2005

Multiwavelength monitoring of BD+53 2790, the optical counterpart to 4U~2206+54

We present the results of our long-term monitoring of BD+53 2790, the optical counterpart to the X-ray source 4U~2206+54. Unlike previous studies that classify the source as a Be/X-ray binary, we find that its optical and infrared properties differ from those of typical Be stars: the variability of the V/R ratio is not cyclical; there are variations in the shape and strength of the H$α$ emission line on timescales less than 1 day; and no correlation between the EW and the IR magnitudes or colors is seen. Our observations suggest that BD+53 2790 is very likely a peculiar O9.5V star. In spite of exhaustive searches we cannot find any significant modulation in any emission line parameter or optical/infrared magnitudes. Spectroscopy of the source extending from the optical to the K-band confirms the peculiarity of the spectrum: not only are the He lines stronger than expected for an O9.5V star but also there is no clear pattern of variability. The possibility that BD+53 2790 is an early-type analogue to He-strong stars (like theta^1 Ori C) is discussed.

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arXiv2005

SXP 323 - an unusual X-ray binary system in the Small Magellanic Cloud

Spectroscopic observations taken with the VLT/UVES telescope/instrument are presented of the unusual Small Magellanic Cloud (SMC) X-ray binary system SXP 323 = AX J0051-733. This system shows a clear modulation at 0.71d in long term optical photometry which has been proposed as the binary period of this system. The high resolution optical spectra, taken at a range of phases during the 0.71d cycle, rule out this possibility. Instead it is suggested that this long-term effect is due to Non Radial Pulsations (NRP) in the Be star companion to SXP 323. In addition, the spectra show clear evidence for major changes in the (V/R) ratio of the double peaks of the Balmer lines indicative of asymmetries in the circumstellar disk. The complex structure of the interstellar lines are also discussed in the context of the SMC structure.

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arXiv2005

Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431

We present the first long-term study of the optical counterpart to the X-ray pulsar LS V +44 17/RX J0440.9+4431. The results of our photometric and spectroscopic analysis show that LS V +44 17/RX J0440.9+4431 contains a moderately reddened, E(B-V)=0.65+-0.05, B0.2V star located at about 3.3 kpc. The Ha line consistently shows a double-peak profile varying from symmetric shape to completely distorted on one side (V/R phases). A correlation between the equivalent width of the Ha line and the infrared magnitudes is seen: as the EW(Ha) decreases the IR magnitudes become fainter. This long-term optical/IR variability is attributed to structural changes in the Be star's circumstellar disc. The observations include a recent decline in the circumstellar disc and subsequent recovery. We have witnessed the cessation of a global oscillation due to the decline of the circumstellar disc. If the present disc growth rate continues we predict the onset of another episode of V/R variability by the end of 2006. We have investigated the typical time scales for disc variability of various Be/X-ray binaries and found a correlation with the orbital period. This correlation is hard to establish due to the difficulty in defining the exact duration of the various activity states, but it is seen both in the duration of the disc growth/dissipation phase and the value of the Ha equivalent width prior to the appearance of asymmetric profiles. These relationships provide further evidence for the interaction of the neutron star with the circumstellar disc of the Be star's companion and confirms the need of a fully developed disc for the V/R variability to be observed.

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arXiv2005

Long Term Behavior of X-ray Pulsars in the Small Magellanic Cloud

Results of a 4 year monitoring campaign of the SMC using the Rossi X-ray timing Explorer (RXTE) are presented. This large dataset makes possible detailed investigation of a significant sample of SMC X-ray binaries. 8 new X-ray pulsars were discovered and a total of 20 different systems were detected. Spectral and timing parameters were obtained for 18. In the case of 10 pulsars, repeated outbursts were observed, allowing determination of candidate orbital periods for these systems. We also discuss the spatial and pulse period distributions of the SMC pulsars.

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arXiv2005

The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504

A probable binary period has been detected in the optical counterpart to the X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul 2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is coincident with an Optical Gravitational Lensing (OGLE) object in the lightcurves of which several optical outburst peaks are visible at ~ 268 day intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99% significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s pulse-period has revealed detections which correspond closely with predicted or actual peaks in the optical data. The relationship between this orbital period and the pulse period of 504s is within the normal variance found in the Corbet diagram.

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arXiv2005

An estimate of the SN kick velocities for High Mass X-ray Binaries in the SMC

This work investigates the possible supernova kick velocities imposed on HMXB systems in the Small Magellanic Cloud. Comparisons are made between the location of such systems and the locations of young, stellar clusters on the premise that these may represent the birthplace of many of these systems. Measurements of the separation of clusters and HMXBs,and an estimate of the typical lifetimes of these systems, leads to a minimum average space velocity 30 km/s. This value is compared to theoretical estimates.

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arXiv2004

Discovery of a Be/X-ray Binary Consistent with the Position of GRO J2058+42

GRO J2058+42 is a 195 s transient X-ray pulsar discovered in 1995 with BATSE. In 1996, RXTE located GRO J2058+42 to a 90% confidence error circle with a 4' radius. On 20 February 2004, the region including the error circle was observed with Chandra ACIS-I. No X-ray sources were detected within the error circle, however, 2 faint sources were detected in the ACIS-I field-of-view. We obtained optical observations of the brightest object, CXOU J205847.5+414637, that had about 64 X-ray counts and was just 0.3' outside the error circle. The optical spectrum contained a strong H alpha line and corresponds to an infrared object in the 2MASS catalog, indicating a Be/X-ray binary system. Pulsations were not detected in the Chandra observations, but similar flux variations and distance estimates suggest that CXOU J205847.5+414637 and GRO J2058+42 are the same object. We present results from the Chandra observation, optical observations, new and previously unreported RXTE observations, and a reanalysis of a ROSAT observation.

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arXiv2004

Optical properties of SMC X-ray binaries

This work represent the first major study of the optical and IR characteristics of the mass donor companions to the X-ray pulsars in the Small Magellanic Cloud (SMC). In this work several new counterparts have been identified, and possible ones confirmed, as companions to X-ray pulsars in the SMC giving a total of 34 such objects now identified. In addition this work presents three new binary periods and confirms two X-ray periods using optical data for objects in this group. This homogeneous sample has been studied as a group to determine important general characteristics that may offer insight into the evolution of such systems. In particular, the spectral class distribution shows a much greater agreement with those of isolated Be stars, and appears to be in some disagreement with the galactic population of Be stars in Be/X-ray binaries. Studies of the long term optical modulation of the Be star companions reveal an extremely variable group of objects, a fact which will almost certainly make a major contribution to the pronounced X-ray variability. The spatial distribution of these systems within the SMC is investigated and strongly suggests a link between massive star formation and the HI density distribution. Finally, studies of the circumstellar disk characteristics reveal a strong link with optical variability offering important clues into the long-term stability of such disks.

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arXiv2004

Three new X-ray pulsars detected in the Small Magellanic Cloud and the positions of two other known pulsars determined

Three new X-ray pulsars have been detected in the Small Magellanic Cloud (SMC) and the positions of two others have been determined, with archive Chandra data. A series of five observations of the SMC took place between May and October 2002. Analysis of these data has revealed three previously unknown X-ray pulsars at pulse periods of 34, 503 and 138 seconds. The position of pulsar XTE J0052-725, which was originally detected by RXTE on June 19 2002, was also accurately determined and a previously detected 7.78s RXTE pulsar was identified as the source SMC X-3.

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arXiv2004

Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud

Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray pulsars. Together with discoveries from other satellites at least 45 SMC pulsars are now known. One of these sources, a pulsar with a period of approximately 7.8 seconds, was first detected in early 2002 and since discovery it has been found to be in outburst nine times. The outburst pattern clearly shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of this system. Candidate outburst periods have also been obtained for nine other pulsars and continued monitoring will enable us to confirm these. This large number of pulsars, all located at approximately the same distance, enables a wealth of comparative studies. In addition, the large number of pulsars found (which vastly exceeds the number expected simply by scaling the relative mass of the SMC and the Galaxy) reveals the recent star formation history of the SMC which has been influenced by encounters with both the LMC and the Galaxy.

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arXiv2004

AX J0049.4-7323 - a close look at a neutron star interacting with a circumstellar disk

Detailed evidence on the system AX J0049.4-7323 is presented here to show how the passage of the neutron star in the binary system disrupts the circumstellar disk of the mass donor Be star. A similar effect is noted in three other Be/X-ray binary systems. Together the observational data should provide valuable tools for modelling these complex interactions.

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arXiv2004

A major outburst from the X-ray binary RX J0520.5-6932

We report on the analysis of 8 years of MAssive Compact Halo Objects (MACHO) data for the source RX J0520.5-6932. A regular period of 24.4 days has been confirmed, however this is manifest almost entirely in the red part of the spectrum. A major outburst, lasting approximately 200 days, was observed which increased the apparent brightness of the object by approximately 0.15 magnitudes without significantly altering its V-R colour index. This outburst was also seen in X-ray data. The evidence from this analysis points to the identification of this object as a Be/X-ray binary with a periodically variable circumstellar disk and a very early optical counterpart.

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arXiv2003

XTE J0111.2-7317 : a nebula-embedded X-ray binary in the SMC

The observed characteristics of the nebulosity surrounding the SMC High Mass X-ray Binary XTE J0111.2-7317 are examined in the context of three possible nebular types: SNR, bowshock and HII region. Observational evidence is presented which appears to support the interpretation that the nebulosity surrounding XTE J0111.2-7317 is an HII region. The source therefore appears to be a normal SMC Be X-ray binary (BeXRB) embedded in a locally enhanced ISM which it has photoionised to create an HII region. This is supported by observations of the X-ray outburst seen with BATSE and RXTE in 1998-1999. It exhibited characteristics typical of a giant or type II outburst in a BeXRB including large spin-up rates, Lx~10E38 erg/sq.cm-s, and a correlation between spin-up rate and pulsed flux. However, the temporal profile of the outburst was unusual, consisting of two similar intensity peaks, with the first peak of shorter duration than the second.

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arXiv2003

Cyclical behaviour and disc truncation in the Be/X-ray binary A0535+26

A0535+26 is shown to display quantised IR excess flux states, which are interpreted as the first observational verification of the resonant truncation scheme proposed by Okazaki and Negueruela (2001) for BeXRBs. The simultaneity of X-ray activity with transitions between these states strongly suggests a broad mechanism for outbursts, in which material lost from the disc during the reduction of truncation radius is accreted by the NS. Furthermore changes between states are shown to be governed by a 1500 day period, probably due to precession of the Be disc, which profoundly dictates the global behaviour of the system. Such a framework appears to be applicable to BeXRBs in general.

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arXiv2003

Long-term Properties of Accretion Disks in X-ray Binaries: II. Stability of Radiation-Driven Warping

A significant number of X-ray binaries are now known to exhibit long-term ``superorbital'' periodicities on timescales of $\sim$ 10 - 100 days. Several physical mechanisms have been proposed that give rise to such periodicities, in particular warping and/or precession of the accretion disk. Recent theoretical work predicts the stability to disk warping of X-ray binaries as a function of the mass ratio, binary radius, viscosity and accretion efficiency, and here we examine the constraints that can be placed on such models by current observations. In paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray datasets (CGRO, RXTE), focusing on the remarkable, smooth variations in the superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to investigate the stability of the superorbital periodicities in the neutron star X-ray binaries Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability predictions with observation. We find that the period and nature of superorbital variations in these sources is consistent with the predictions of warping theory. We also use a dynamic lightcurve analysis to examine the behaviour of Her X-1 as it enters and leaves the 1999 Anomalous Low State (ALS). This reveals a significant phase shift some 15 cycles before the ALS, which indicates a change in the disk structure or profile leading into the ALS.

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arXiv2002

XTE J1946+274 = GRO J1944+26: An Enigmatic Be/X-ray Binary

XTE J1946+274 = GRO J1944+26 is a 15.8 s Be/X-ray pulsar discovered simultaneously in 1998 September with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) and the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE). Here we present new results from BATSE and {\em RXTE} including a pulse timing analysis, spectral analysis, and evidence for an accretion disk. Our pulse timing analysis yielded an orbital period of 169.2 days, a moderate eccentricity of 0.33, and implied a mass function of 9.7 M_sun. We observed evidence for an accretion disk, a correlation between measured spin-up rate and flux, which was fitted to obtain a distance estimate of 9.5 +/- 2.9 kpc. XTE J1946+274 remained active from 1998 September - 2001 July, undergoing 13 outbursts that were not locked in orbital phase. Comparing RXTE PCA observations from the initial bright outburst in 1998 and the last pair of outbursts in 2001, we found energy and intensity dependent pulse profile variations in both outbursts and hardening spectra with increasing intensity during the fainter 2001 outbursts. In 2001 July, optical Halpha observations indicate a density perturbation appeared in the Be disk as the X-ray outbursts ceased. We propose that the equatorial plane of the Be star is inclined with respect to the orbital plane in this system and that this inclination may be a factor in the unusual outburst behavior of the system.

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