The Astrophysical Journal, 563:L139–L142, 2001 December 20 䉷 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
SIMULTANEOUS CHANDRA AND HUBBLE SPACE TELESCOPE OBSERVATIONS OF SMC X-11 S. D. Vrtilek and J. C. Raymond Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138;
[email protected],
[email protected]
B. Boroson and T. Kallman NASA Goddard Space Flight Center, Code 666, Greenbelt, MD 20771
H. Quaintrell Open University, Walton Hall, Milton Keynes MK7 6AA, England, UK
and R. McCray JILA, University of Colorado, Campus Box 440, Boulder, CO 80309-0440 Received 2001 August 24; accepted 2001 November 15; published 2001 December 4
ABSTRACT We present first results from simultaneous ultraviolet (Hubble Space Telescope/STIS) and X-ray (Chandra/ ACIS) observations of the SMC X-1/SK 160 eclipsing binary system. Observations covering four orbital phases during each of the X-ray high and X-ray low states were taken in 2000 October–November and 2001 April. The ultraviolet P Cygni lines show strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line of sight. The effect is visible during both the X-ray high and X-ray low states; the UV continuum flux remains roughly constant in spite of more than an order-of-magnitude reduction in X-ray flux, as expected if the X-ray flux reduction is due to occultation of the X-ray source by a precessing disk rather than an intrinsic change in X-ray luminosity. The X-ray spectra are dominated by continuum emission in the X-ray high state. Occultation of the neutron star by the disk during the low state also implies that X-ray emission from the disk surface should be present, and the low-state spectra do show strong emission lines. During eclipse and during the X-ray low state, the continuum emission largely disappears, and we see line emission from O, Ne, Mg, and Fe and possibly from Si and S. The emission lines are consistent with recombination lines from mostly hydrogenic and helium-like species, which could be produced by photoionization in an extended stellar wind. Subject headings: accretion, accretion disks — binaries: close — pulsars: individual (SMC X-1) — ultraviolet: stars — X-rays: stars pulse-phased spectroscopy of the X-ray emission. With our multiwavelength observations we can analyze the density structure and composition of the wind; set limits on the size, shape, rotation, and precession of the disk; and determine the effects of X-ray illumination on the disk, the star, and the stellar wind. Detailed papers (in preparation) on the above goals will be presented later. Here we report on our successful search for the “Hatchett-McCray” effect: a correlation between orbital phase and “bleaching” of important ultraviolet lines by X-ray photoionization (Hatchett & McCray 1977), as well as on the first positive detection (in SMC X-1) of recombination lines during the eclipse and low-state X-ray observations.
1. INTRODUCTION
SMC X-1 is the most rapidly spinning and at times the most luminous of the persistent accretion-powered X-ray pulsars. In addition to showing X-ray pulses with a 0.71 s period and orbital eclipses every 3.89 days, the uneclipsed X-ray flux of SMC X-1 exhibits aperiodic variabilities that include quasiperiodic oscillation at 0.06 Hz (Wojdowski et al. 1998), X-ray bursts (Angelini, Stella, & White 1991), and a “superorbital” period of 50–60 days, reported by Gruber & Rothschild (1984) and confirmed by several X-ray observatories (Levine et al. 1996; Wojdowski et al. 1998). In 2000 October–November and 2001 April we undertook simultaneous observations of SMC X-1 and its B0 companion Sk 160 with the Hubble Space Telescope (HST ), Chandra, and ground-based optical telescopes. The HST/Space Telescope Imaging Spectrograph (STIS) observations described here represent the first ultraviolet data of SMC X-1 that are of sufficient spectral and temporal resolution to generate Doppler tomograms and search for ultraviolet QPOs. The time-tagged HST data also allow us to search for the aperiodic variability near 0.1–0.2 Hz reported by Wojdowski et al. (1998) throughout the lines and continuum and for the 0.7 s X-ray pulsar period. The Chandra Advanced CCD Imaging Spectrometer-S (ACISS) in continuous clocking (CC) mode enables us to conduct
2. OBSERVATIONS
SMC X-1/SK 160 was observed on 10 separate occasions: four times during the high state of the superorbital cycle and six times during the low state. The location of the observations relative to the superorbital period are marked with arrows in Figure 1. These indicate that the 2000 October observations took place during a high state of the ∼55 day cycle; the 2000 November and 2001 April observations took place during low states. Orbital phases were calculated using the period and ephemeris given by Wojdowski et al. (1998). HST/STIS observed for 20 HST orbits with the E140M echelle spectrometer. The STIS instrument design and in-orbit performance have been described by Woodgate et al. (1998) and Kimble et al. (1998). The E140M grating provided a resolving ˚ . The power of 6 km s⫺1 in the wavelength range 1150–1736 A E140M exposures were taken with the TIMETAG mode, which
1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
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Fig. 1.—1 day averages of the flux observed from SMC X-1 with the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) provided courtesy of the RXTE/ASM team. The upper panel shows the light-curve history since the launch of RXTE. The lower panel shows an expanded view of the times encompassing our observations. The arrows indicate the times of simultaneous HST/STIS and Chandra/ACIS observations. The error bars represent 1 j counting statistics.
stamps each photon detected with a time accurate to 125 ms. The data were reduced using the STSDAS routines available through IRAF. Simultaneous observations with the Chandra ACIS (Garmire et al. 1992) in CC mode were conducted for the first six of the eight HST observations conducted in 2000 and both HST observations conducted in 2001. The CC mode was used to allow 3 ms timing at the expense of one dimension of spatial resolution and also to mitigate effects due to pileup. A backilluminated ACIS chip (S3) was used to obtain the best lowenergy response. Data were extracted with the Chandra X-Ray Center (CXC) CIAO v2.1 software using the latest gain map for S3 (CALDB 2.7) and fitted with Sherpa, the CXC modeling and fitting program. 2.1. X-Ray Spectra The pulse-averaged X-ray (0.1–200 keV) spectrum of SMC X-1 during the high state of the superorbital cycle has been described as a power law with a high-energy cutoff, a thermal component at low energies and Fe K-shell emission (Woo et al. 1995), or a power law with photon index (g p ⫺0.94) and a high-energy cutoff at 10 keV with a 15 keV folding energy as determined by Wojdowski, Clark, & Kallman (2000, hereafter WCK) using ASCA data. Since the Fe emission from SMC X-1 is very weak, we use the WCK model, which provides acceptable fits to the continuum emission. The Chandra ACIS-S source counts were extracted from a rectangular area of 4 # 17 pixel2 from the CC-mode image, and an equivalent area spaced 100 pixels away was used for the background (Fig. 2). The background was negligible during the high state and less than 3% of the source counts during the low state. The high-state out-of-eclipse count rates (40–54 counts s⫺1) translate to 0.4–0.5 counts frame⫺1 (for CC mode the frame time is 3 # 0.00285 p 0.00855 s), corresponding to pileup fractions of 15%–20%. During eclipse and the low state we had fewer than 0.03 counts frame⫺1, and pileup is negligible. Since pileup distorts the spectra by decreasing counts at low energies and adding them at high energies, we restricted our fits during the high state to the WCK model noted above. Here we use these fits only to estimate the fluxes in the 0.3–7.5 keV band; these ranged from 6 # 10⫺12 ergs cm⫺2 s⫺1 during orbital phase 0.03 in the X-ray low state to 1 # 10⫺9 ergs cm⫺2 s⫺1 at orbital phase 0.5 during the X-ray high state. For a distance of 59 kpc (Mathewson, Ford, & Visvanathan
Fig. 2.—Chandra ACIS-S spectra. Orbital phase IS are indicated for each spectrum. (a)–(d) are during the X-ray high state; (e)–(h) are during the X-ray low state. The integration times are typically ∼8 ks.
1986), this corresponds to an X-ray luminosity for SMC X-1 of L X p 4.3 # 10 38 ergs s⫺1 during the high state. During the low state and during X-ray eclipse, the continuum is reduced by more than an order of magnitude, and emission lines due to O, Ne, Mg, Si, S, and Fe become prominent (Fig. 2). As a first attempt in fitting our eclipse spectra, we used the same continuum model as for the high state (as appropriate for electron scattering of the uneclipsed spectrum) and augmented it with bright recombination lines determined by the XSTAR code (Kallman & Krolik 1999) using the same input spectrum as WCK for different values of the ionization parameter y p L X /nr 2 (Tarter, Tucker, & Salpeter 1969; where L X is the X-ray luminosity, n is the proton number density, and r is the distance from the ionizing source). We used a metal abundance of 15 solar as expected for SMC X-1. Our observations are consistent with line emission from a photoionized plasma when the ionization parameter is in the range 1.0 ≤ log y ≤ 1.5. This range of ionization parameters is consistent with the density distribution derived from a hydrodynamic simulation of the X-ray–perturbed wind in SMC X-1 (Blondin & Woo 1995; Woo et al. 1995): our measured values of L and y, and a value of r (2 # 10 12 cm) reasonable for SMC X-1 yields densities of (3–10) # 10 12 cm⫺3. Some strong lines identified in the simulations and their strengths as measured in our spectra are listed in Table 1. In Figure 3 we show the best fits to our eclipse observations: a reduced x 2 of 0.9 for 117 degrees of freedom at orbital phase 0.03 in the X-ray low state and a reduced x 2 of 1.1 for 177 degrees of freedom at orbital phase 0.0 in the X-ray high state. The line energies were held constant at the expected values and the line widths constrained to 50 eV. The line fluxes measured from these fits are listed in Table 1. While only five of the lines are detected at greater than 3 j, the fact that we detect all the lines in each of five separate observations taken days and months apart gives us confidence that these features are real. Both models (Jimenez-Garate et al. 2001) and observations of other systems (Sako et al. 1999) indicate that radiative recombination continua (RRCs) should be present at strengths up to half the intensities of the Lyman lines. The Ne ix feature,
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TABLE 1 Measured X-Ray Line Intensities during Eclipse Flux (photons cm⫺2 s⫺1)
Ion
Line Energy (keV)
High State
Low State
O vii . . . . . . . . O viii . . . . . . . O viii . . . . . . . Ne ix . . . . . . . . Ne x . . . . . . . . Mg xi . . . . . . . Si xiii . . . . . . . S x .......... Ca xiii . . . . . . Fe xxv . . . . . .
0.57 0.65 0.84 0.91 1.02 1.30 1.85 2.40 3.83 6.65
… (1.1 Ⳳ 0.1)E⫺4 … (1.2 Ⳳ 0.1)E⫺4 (7.8 Ⳳ 0.8)E⫺5 (3.2 Ⳳ 0.6)E⫺5 !5.0E⫺6 … … (2.8 Ⳳ 0.8)E⫺5
(4.1 Ⳳ 1.9)E⫺5 (3.9 Ⳳ 1.0)E⫺5 (4.4 Ⳳ 0.9)E⫺5 (5.6 Ⳳ 0.8)E⫺5 (1.6 Ⳳ 0.5)E⫺5 (1.3 Ⳳ 0.4)E⫺5 !5.0E⫺6 (7.0 Ⳳ 5.0)E⫺6 (9.7 Ⳳ 4.4)E⫺6 (2.5 Ⳳ 0.8)E⫺5
Notes.—Errors (1 j) were calculated using the upper and lower bounds provided by the command uncertainty in Sherpa. If no flux is listed, the line was not used for the fit.
which is broad and surprisingly strong compared to Ne x, may contain a significant contribution from the O viii RRC. The low state is believed to occur when the precessing accretion disk occults the neutron star, and the observed X-ray emission arises from reprocessing in the visible part of the disk and perhaps the surface of the companion. Jimenez-Garate et al. (2001) computed model X-ray spectra for a low-mass X-ray binary accretion disk illuminated by a central source. The models are qualitatively similar to the observed low-state line spectra outside eclipse when scaled to the distance of the SMC, in spite of the low SMC elemental abundances. Comparison of the low-state spectra in and out of eclipse suggests that both the photoionized wind and the illuminated disk contribute to the out-of-eclipse spectra. The factor of 2 increase in brightness between phases 0.2 and 0.6 suggests that the heated face of the companion makes a substantial contribution. 2.2. Ultraviolet Spectra The ultraviolet spectra of SMC X-1/Sk 160 show absorptionline features that are typical for hot stars with equivalent widths comparable to other sources in the Magellanic Clouds. The equivalent widths are weaker than in our Galaxy, as expected owing to the lower metallicity in the Clouds. In Figure 4 we show STIS observations of N v, Si iv, and C iv during orbital phases near 0.0 and 0.5 which represent the extremes as expected: at phase 0.5, the X-ray source has the greatest impact on the stellar wind, and ionization of the wind reduces the absorption lines; at phase 0.0 (X-ray eclipse), the X-ray source has the least influence, and we see mostly wind absorption from the giant companion. Interstellar absorption lines from S ii ll1250.5, 1253.8 are visible near N ll1238, 1242. The residual absorption remaining at phase 0.5 for C iv and Si iv is likely due to a combination of interstellar absorption and the photospheric absorption of the B0 supergiant. Our identification of these features will be presented in a later paper. These lines were also detected in our observations of LMC X-4 with velocities consistent with an origin in our Galaxy. It is also clear that the X-ray high and low states exert very little influence on the ultraviolet emission, as expected if the X-ray states are caused by obscuration from a precessing accretion disk rather than intrinsic luminosity variations. 3. DISCUSSION AND SUMMARY
In systems with high-mass OB companions, gravitational capture of the stellar wind contributes significantly to the ac-
Fig. 3.—Chandra ACIS-S spectra. (a) X-ray high state; forb p 0.00. (b) Xray low state; forb p 0.03. The data are represented as histograms; the fits (smooth lines) are described in § 2.2.
cretion flow that powers the X-ray emission. In principle, X-ray emission from such systems can be used to measure directly the density of the stellar wind along a variety of lines of sight. In practice, there is an additional complication, since X-rays from the compact object photoionize the wind material, changing the radiative coupling between some of the wind material and the stellar UV flux that drives the outflow. Observations of such systems give us an important probe of the dynamics of stellar winds and of accretion flows. During our simultaneous ultraviolet and X-ray observations of SMC X-1, the ultraviolet P Cygni lines show dramatic changes with orbital phase, with strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is
Fig. 4.—HST/STIS spectra. Solid black lines indicate orbital phases near 0.5 and dotted lines indicate orbital phases near 0.0. (a) N v during X-ray low state. (b) Si iv during X-ray low state. (c) C iv during X-ray low state. (d) N v during X-ray high state. (e) Si iv during X-ray high state. ( f ) C iv during X-ray high state.
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in the line of sight. A clear example of the Hatchett-McCray effect. The effect is visible during both the X-ray high and Xray low states; the ultraviolet continuum flux remains roughly constant in spite of more than an order-of-magnitude reduction in X-ray flux probably because the B0 star dominates the ultraviolet flux. The X-ray spectra are dominated by continuum emission in the X-ray high state. During eclipse and during the X-ray low state, the continuum largely disappears and we see line emission from O, Ne, Mg, and Fe and possibly Si and S. The continuum spectra in and out of eclipse have the same form and are consistent with the eclipse continuum due to nonthermal emission from the neutron star that is scattered into our line of sight by free electrons in the wind. WCK reported no significant line emission during eclipse from ASCA observations of SMC X-1, with the possible exception of a feature that they noted was near the energy of the fluourscence line of neutral Si, 1.74 keV. However, WCK included substantial out-of-eclipse data (with count rates a factor of 3 higher than during the actual eclipse) in order to improve their signal-to-noise ratio; they also combined the SIS0 and SIS1 instruments since the systematic calibration errors between the two detectors were smaller than the statistical errors due to the low count rate. Since the observed lines are weak relative to the continuum outside eclipse, the improvement in total signal-to-noise ratio achieved by using out-of-eclipse data may have, in fact, reduced their ability to detect line features. We note that our eclipse spectra are very similar to the synthetic spectra produced from
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the Blondin & Woo (1995) hydrodynamic wind model by WCK (their Figs. 5 and 14) for log y p 2.5. Sako et al. (1999) required both recombination lines and fluorescent lines to fit their ASCA observations of Vela X-1, and they invoked cold clumped material to produce the latter, in addition to the diffuse hot gas that produces the recombination lines. Our observations show several significant emission lines that are attributable to recombination features from intermediate ionized species; the feature mentioned by WCK as requiring the presence of low ionization states could in fact be the recombination line of Si xiii at 1.85 keV for which we have marginal evidence. The presence of recombination lines from intermediate ionization states and lack of fluorescence lines in our observations is in accord with the three-dimensional hydrodynamic simulation of Blondin & Woo (1995), which requires a substantial amount of material with 1 ≤ log y ≤ 3 and no material at low enough ionization to produce fluorescence lines. Definitive detection of the X-ray lines described here should be possible with the grating spectrometers on either Chandra or XMM. We are extremely grateful to the mission planning teams of the Chandra X-Ray Observatory and Hubble Space Telescope for a magnificent job coordinating the two satellites for this project. S. D. V. would like to acknowledge support from NASA grants NAG 5-6711 and GO0-8566.01-A through STScI and GO1-2028X through the Chandra X-Ray Center.
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