arXiv subjects

P. A. Charles

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

At least 73 records · Page 4Linked to original sources

arXiv2008

Phase resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during the 2008 outburst

aims: We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K_2), and derive estimates on the pulsar mass. methods: Using Doppler images of the Bowen region we find a significant (>8sigma) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure K_em=248+/-20 km/s (1sigma confidence) which represents a strict lower limit to K_2. Also, the Doppler map of He II lambda4686 shows the characteristic signature of the accretion disk, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disk that are expected in very low mass ratio interacting binaries. results: The lower-limit on K_2 leads to a lower-limit on the mass function of f(M_1)>0.10M_sun. Applying the maximum K-correction gives 228<K_2<322 km/s and a mass ratio of 0.051<q<0.072. conclusions: Despite the limited S/N of the data we were able to detect a signature of the donor star in SAX J1808.4-3658, although future observations during a new outburst are still warranted to confirm this. If the derived K_em is correct, the largest uncertainty in the determination of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.

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arXiv2008

On the triple nature of the X-ray source 4U2129+47 (= V1727 Cyg)

Context. In quiescence, the proposed optical counterpart to the neutron star low mass X-ray binary 4U 2129+47 (V1727 Cyg) shows a spectrum consistent with a late F-type subgiant and no radial velocity variations on the 5.24 hour binary period. This could imply that V1727 Cyg is a chance line of sight interloper. Radial velocity measurements, however, showed evidence for a longer term ~40 km/s shift, which suggested that 4U 2129+47 could be a hierarchical triple system, with the F-type star in a wide orbit about the inner low mass X-ray binary. Aims. In order to confirm the long-term radial velocity shift reported in Garcia et al. (1989) and its amplitude, we obtained spectroscopic observations of V1727 Cyg during 1996 and 1998 with the William Herschel Telescope using the ISIS spectrograph. Methods. We determined radial velocities from the ISIS spectra by means of the cross-correlation technique with a template spectrum. Results. The resulting radial velocities show variations with a maximum amplitude of ~40 km/s, confirming previous results and supporting the F-type star as being the third body in a hierarchical triple system. The odds that this star could be an interloper are ~3e-6

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arXiv2008

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VI. A multi-observatory identification campaign

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

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arXiv2007

Unveiling the core of the Globular Cluster M15 in the Ultraviolet

We have obtained deep far- (FUV) and near-ultraviolet (NUV) images of the inner region of the dense globular cluster M15 with the Advanced Camera for Surveys on board the Hubble Space Telescope. The FUV-NUV colour-magnitude diagram shows a well defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main sequence turn-off is clearly visible at FUV~23.5 mag and FUV-NUV~3 mag, and the main sequence stars form a prominent track that extends at least two magnitudes below the main sequence turn-off. As such, this is the deepest FUV-NUV colour-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically-formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colours and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variables, one known and one probable SX Phoenicis star, and the well known low-mass X-ray binary AC211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyraes and SX Phoenicis exhibit massive variability amplitudes in this waveband (several mags).

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arXiv2007

Bowen blend echo-tomography of low mass X-ray binaries

We present simultaneous high time resolution (1-10 Hz) X-ray and optical observations of the persistent LMXBs Sco X-1 and V801 Ara(=4U 1636-536). In the case of Sco X-1 we find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the donor star. We also present the detection of three correlated X-ray/optical bursts in V801 ara. Although this latter project is still in progress our preliminary results obtained by subtracting the Continuum light-curve from the Bowen/HeII data provide evidence of orbital phase dependent echoes from the companion star.

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arXiv2007

A signature of the donor star in the extra-galactic X-ray binary LMC X-2

Two nights of phase-resolved medium resolution VLT spectroscopy of the extra-galactic low mass X-ray binary LMC X-2 have revealed a 0.32+/-0.02 day spectroscopic period in the radial velocity curve of the HeII lambda4686 emission line that we interpret as the orbital period. However, similar to previous findings, this radial velocity curve shows a longer term variation that is most likely due to the presence of a precessing accretion disk in LMC X-2. This is strengthened by HeII lambda4686 Doppler maps that show a bright spot that is moving from night to night. Furthermore, we detect narrow emission lines in the Bowen region of LMC X-2,with a velocity of K_em=351+/-28 km/s, that we tentatively interpret as coming from the irradiated side of the donor star. Since K_em must be smaller than K_2, this leads to the first upper-limit on the mass function of LMC X-2 of f(M_1)>=0.86Msun (95% confidence), and the first constraints on its system parameters.

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arXiv2007

Kinematical Studies of the Low Mass X-Ray Binary GR Mus (XB1254-690)

We present simultaneous high-resolution optical spectroscopy and X-ray data of the X-ray binary system GR Mus (XB1254-690), obtained over a full range of orbital phases. The X-ray observations are used to re-establish the orbital ephemeris for this source. The optical data includes the first spectroscopic detection of the donor star in this system, through the use of the Doppler Tomography technique on the Bowen fluorescence blend (~4630-4650 A). In combination with an estimate for the orbital parameters of the compact object using the wings of the He II 4686 emission line, dynamical mass constraints of 1.20 < M_X/M_{sun} < 2.64 for the neutron star and 0.45 < M_2/M_{sun} < 0.85 for the companion are derived.

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arXiv2007

Optical spectroscopy of the low mass X-ray binary GX9+9

Phase-resolved medium resolution VLT spectroscopy of the low mass X-ray binary GX9+9 has revealed narrow CIII emission lines that move in phase relative to our new estimate of the ephemeris, and show a velocity amplitude of 230+/-35 km/s. We identify the origin of these lines as coming from the surface of the donor star, thereby providing the first estimate of the mass function of f(M_1)>=0.22M_sun. Rotational broadening estimates together with assumptions for the mass donor give 0.07<q<0.35 and 182<K_2<406 km/s. Despite a low mass ratio, there is no evidence for a superhump in our dataset. Doppler maps of GX9+9 show the presence of a stream overflow, either in the form of material flowing downward along the accretion disk rim or in a similar fashion as occurs in high mass transfer rate cataclysmic variables known as the SW Sex stars. Finally we note that the Bowen region in GX9+9 is dominated by CIII instead of NIII emission as has been the case for most other X-ray binaries.

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arXiv2007

IGR J16194-2810: a new symbiotic X-ray binary

We here report on the multiwavelength study which led us to the identification of X-ray source IGR J16194-2810 as a new Symbiotic X-ray Binary (SyXB), that is, a rare type of Low Mass X-ray Binary (LMXB) composed of a M-type giant and a compact object. Using the accurate X-ray position allowed by Swift/XRT data, we pinpointed the optical counterpart, a M2 III star. Besides, the combined use of the spectral information afforded by XRT and INTEGRAL/IBIS shows that the 0.5-200 keV spectrum of this source can be described with an absorbed Comptonization model, usually found in LMXBs and, in particular, in SyXBs. No long-term (days to months) periodicities are detected in the IBIS data. The time coverage afforded by XRT reveals shot-noise variability typical of accreting Galactic X-ray sources, but is not good enough to explore the presence of X-ray short-term (seconds to hours) oscillations in detail. By using the above information, we infer important parameters for this source such as its distance (about 3.7 kpc) and X-ray luminosity (about 1.4e35 erg/s in the 0.5-200 keV band), and we give a description for this system (typical of SyXBs) in which a compact object (possibly a neutron star) accretes from the wind of its M-type giant companion. We also draw some comparisons between IGR J16194-2810 and other sources belonging to this subclass, finding that this object resembles SyXBs 4U 1700+24 and 4U 1954+31.

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arXiv2007

Echoes from the companion star in Sco X-1

We present simultaneous X-ray (RXTE) and optical (ULTRACAM) narrow band (Bowen blend/HeII and nearby continuum) observations of Sco X-1 at 2-10 Hz time resolution. We find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of ~11-16s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ~0.5 when Sco X-1 is at the top of the Flaring Branch. Evidence of echoes is also seen at the bottom of the Flaring Branch but with time-lags of 5-10s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.

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arXiv2006

A detection of the donor star of Aquila X-1 during its 2004 outburst?

Phase-resolved high resolution optical spectroscopy has revealed narrow N III and He II emission lines from the soft X-ray transient Aquila X-1 during its 2004 outburst that move as a function of the orbit consistent with the phasing of the donor star. Under the assumption that these lines come from the irradiated side of the donor star, we can constrain its K_2 velocity to >247+/-8 km/s, and derive a mass function of f(M_1)>1.23+/-0.12M_sun. Estimates for the rotational broadening based on the emission components suggest a possible massive neutron star of >1.6M_sun (at 95% confidence). However, an updated ephemeris and additional high resolution spectroscopy of Aql X-1 during a future outburst are warranted in order to confirm that the narrow lines indeed originate on the donor star surface, and reliably characterise the system parameters of this important X-ray binary. Spectra taken during the end of the outburst show that the morphology of the emission lines changed dramatically. No donor star signature was present anymore, while the presence of narrow low-velocity emission lines became clear, making Aql X-1 a member of the slowly growing class of low-velocity emission line sources.

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arXiv2006

Detection of the Irradiated Donor in the LMXBs 4U 1636-536 (=V801 Ara) and 4U 1735-444 (=V926 Sco)

Phase-resolved VLT spectroscopy of the bursting Low Mass X-ray Binaries 4U 1636-536/V801 Ara and 4U 1735-444/V926 Sco is presented. Doppler images of the NIII 4640 Bowen transition reveal compact spots which we attribute to fluorescent emission from the donor star and enable us to define a new set of spectroscopic ephemerides. We measure Kem=277+-22 km/s and Kem=226+-22 km/s from the NIII spots in V801 Ara and V926 Sco respectively which represent strict lower limits to the radial velocity semi-amplitude of the donor stars. Our new ephemerides provide confirmation that lightcurve maxima in V801 Ara and likely V926 Sco occur at superior conjunction of the donor star and hence photometric modulation is caused by the visibility of the X-ray heated donor. The velocities of HeII 4686 and the broad Bowen blend are strongly modulated with the orbital period, with phasing supporting emission dominated by the disc bulge. In addition, a reanalysis of burst oscillations in V801 Ara, using our spectroscopic T0, leads to K1=90-113 km/s. We also estimate the K-corrections for all possible disc flaring angles and present the first dynamical constraints on the masses of these X-ray bursters. These are K2=360+-74 km/s, f(M)=0.76+-0.47 Msun and q=0.21-0.34 for V801 Ara and K2=298+-83 km/s, f(M)=0.53+-0.44 Msun and q=0.05-0.41 for V926 Sco. Disc flaring angles alpha>12 deg and q~0.26-0.34 are favoured for V801 Ara whereas the lack of K1 constraint for V926 Sco prevents tight constraints on this system. Although both binaries seem to have intermediate inclinations, the larger equivalent width of the narrrow NIII line in V801 Ara at phase 0.5 relative to phase 0 suggests that it has the higher inclination of the two.

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arXiv2006

A Long, Hard Look at the Low-Hard State in Accreting Black Holes

We present the first results of coordinated multi-wavelength observations of the Galactic black hole GX 339-4 in a canonical low-hard state, obtained during its 2004 outburst. XMM-Newton observed the source for 2 revolutions, or approximately 280 ksec; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low-hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component, and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low-hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially-recessed disk in the low-hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona which is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes.

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arXiv2006

First science with SALT: peering at the accreting polar caps of the eclipsing polar SDSS J015543.40+002807.2

We describe briefly the properties of the recently completed Southern African Large Telescope (SALT), along with its first light imager SALTICAM. Using this instrument, we present 4.3 hr of high speed unfiltered photometric observations of the eclipsing polar SDSSJ015543.40+002807.2 with time resolution as short as 112 ms, the highest quality observations of this kind of any polar to date. The system was observed during its high luminosity state. Two accreting poles are clearly seen in the eclipse light curve. The binary system parameters have been constrained: the white dwarf mass is at the low end of the range expected for cataclysmic variables. Correlations between the positions of the accretion regions on or near the surface of the white dwarf and the binary system parameters were established. The sizes of the accretion regions and their relative movement from eclipse to eclipse were estimated: they are typically 4-7 deg depending on the mass of the white dwarf. The potential of these observations will only fully be realised when low state data of the same kind are obtained and the contact phases of the eclipse of the white dwarf are measured.

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arXiv2006

Multiwavelength Observations of EXO 0748-676 - I. Reprocessing of X-ray Bursts

We present the first high time-resolution simultaneous X-ray, ultraviolet, and optical observations of X-ray bursts in UY Vol, the optical counterpart of the low mass X-ray binary EXO 0748-676, obtained with RXTE, HST, and Gemini-S. Strong reprocessed signals are present in the ultraviolet (a factor of 4) and optical (a factor of 2.5). These signals are lagged with respect to the X-rays and appear significantly smeared as well. The addition of far-ultraviolet coverage for one burst, together with the high quality of the dataset, allow much tighter constraints upon the temperature and geometry of the reprocessing region than previously possible. A single-zone black body reprocessing model for this burst suggests a rise in temperatures during the burst from 18,000 to 35,000K and an emitting area comparable to that expected for the disk and/or irradiated companion star. The lags, a mean of 4.0s and range of 2.5s, are consistent with those expected within the binary. The single-zone black body model cannot reproduce the ratio of optical to ultraviolet flux during the burst, however. The discrepancy, corresponding to underpredicting the optical by more than a factor of two, seems too large to explain with deviations from a local black body spectrum and more likely indicates that a range of reprocessing temperatures are required, as would be expected, with cooler regions not contributing to the UV. Comparable results are derived from other bursts, and in particular the lag and smearing both appear shorter when the companion star is on the near side of the disk as predicted. The burst observed by HST also yielded a spectrum of the reprocessed light. [Abridged]

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. III. Observations of seven southern sources

Context. The detection, thanks to the INTEGRAL satellite, of a number of new hard X-ray sources calls for the identification of their nature by means of observations at longer wavelengths. Aims. In our continuing campaign to identify longer-wavelength counterparts of these newly-discovered hard X-ray sources detected by INTEGRAL, we observed the putative optical counterparts of seven southern sources at the South African Astronomical Observatory and at the European Southern Observatory. Methods. Optical spectroscopy was performed on all candidates of these X-ray sources. For two of these objects, optical photometry was also acquired. Results. These observations firmly established the nature of four of these sources: we found that IGR J10404-4625 (=LEDA 93974), 4U 1344-60 and IGR J16482-3036 are Active Galactic Nuclei (AGNs) at redshifts z = 0.0237, 0.013 and 0.0313, respectively, and that 2RXP J130159.6-635806 is a Galactic High-Mass X-ray Binary (HMXB). We also give possible optical identifications for three further objects, namely IGR J11215-5952, IGR J11305-6256 and IGR J16207-5129, which are consistent with being Galactic HMXBs. Physical parameters for these objects are also evaluated by collecting and discussing the available multiwavelength information. Conclusions. The detection of four definite or likely HMXBs out of seven objects in our sample further stresses INTEGRAL's crucial contribution in hunting this class of object. Also, the determination of the extragalactic nature of a substantial fraction of the INTEGRAL survey sources underlines the importance of hard X-ray observations for the study of background AGNs located beyond the 'Zone of Avoidance' of the Galactic Plane.

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arXiv2006

Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)

We report on the results of the first simultaneous X-ray (RXTE) and optical (SAAO) observations of the luminous low mass X-ray binary (LMXB) GX 9+9 in 1999 August. The high-speed optical photometry revealed an orbital period of 4.1958 hr and confirmed previous observations, but with greater precision. No X-ray modulation was found at the orbital period. On shorter timescales, a possible 1.4-hr variability was found in the optical light curves which might be related to the mHz quasi-periodic oscillations seen in other LMXBs. We do not find any significant X-ray/optical correlation in the light curves. In X-rays, the colour-colour diagram and hardness-intensity diagram indicate that the source shows characteristics of an atoll source in the upper banana state, with a correlation between intensity and spectral hardness. Time-resolved X-ray spectroscopy suggests that two-component spectral models give a reasonable fit to the X-ray emission. Such models consist of a blackbody component which can be interpreted as the emission from an optically thick accretion disc or an optically thick boundary layer, and a hard Comptonized component for an extended corona.

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arXiv2005

An ultracompact X-ray binary in the globular cluster M15 (NGC7078)

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) waveband. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semi-amplitude and we clearly detect a period of 22.5806+/-0.0002 min. We have carried out extensive Monte Carlo simulations which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV bright (approx. 17 mag) and is characterized by an extremely blue spectral energy distribution (F_lambda ~ lambda^{-2.0}). We also find evidence for an excess of flux between 1500 and 1600 Åand probably between 1600 and 2000 Å, which might be due to CIV 1550 and HeII 1640 emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is approx. 34 degree. We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.

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