arXiv subjects

R. I. Hynes

Publications and source records attributed to R. I. Hynes.

At least 73 records · Page 4Linked to original sources

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.

astro-ph↗

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).

astro-ph↗

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.

astro-ph↗

arXiv2004

Discovery of Optical Bursts from MS1603.6+2600 = UW CrB

We report the discovery of several optical burst-like events from the low-mass X-ray binary MS1603.6+2600 (UW CrB). The events last for a few tens of seconds, exhibit a very fast rise and slow decay, and involve optical brightening of a factor of 2-3. The flares appear distinct from the lower level flickering and instead strongly resemble reprocessed type-I X-ray bursts as seen in a number of other neutron star low-mass X-ray binaries. In conjunction with the previously reported candidate X-ray burst, these confirm that the compact object in UW CrB is a neutron star. We examine the optical burst brightness and recurrence times and discuss how the nature of the system can be constrained. We conclude that the source is most likely an accretion disk corona source at an intermediate distance, rather than a nearby quiescent system or very distant dipper.

astro-ph↗

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.

astro-ph↗

arXiv2004

The Distance and Interstellar Sight Line to GX339-4

The distance to the black hole binary GX339-4 remains a topic of debate. We examine high-resolution optical spectra of the NaD lines resolving the velocity structure along the line of sight. We find this to be complex, with at least nine components, mostly blue-shifted, spanning a velocity range of nearly 200km/s. The presence of components with a large blue-shift rules out a nearby location and requires that the binary be located at or beyond the tangent point, implying a lower limit to the distance of ~6kpc. The presence of a significant red-shifted component at +30km/s is even more intriguing as GX339-4 also has a slightly positive systemic velocity, suggesting that the source, and this cloud, could be on the far side of the Galaxy, where the radial velocities due to Galactic rotation become positive again. If this is the case, we require a distance of ~15kpc. This is less secure than the 6kpc lower limit however. We discuss the implications of these possible distances for the outburst and quiescent luminosities, and the nature of the companion star, and argue that a large distance is consistent with these characteristics. In particular, it would explain the non-detection of the companion star during the faintest states.

astro-ph↗

arXiv2004

Bowen Fluorescence from Companion Stars in X-ray Binaries

This paper will review a new technique of detecting companion stars in LMXBs and X-ray transients in outburst using the Bowen fluorescence lines at 4634-4640 Angs. These lines are very efficiently reprocessed in the atmospheres of the companion stars, and thereby provide estimates of the K2 velocities and mass functions. The method has been applied to Sco X-1, X1822-371 and GX339-4 which, in the latter case, provides the first dynamical evidence for the presence of an accreting black hole. Preliminary results from a VLT campaign on V801 Ara, V926 Sco and XTE J1814-338 are also presented.

astro-ph↗

arXiv2004

SS433:the microquasar link with ULXs?

SS433 is the prototype microquasar in the Galaxy and may even be analogous to the ULX sources if the jets' kinetic energy is taken into account. However, in spite of 20 years of study, our constraints on the nature of the binary system are extremely limited as a result of the difficulty of locating spectral features that can reveal the nature and motion of the mass donor. Newly acquired, high resolution blue spectra taken when the (precessing) disc is edge-on suggest that the binary is close to a common-envelope phase, and hence providing kinematic constraints is extremely difficult. Nevertheless, we do find evidence for a massive donor, as expected for the inferred very high mass transfer rate, and we compare SS433's properties with those of Cyg X-3.

astro-ph↗

arXiv2003

Spectroscopy of the Optical Counterpart to Ser X-1

We present WHT and VLT spectroscopy of MM Ser, the optical counterpart to Ser X-1. We deblend the red spectra of the two close stars identified by Wachter (1997) and show that the brighter of the two is responsible for the Halpha and HeI emission, hence confirming that this is the true counterpart of the X-ray source. We also identify several HeII and NIII lines in the blue spectrum. The isolated emission lines are all remarkably narrow, with FWHM 200-300 km/s. The Bowen blend has structure suggesting that the individual components are also narrow. These narrow lines could be from the disc if the binary inclination is quite low, or they could come from a more localised region such as the heated face of the companion star. Several interstellar lines are detected and indicate that the reddening is moderate, and consistent with the neutral hydrogen column density inferred in X-rays.

astro-ph↗

arXiv2003

Multicolour observations of V404 Cyg with ULTRACAM

We present high time-resolution multicolour observations of the quiescent soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the secondary star's ellipsoidal modulation are large flares on timescales of a few hours, as well as several distinct rapid flares on timescales of tens of mins. The rapid flares, most of which show further variability and unresolved peaks, cover shorter timescales than those reported in previous observations. The power density spectrum (PDS) of the 5 s time-resolution data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this periodicity represents the Keplerian period at the transition between the thin and advective disc regions, we determine the transition radius. We discuss the possible origins for the QPO feature in the context of the advection-dominated accretion flow model. We determine the colour of the large flares and find that the i' band flux per unit frequency interval is larger than that in the g' band. The colour is consistent with optically thin gas with a temperature of ~8000 K arising from a region with an equivalent blackbody radius of at least 2 Ro, which covers 3 percent of the accretion disc's surface. Our timing and spectral analysis results support the idea that the rapid flares (i.e. the QPO feature) most likely arise from regions near the transition radius.

astro-ph↗

arXiv2003

Multiwavelength observations revealing the evolution of the outburst of the black hole XTE J1118+480

We report multiwavelength observations of the soft X-ray transient (SXT) XTE J1118+480, which we observed with UKIRT, HST, RXTE, EUVE and many other instruments and facilities. Adding radio (Ryle Telescope, VLA), sub-millimeter (JCMT) and X-ray (Chandra and SAX) data from the literature, we assembled the most complete spectral energy distribution (SED) of this source yet published. We followed the evolution of this source for 1 year, including 6 observations performed during the outburst, and one observation at the end of the outburst. Because of XTE J1118+480's unusually high galactic latitude, it suffers from very low extinction, and its SED is near-complete, including EUV (Extreme Ultraviolet) observations. XTE J1118+480 exhibits an unusually low low-hard state (estimated inner radius 350Rs) and a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We discuss the interstellar column density and show that it is low, between 0.80 and 1.30 x 10^20 cm-2. We analyse the evolution of the SED during the outburst, including the contributions from the companion star, the accretion disc, the outflow, and relating irradiation and variability of the source in different bands to the SED. We find no significant spectral variability during the outburst evolution, consistent with the presence of a steady outflow. Analysis of its outflow to accretion energy ratio suggests that the microquasar XTE J1118+480 is analogous to radio-quiet quasars. This, combined with the inverted spectrum from radio to optical, makes XTE J1118+480 very similar to other microquasar sources, e.g. GRS 1915+105 and GX 339-4 in their low/hard state. We model the high-energy emission with a hot disc model, and discuss different accretion models for XTE J1118+480's broad band spectrum.

astro-ph↗

arXiv2003

The remarkable rapid X-ray, ultraviolet, optical, and IR variability in the black hole XTE J1118+480

The transient black hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April-July outburst. We examine time-resolved X-ray, ultraviolet, optical, and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the UV variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in HST timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star - the lack of variation in the lags is not consistent with this given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the IR flux as due to synchrotron emission rather than thermal disc emission. In fact the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar indicating a common origin.

astro-ph↗

arXiv2003

Bowen Fluorescence from the Companion Star in X1822-371

We present a clear evidence for the motion of the companion star in the Low Mass X-Ray Binary (LMXB) X1822-371. We detect NIII lambda 4640 emission moving in antiphase with the radial velocity curve of the neutron star and produced on the X-ray heated hemisphere of the donor star. From the motion of this feature we derive a lower limit to the radial velocity semi-amplitude of the companion star K2 > 300 +_ 8 km/s, which, combined with a previous determination of the inclination angle and the pulsar's radial velocity curve, yield M2 > 0.36(2) Msun and M1 > 1.14(6) Msun. The HeI lambda 4471 absorption line moves at lower velocities (~ 225 km/s) and with a -0.05 phase shift, suggests a likely origin on the gas stream near the L1 point. In addition, we detect an S-wave emission of OVI lambda 3811 produced by illumination of the hot-spot bulge by the central source. The Balmer lines are dominated by broad absorptions probably due to obscuration of the accretion disc by vertically extended cool material from the splash region and overflowing stream. We also derive a more accurate, and significantly different (compared to earlier work) systemic velocity of gamma = -44 +_ 5 km/s based on the motion of the HeII lambda 4686 wings and Doppler tomography. This work confirms the power of imaging the companion stars in LMXBs and outbursting transients using the Bowen fluorescence transitions.

astro-ph↗

arXiv2003

Dynamical Evidence for a Black Hole in GX 339-4

We present outburst spectroscopy of GX339-4 which may reveal the motion of its elusive companion star. NIII lines exhibit sharp emission components moving over ~300 km/s in a single night. The most plausible interpretation of these components is that they are formed by irradiation of the companion star and the velocities indicate its orbital motion. We also detect motion of the wings of the HeII 4686A line and changes in its morphology. No previously proposed period is consistent with periodic behavior of all of these measures. However, consistent and sensible solutions are obtained for periods around 1.7 days. For the best period, 1.7557 days, we estimate a mass function of 5.8 +/- 0.5 Msun. Even allowing for aliases, the 95% confidence lower-limit on the mass function is 2.0 Msun. GX339-4 can therefore be added to the list of dynamical black hole candidates. This is supported by the small motion in the wings of the HeII line; if the compact object velocity is not larger than the observed motion then the mass ratio is q < 0.08, similar to other systems harboring black holes. Finally, we note that the sharp components are not always present, but do seem to occur within a repeating phase range. This appears to migrate between our epochs of observation, and may indicate shielding of the companion star by a variable accretion geometry such as a warp.

astro-ph↗

arXiv2002

Fast Photometry of Quiescent Soft X-ray Transients with the Gemini-South Acquisition Camera

We present a compilation of high time-resolution photometric observations of quiescent soft X-ray transients obtained with the acquisition camera of Gemini-South. A0620-00 was observed with a short cycle time and high precision. Superimposed on the ellipsoidal modulation we find several prominent flares together with weaker continual variability. The flares seen sample shorter timescale than those reported in previous observations, with rise times as low as 30s or less; most flares show unresolved peaks. The power density spectrum (PDS) of A0620-00 appears to exhibit band-limited noise closely resembling the X-ray PDS of black hole candidates in their low states, but with the low-frequency break at a lower frequency. X-ray Nova Mus 1991 shows much larger amplitude flares than A0620-00 and if a break is present it is at a lower frequency. X-ray Nova Vel 1993 shows very little flaring and is, like A0620-00, dominated by the ellipsoidal modulation. We discuss the possible origins for the flares. They are clearly associated with the accretion flow rather than an active companion, but whether they originate in the outer disc, or are driven by events in the inner region is not yet resolved. The similarities of the PDS to those of low/hard state sources would support the latter interpretation, and the low break frequency is as would be expected if this frequency approximately scales with the size of an inner evaporated region. We also report the discovery of a new variable star only 14arcsec from XN Mus 1991. This appears to be a W UMa star, with an orbital period of about 6hrs.

astro-ph↗

arXiv2002

Discovery of 15-second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst

We report the discovery of 15-s oscillations in ultraviolet observations of WZ Sge obtained with the Hubble Space Telescope approximately one month after the peak of the 2001 outburst. This is the earliest detection of oscillations in WZ Sge following an outburst and the first time that a signal near 15 s has been seen to be dominant. The oscillations are quite strong (amplitude about 5%), but not particularly coherent. In one instance, the oscillation period changed by 0.7 s between successive observations separated by less than 1 hour. We have also found evidence for weaker signals with periods near 6.5 s in some of our data. We discuss the implications of our results for the models that have been proposed to account for the 28-s oscillations seen in quiescence. If the periods of the 15-s oscillations can be identified with the periods of revolution of material rotating about the white dwarf, the mass of the white dwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-s signals is M_WD > 1.03 M_sun.

astro-ph↗

arXiv2002

The system parameters of DW Ursae Majoris

We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the Hubble Space Telescope. Our data were obtained during a low state of the system, in which the UV light was dominated by the hot white dwarf (WD) primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set firm limits on the masses and radii of the system components and the distance between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014, M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5 the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's mass-period relation for CV secondaries, our estimates for the system parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001, M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82 \pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160 pc, from time-resolved I- and K-band photometry. Finally, we have repeated Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis. In this way we obtained a second estimate for the distance, d = 590 \pm 100 pc, which allows us to obtain a second estimate for the spectral type of the secondary, M7 \pm 2.0. We conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods.

astro-ph↗

arXiv2002

Spectroscopic observations of the candidate sgB[e]/X-ray binary CI Cam

We present a compilation of spectroscopic observations of the sgB[e] star CI Cam. This includes data from before, during, and after its 1998 outburst. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with later data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with FeII and [NII] lines still a factor of ~2 above the pre-outburst level and HeI, HeII, and NII lines suppressed by a factor of 2-10. We find that the spectral properties of CI Cam are similar to other sgB[e] stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other objects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. We suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. [Abbreviated]

astro-ph↗