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K. E. McGowan

Publications and source records attributed to K. E. McGowan.

At least 19 recordsLinked to original sources

Properties of X-ray Binaries in the Magellanic Clouds from RXTE and Chandra Observations

The X-ray binary population of the SMC is very different from that of the Milky Way consisting, with one exception, entirely of transient pulsating Be/neutron star binaries. We have now been monitoring these SMC X-ray pulsars for over 10 years using the Rossi X-ray Timing Explorer with observations typically every week. The RXTE observations have been complemented with surveys made using the Chandra observatory. The RXTE observations are non-imaging but enable detailed studies of pulsing sources. In contrast, Chandra observations can provide precise source locations and detections of sources at lower flux levels, but do not provide the same timing information or the extended duration light curves that RXTE observations do. We summarize the results of these monitoring programs which provide insights into both the differences between the SMC and the Milky Way, and the details of the accretion processes in X-ray pulsars.

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High mass X-ray binary SXP18.3 undergoes the longest Type II outburst ever seen in the Small Magellanic Cloud

On 30th August 2006 SXP18.3 a high mass X-ray binary (HMXB) in the Small Magellanic Cloud (SMC) with an 18.3s pulse period was observed by Rossi X-ray Timing Explorer (RXTE). The source was seen continuously for the following 36 weeks. This is the longest Type II outburst ever seen from a HMXB in the SMC. During the outburst SXP18.3 was located from serendipitous XMM-Newton observations. The identification of the optical counterpart has allowed SXP18.3 to be classified as a Be/X-ray binary. This paper will report on the analysis of the optical and weekly RXTE X-ray data that span the last 10 years. The extreme length of this outburst has for the first time enabled us to perform an extensive study of the pulse timing of an SMC Be/X-ray binary. We present a possible full orbital solution from the pulse timing data. An orbital period of 17.79d is proposed from the analysis of the OGLE III light curve placing SXP18.3 on the boundary of known sources in the Corbet diagram.

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Optical properties of High-Mass X-ray Binaries (HMXB) in the Small Magellanic Cloud

The SMC represents an exciting opportunity to observe the direct results of tidal interactions on star birth. One of the best indicators of recent star birth activity is the presence of signicant numbers of High-Mass X-ray Binaries (HMXBs) - and the SMC has them in abundance! We present results from nearly 10 years of monitoring these systems plus a wealth of other ground-based optical data. Together they permit us to build a picture of a galaxy with a mass of only a few percent of the Milky Way but with a more extensive HMXB population. However, as often happens, new discoveries lead to some challenging puzzles - where are the other X-ray binaries (eg black hole systems) in the SMC? And why do virtually all the SMC HMXBs have Be starcompanions? The evidence arising from these extensive optical observations for this apparently unusual stellar evolution are discussed.

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Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud

The spectral distributions of Be/X-ray binaries in the Large Magellanic Cloud and Galaxy have been shown to differ significantly from the distribution of isolated Be stars in the Galaxy. Population synthesis models can explain this difference in spectral distributions through substantial angular momentum loss from the binary system. In this work we explore the spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud (SMC) using high signal-to-noise spectroscopy of a sample of 37 optical counterparts to known X-ray pulsars. Our results show that the spectral distribution of Be/X-ray binaries in the SMC is consistent with that of the Galaxy, despite the lower metallicity environment of the SMC. This may indicate that, although the metallicity of the SMC is conducive to the formation of a large number of HMXBs, the spectral distribution of these systems is likely to be most strongly influenced by angular momentum losses during binary evolution, which are not particularly dependent on the local metallicity.

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Optical and X-ray variability of two Small Magellanic Cloud X-ray binary pulsars - SXP46.6 and SXP6.85

We present long-term optical and RXTE data of two X-ray binary pulsars in the Small Magellanic Cloud, SXP46.6 and SXP6.85. The optical light curves of both sources show substantial (~0.5-0.8 mag) changes over the time span of the observations. While the optical data for SXP6.85 do not reveal any periodic behaviour, by detrending the optical measurements for SXP46.6 we find an orbital period of ~137 days, consistent with results from the X-ray data. The detection of Type I X-ray outbursts from SXP46.6, combined with the fact that we also see optical outbursts at these times, implies that SXP46.6 is a high orbital eccentricity system. Using contemporaneous optical spectra of SXP46.6 we find that the equivalent width of the H_alpha emission line changes over time indicating that the size of the circumstellar disc varies. By studying the history of the colour variations for SXP6.85 we find that the source gets redder as it brightens which can also be attributed to changes in the circumstellar disc. We do not find any correlation between the X-ray and optical data for SXP6.85. The results for SXP6.85 suggest that it is a low eccentricity binary and that the optical modulations are due to the Be phenomenon.

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The Chandra Small Magellanic Cloud Wing Survey - the search for X-ray Binaries

We have detected 523 sources in a survey of the Small Magellanic Cloud (SMC) Wing with Chandra. By cross-correlating the X-ray data with optical and near-infrared catalogues we have found 300 matches. Using a technique that combines X-ray colours and X-ray to optical flux ratios we have been able to assign preliminary classifications to 265 of the objects. Our identifications include four pulsars, one high-mass X-ray binary (HMXB) candidate, 34 stars and 185 active galactic nuclei (AGNs). In addition, we have classified 32 sources as 'hard' AGNs which are likely absorbed by local gas and dust, and nine 'soft' AGNs whose nature is still unclear. Considering the abundance of HMXBs discovered so far in the Bar of the SMC the number that we have detected in the Wing is low.

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INTEGRAL observations of the Small Magellanic Cloud

The first INTEGRAL observations of the Small Magellanic Cloud (carried out in 2003) are reported in which two sources are clearly detected. The first source, SMC X-1, shows a hard X-ray eclipse and measurements of its pulse period indicate a continuation of the long-term spin-up now covering ~30 years. The second source is likely to be a high mass X-ray binary, and shows a potential periodicity of 6.8s in the IBIS lightcurve. An exact X-ray or optical counterpart cannot be designated, but a number of proposed counterparts are discussed. One of these possible counterparts shows a strong coherent optical modulation at ~2.7d, which, together with the measured hard X-ray pulse period, would lead to this INTEGRAL source being classified as the fourth known high mass Roche lobe overflow system.

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Optical Follow-up of New SMC Wing Be/X-ray Binaries

We investigate the optical counterparts of recently discovered Be/X-ray binaries in the Small Magellanic Cloud. In total four sources, SXP101, SXP700, SXP348 and SXP65.8 were detected during the Chandra Survey of the Wing of the SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground based telescopes have been utilised in the optical follow-up, providing coverage in both the red and blue bands. This has led to the classification of all of the counterparts as Be stars and confirms that three lie within the Galactic spectral distribution of known Be/X-ray binaries. SXP101 lies outside this distribution becoming the latest spectral type known. Monitoring of the Halpha emission line suggests that all the sources bar SXP700 have highly variable circumstellar disks, possibly a result of their comparatively short orbital periods. Phase resolved X-ray spectroscopy has also been performed on SXP65.8, revealing that the emission is indeed harder during the passage of the X-ray beam through the line of sight.

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X-ray bright sources in the Chandra Small Magellanic Cloud Wing Survey - detection of two new pulsars

We investigate the X-ray and optical properties of a sample of X-ray bright sources from the Small Magellanic Cloud (SMC) Wing Survey. We have detected two new pulsars with pulse periods of 65.8 s (CXOU J010712.6-723533) and 700 s (CXOU J010206.6-714115), and present observations of two previously known pulsars RX J0057.3-7325 (SXP101) and SAX J0103.2-7209 (SXP348). Our analysis has led to three new optical identifications for the detected pulsars. We find long-term optical periods for two of the pulsars, CXOU J010206.6-714115 and SXP101, of 267 and 21.9 d, respectively. Spectral analysis of a sub-set of the sample shows that the pulsars have harder spectra than the other sources detected. By employing a quantile-based colour-colour analysis we are able to separate the detected pulsars from the rest of the sample. Using archival catalogues we have been able to identify counterparts for the majority of the sources in our sample. Combining this with our results from the temporal analysis of the Chandra data and archival optical data, the X-ray spectral analysis, and by determining the X-ray to optical flux ratios we present preliminary classifications for the sources. In addition to the four detected pulsars, our sample includes two candidate foreground stars, 12 probable active galactic nuclei, and five unclassified sources.

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The early and late-time spectral and temporal evolution of GRB 050716

We report on a comprehensive set of observations of Gamma Ray Burst 050716, detected by the Swift satellite and subsequently followed-up rapidly in X-ray, optical and near infra-red wavebands. The prompt emission is typical of long-duration bursts, with two peaks in a time interval of T90 = 68 seconds (15 - 350 keV). The prompt emission continues at lower flux levels in the X-ray band, where several smaller flares can be seen, on top of a decaying light curve that exhibits an apparent break around 220 seconds post trigger. This temporal break is roughly coincident with a spectral break. The latter can be related to the extrapolated evolution of the break energy in the prompt gamma-ray emission, and is possibly the manifestation of the peak flux break frequency of the internal shock passing through the observing band. A possible 3 sigma change in the X-ray absorption column is also seen during this time. The late-time afterglow behaviour is relatively standard, with an electron distribution power-law index of p = 2 there is no noticable temporal break out to at least 10 days. The broad-band optical/nIR to X-ray spectrum indicates a redshift of z ~> 2 for this burst, with a host-galaxy extinction value of E(B-V) ~ 0.7 that prefers an SMC-like extinction curve.

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Swift-UVOT Observations of the X-Ray Flash 050406

We present Swift-UVOT data on the optical afterglow of the X-ray flash of 2005 April 6 (XRF 050406) from 88s to \sim 10^5s after the initial prompt gamma-ray emission. Our observations in the V, B and U bands are the earliest that have been taken of an XRF optical counterpart. Combining the early -time optical temporal and spectral properties with γ- and simultaneous X-ray data taken with the BAT and XRT telescopes on-board Swift, we are able to constrain possible origins of the XRF. The prompt emission had a FRED profile (fast-rise, exponential decay) with a duration of T_90 = 5.7\pm 0.2s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption red-ward of 4000 Åin the UV/optical spectrum provides a firm upper limit of z\leq 3.1 on the redshift, thus excluding a high redshift as the sole reason for the soft spectrum. The optical light curve is consistent with a power-law decay with slope alpha = -0.75\pm 0.26 (F_ν\propto t^α), and a maximum occurring in the first 200s after the initial gamma-ray emission. The softness of the prompt emission is well described by an off-axis structured jet model, which is able to account for the early peak flux and shallow decay observed in the optical and X-ray bands.

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The 2005 outburst of GRO J1655-40: spectral evolution of the rise, as observed by Swift

We present Swift observations of the black hole X-ray transient, GRO J1655-40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well-suited for monitoring the spectral evolution of such an event. GRO J1655-40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the ``canonical'' low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3-10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending $\sim 3$ weeks in the very high state. The hard X-rays (14-150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet lightcurve behaved very differently from that of the X-rays; this might suggest that the soft X-ray lightcurve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona).

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A Link between Prompt Optical and Prompt Gamma-Ray Emission in Gamma-Ray Bursts

The prompt optical emission that arrives with gamma-rays from a cosmic gamma-ray burst (GRB) is a signature of the engine powering the burst, the properties of the ultra-relativistic ejecta of the explosion, and the ejecta's interactions with the surroundings. Until now, only GRB 990123 had been detected at optical wavelengths during the burst phase. Its prompt optical emission was variable and uncorrelated with the prompt gamma-ray emission, suggesting that the optical emission was generated by a reverse shock arising from the ejecta's collision with the surrounding material. Here we report prompt optical emission from GRB 041219a. It is variable and correlated with the prompt gamma-rays, indicating a common origin for the optical light and the gamma-rays. Within the context of the standard fireball model of GRBs, we attribute this new optical component to internal shocks driven into the burst ejecta by variations of the inner engine. The correlated optical emission is a direct probe of the jet isolated from the medium. The timing of the uncorrelated optical emission is strongly dependent on the nature of the medium.

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Limits on I-band microvariability of the Galactic Bulge Miras

We search for microvariability in a sample of 485 Mira variables with high quality I-band light curves from the second generation Optical Gravitational Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1 mag range lasting hours to days were discovered in Hipparcos data by de Laverny et al. (1998). Our search is primarily sensitive to events with time-scales of about 1 day, but retains a few percent efficiency (per object) for detecting unresolved microvariability events as short as 2 hours. We do not detect any candidate events. Assuming that the distribution of the event time profiles is identical to that from the Hipparcos light curves we derive the 95% confidence level upper limit of 0.038 per year per star for the rate of such events (1 per 26 years per average object of the ensemble). The high event rates of the order of 1 per year per star implied by the Hipparcos study in the H_P band are excluded with high confidence by the OGLE-II data in the I band. Our non-detection could still be explained by much redder spectral response of the I filter compared to the H_P band or by population differences between the bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide the first limit on the rate of the postulated microvariability events in Mira stars and offer new quantitative constraints on their properties. Similar limits are obtained for other pulse shapes and a range of the assumed time-scales and size-frequency distributions.

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Northern Sky Variability Survey (NSVS): Public data release

The Northern Sky Variability Survey (NSVS) is a temporal record of the sky over the optical magnitude range from 8 to 15.5. It was conducted in the course of the first generation Robotic Optical Transient Search Experiment (ROTSE-I) using a robotic system of four co-mounted unfiltered telephoto lenses equipped with CCD cameras. The survey was conducted from Los Alamos, NM, and primarily covers the entire northern sky. Some data in southern fields between declinations 0 and -38 deg is also available, although with fewer epochs and noticeably lesser quality. The NSVS contains light curves for approximately 14 million objects. With a one year baseline and typically 100-500 measurements per object, the NSVS is the most extensive record of stellar variability across the bright sky available today. In a median field, bright unsaturated stars attain a point to point photometric scatter of ~0.02 mag and position errors within 2 arcsec. At Galactic latitudes |b| < 20 deg the data quality is limited by severe blending due to ~14 arcsec pixel size. We present basic characteristics of the data set and describe data collection, analysis, and distribution. All NSVS photometric measurements are available for on-line public access from the Sky Database for Objects in Time-Domain (SkyDOT; http://skydot.lanl.gov) at LANL. Copies of the full survey photometry may also be requested on tape.

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The ROTSE-III Robotic Telescope System

The observation of a prompt optical flash from GRB990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow-up observations of gamma-ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next-generation instrument, ROTSE-III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing & protection and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.

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MACHO Photometry of Two LMC Be X-ray Transients, EXO 0531-66 and H 0544-665

Long-term variations are well-known in Be X-ray binaries, and are attributed to non-orbital changes in the structure of the Be circumstellar (equatorial) disc. However, the timescales involved are so long (tens of days, to years) that systematic studies have been very restricted. The ~8 year MACHO monitoring of the Large Magellanic Cloud (LMC) therefore presents an ideal opportunity to undertake such studies of Be X-ray systems that lie within the monitored fields. Here we present MACHO observations of two LMC Be X-ray transients, EXO 0531-66 and H 0544-665, the light curves of which show substantial (~0.5 mag) long-term variations. However, our analysis shows little evidence for any periodic phenomena in the light curves of either source. We find an upper limit for detection of a short (1-100 d) periodicity in the V- and R-band light curves of EXO 0531-66 of 0.041 mag and 0.047 mag semi-amplitude, respectively. The upper limits for the V- and R-band data of H 0544-665 are 0.054 mag and 0.075 mag semi-amplitude, respectively. Both EXO 0531-66 and H 0544-665 become redder as they brighten, possibly due to variations in the structure of the equatorial disc around the Be star. Spectra of both sources show H$α$ emission; for EXO 0531-66 we find the emission varies over time, thereby confirming its optical identification.

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A 421 d Activity Cycle in the BeX Recurrent Transient A0538-66 from MACHO monitoring

We present a ~ 5-yr optical light curve of the recurrent Be/X-ray transient A0538-66 obtained as a by-product of the MACHO Project. These data reveal both a long-term modulation at P = 420.8 +/- 0.8 d and a short-term modulation at 16.6510 +/- 0.0022 d which, within errors, confirms the previously found orbital period. Furthermore, the orbital activity is only seen at certain phases of the 421 d cycle suggesting that the long-term modulation is related to variations in the Be star envelope.

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