arXiv subjects

P. A. Charles

Publications and source records attributed to P. A. Charles.

At least 91 records · Page 5Linked to original sources

arXiv2005

On the Origin of the Absorption Features in SS433

We present high-resolution optical spectroscopy of the X-ray binary system SS433, obtained over a wide range of orbital phases. The spectra display numerous weak absorption features, and include the clearest example seen to date of those features, resembling a mid-A type supergiant spectrum, that have previously been associated with the mass donor star. However, the new data preclude the hypothesis that these features originate solely within the photosphere of the putative mass donor, indicating that there may be more than one region within the system producing an A supergiant-like spectrum, probably an accretion disc wind. Indeed, whilst we cannot confirm the possibility that the companion star is visible at certain phase combinations, it is possible that all supergiant-like features observed thus far are produced solely in a wind. We conclude that great care must be taken when interpreting the behaviour of these weak features.

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arXiv2005

A "Combination Nova" Outburst in Z Andromedae: Nuclear Shell Burning Triggered by a Disk Instability

We describe observational evidence for a new kind of interacting-binary-star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly two magnitudes in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far ultraviolet with FUSE, and in the X-rays with both Chandra and XMM. The two-year-long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion-disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10^4 Lsun for approximately one year. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.

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arXiv2005

Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence

We present ESO-NTT low resolution spectroscopy of the neutron star X-ray transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0 km s^{-1}. This, combined with a previous determination of the inclination angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/- 1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus undermassive for the inferred spectral type indicating it is probably evolved, in agreement with previous studies. Doppler tomography of the H_alpha line shows prominent emission located on the companion and a slightly asymmetric accretion disc distribution similar to that seen in systems with precessing eccentric discs. Strong H_alpha emission from the companion can be explained by X-ray irradiation from the primary. No evidence is found for a hot spot in H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This can be interpreted as the hot spot and outer regions of the disc being at a higher temperature than in other systems.

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arXiv2005

ULTRACAM observations of the black hole X-ray transient XTE J1118+480 in quiescence

We present high time-resolution multicolour observations of the quiescent soft X-ray transient XTE J1118+480 obtained with ULTRACAM. Superimposed on the double-humped continuum g' and i'-band lightcurves are rapid flare events which typically last a few minutes. The power density spectrum of the lightcurves can be described by a broken power-law model with a break frequency at ~2 mHz or a power-law model plus a broad quasi-periodic oscillation (QPO) at ~2 mHz. In the context of the cellular-automaton we estimate the size of the quiescent advection-dominated flow (ADAF) region to be ~10^4 Schwarzschild radii, similar to that observed in other quiescent black hole X-ray transients, suggesting the same underlying physics. The similarites between the low/hard and quiescent state PDS suggest a similar origin for the optical and X-ray variability, most likely from regions at/near the ADAF.

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arXiv2005

Multiband Echo Tomography of Sco X-1

We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1-10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines and a red continuum at lambda ~ 6000 Ang were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring Branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc

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arXiv2005

Periodic Variability During the X-ray Decline of 4U 1636-53

We report the onset of a large amplitude, statistically significant periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U 1636-53, the X-ray flux of which has been gradually declining over the last four years. This behaviour is remarkably similar to that observed in the neutron star LMXB KS 1731-260, which is a long-term transient. We also report on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and find that the hard X-ray flux (20-100 keV) indicates an apparent anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is transiting from activity to quiescence, as occurred in KS 1731-260. We also suggest that the variability during the X-ray decline is the result of an accretion rate variability related to the X-ray irradiation of the disc.

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arXiv2005

Echo Tomography of Sco X-1 using Bowen Fluorescence Lines

We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1-10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring Branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star.

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arXiv2004

The X-ray source population of the globular cluster M15: Chandra high resolution imaging

The globular cluster M15 was observed on three occasions with the High Resolution Camera on board Chandra in 2001 in order to investigate the X-ray source population in the cluster centre. After subtraction of the two bright central sources, four faint sources were identified within 50 arcsec of the core. One of these sources is probably the planetary nebula, K648, making this the first positive detection of X-rays from a planetary nebula inside a globular cluster. Another two are identified with UV variables (one previously known), which we suggest are cataclysmic variables (CVs). The nature of the fourth source is more difficult to ascertain, and we discuss whether it is possibly a quiescent soft X-ray transient (qSXT) or also a CV.

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arXiv2004

A New Kind of Nova

We performed extensive, multi-wavelength observations of the prototypical symbiotic star Z Andromedae between 2000 and 2003, during a large eruption. The rise to optical maximum occurred in three distinct stages. During the first stage, the rise was very similar to an earlier, small outburst which we determined was due to an accretion-disk instability. In the second stage, an optically thick shell of material was ejected, and in the third stage, the shell cleared to reveal a white dwarf whose luminosity was roughly 10^4 Lsun. We suggest that the outburst was powered by an increase in the rate of nuclear burning on the white-dwarf surface, triggered by a sudden burst of accretion. This outburst thus combined elements of both dwarf novae and classical novae.

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arXiv2004

The massive neutron star or low-mass black hole in 2S0921-630

We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, that reveals the absorption line radial velocity curve of the K0III secondary star with a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity $γ$=34.9 +/- 3.3 \kms and an orbital period P_orb of 9.0035 +/- 0.0029 day (1-sigma). Given the quality of the data, we find no evidence for the effects of X-ray irradiation. Using the previously determined rotational broadening of the mass donor, and applying conservative limits on the orbital inclination, we constrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out a canonical neutron star at the 99% level. Since the nature of the compact object is unclear, this mass range implies that the compact object is either a low-mass black hole with a mass slightly higher than the maximum neutron star mass (2.9 Msolar) or a massive neutron star. If the compact object is a black hole, it confirms the prediction of the existence of low-mass black holes, while if the object is a massive neutron star its high mass severely constrains the equation of state of nuclear matter.

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arXiv2004

Evidence for a Black Hole in the X-ray transient GS 1354-64 (=BW Cir)

We present the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this X-ray transient which recorded outbursts in 1987 and 1997 (and possibly 1971-2). We identify weak absorption features corresponding to a G0-5III donor star, strongly veiled by a residual accretion disc which contributes 61-65 % of the total light at lambda 6300. The Doppler motions of these features trace an orbit of P=2.54448 days (or its 1-yr alias of P=2.56358 days) and velocity semi-amplitude K2=279 +- 5 km/s (or K2=292 +- 5 km/s). Both solutions are equally possible. The mass function implied by the shorter period solution is f(M)= 5.75 +- 0.30 Msun which, combined with the rotational broadening of the tidally locked companion (V sin i =71 +- 4 km/s), yields a compact object mass of M1 sin3 i = 7.34 +- 0.46 Msun. This is substantially above the mass of a neutron star under any standard equation of state of nuclear matter. The companion star is probably a G subgiant which has evolved off the main sequence in order to fill its Roche lobe. Remarkably, a distance of > 27 kpc is inferred by the companion's luminosity and this is supported by the large observed systemic velocity (gamma = 103 +- 4 km/s) which requires such a distance in order to be consistent with the Galactic rotation curve.

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arXiv2004

Optical spectroscopy of flares from the black hole X-ray transient A0620-00 in quiescence

We present a time-resolved spectrophotometric study of the optical variability in the quiescent soft X-ray transient A0620-00. Superimposed on the double-humped continuum lightcurve are the well known flare events which last tens of minutes. Some of the flare events that appear in the continuum lightcurve are also present in the emission line lightcurves. From the Balmer line flux and variations, we find that the persistent emission is optically thin. During the flare event at phase 1.15 the Balmer decrement dropped suggesting either a significant increase in temperature or that the flares are more optically thick than the continuum. The data suggests that there are two HI emitting regions, the accretion disc and the accretion stream/disc region, with different Balmer decrements. The orbital modulation of H$α$ with the continuum suggests that the steeper decrement is most likely associated with the stream/disc impact region. We construct Doppler images of the H$α$ and H$β$ emission lines. Apart from showing enhanced blurred emission at the region where the stream impacts the accretion disc, the maps also show significant extended structure from the opposite side of the disc. The trailed spectra show characteristic S-wave features that can be interpreted in the context of an eccentric accretion disc (abridged).

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arXiv2004

Correlated X-ray and Optical Variability in V404 Cyg in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of >20 during a 60ks observation. X-ray variations were well correlated with those in Halpha, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Halpha flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outwards to at least the circularization radius. The prompt response in the entire Halpha line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

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arXiv2004

A new mass-ratio for the X-ray Binary X2127+119 in M15?

The luminous low-mass X-ray binary X2127+119 in the core of the globular cluster M15 (NGC 7078), which has an orbital period of 17 hours, has long been assumed to contain a donor star evolving off the main sequence, with a mass of 0.8 solar masses (the main-sequence turn-off mass for M15). We present orbital-phase-resolved spectroscopy of X2127+119 in the H-alpha and He I 6678 spectral region, obtained with the Hubble Space Telescope. We show that these data are incompatible with the assumed masses of X2127+119's component stars. The continuum eclipse is too shallow, indicating that much of the accretion disc remains visible during eclipse, and therefore that the size of the donor star relative to the disc is much smaller in this high-inclination system than the assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I 6678 emission, which has a velocity that implies an association with the stream-disc impact region, remains unchanged through eclipse, implying that material from the impact region is always visible. This should not be possible if the previously-assumed mass ratio is correct. In addition, we do not detect any spectral features from the donor star, which is unexpected for a 0.8 solar-mass sub-giant in a system with a 17-hour period.

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arXiv2004

High resolution Chandra X-ray imaging of the nucleus of M33

We show with a short Chandra HRC-S exposure that the powerful X-ray source coincident with the nucleus of M33 is unresolved at the highest spatial resolution available to date (0"4). The source flux is variable, doubling during the 5 ks exposure. The combination of properties exhibited by M33 X-8 establishes the source as the nearest example of the ultra-luminous X-ray sources that have been uncovered in other nearby galaxies. On short timescales, we set limits of 9% r.m.s. variability for pulsations in the 0.01-1000 Hz range.

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arXiv2004

GRS 1915+105: The first three months with INTEGRAL

GRS 1915+105 is being observed as part of an Open Time monitoring program with INTEGRAL. Three out of six observations from the monitoring program are presented here, in addition to data obtained through an exchange with other observers. We also present simultaneous RXTE observations of GRS 1915+105. During INTEGRAL Revolution 48 (2003 March 6) the source was observed to be in a highly variable state, characterized by 5-minute quasi-periodic oscillations. During these oscillations, the rise is faster than the decline, and is harder. This particular type of variability has never been observed before. During subsequent INTEGRAL revolutions (2003 March-May), the source was in a steady or ''plateau'' state (also known as class $χ$ according to Belloni et al. 2000). Here we discuss both the temporal and spectral characteristics of the source during the first three months of observations. The source was clearly detected with all three gamma-ray and X-ray instruments onboard INTEGRAL.

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arXiv2004

Long-term optical/infrared variability in the quiescent X-ray transient V404 Cyg

We present the results of optical and infrared photometry of the quiescent X-ray transient V404 Cyg during the period 1992-2003. The ellipsoidal modulations extracted from the most complete databases (years 1992, 1998 and 2001) show unequal maxima and minima with relative strength varying from year to year although their peak to peak amplitudes remain roughly constant at 0.24+-0.01 magnitudes. Fast optical variations superimposed on the secondary star's double-humped ellipsoidal modulation were detected every year with a mean amplitude of ~0.07 mags. We have not found significant changes in the activity during this decade which indicates that this variability is probably not connected to the 1989 outburst. We have found periodicities in the 1998 and 2001 data near the 6 hr quasi-periodicity observed in 1992, although we interpret it as consequence of the appearance of a flare event almost every night. Significant variability is also present in the I and near infrared (J and K_short) bands and this decreases slightly or remains approximately constant at longer wavelengths. A cross correlation analysis shows that both the R and I emission are simultaneous down to 40 s.

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arXiv2004

Evidence for Black Hole Spin in GX 339-4: XMM-Newton EPIC-pn and RXTE Spectroscopy of the Very High State

We have analyzed spectra of the Galactic black hole GX 339-4 obtained through simultaneous 76 ksec XMM-Newton/EPIC-pn and 10 ksec RXTE observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe K-alpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe K-alpha emission line profile and disk reflection spectrum rule-out an inner disk radius compatible with a Schwarzschild black hole at more than the 8 sigma level of confidence. The best-fit inner disk radius of 2-3 r_g suggests that GX 339-4 harbors a black hole with a ~ 0.8-0.9 (where r_g = GM/c^2 and a=cJ/GM^2, and assuming that reflection in the plunging region is relatively small). This confirms indications for black hole spin based on a Chandra spectrum obtained later in the outburst. The emission line and reflection spectrum also rule-out a standard power-law disk emissivity in GX 339-4; a broken power-law form with enhanced emissivity inside ~6 r_{g} gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and steep emissivity require a centrally--concentrated source of hard X-rays which can strongly illuminate the inner disk. Hard X-ray emission from the base of a jet - enhanced by gravitational light bending effects - could create the concentrated hard X-ray emission; this process may be related to magnetic connections between the black hole and the inner disk. We discuss these results within the context of recent results from analyses of XTE J1650-500 and MCG-6-30-15, and models for the inner accretion flow environment around black holes.

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