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Register a new userCorrelated X-ray and optical variability in the O-type supergiant zeta Puppis
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Analysis of the recent long exposure Chandra X-ray observation of the early-type O star zeta Pup shows clear variability with a period previously reported in optical photometric studies. These 813 ks of HETG observations taken over a roughly one year time span have two signals of periodic variability: a high significance period of 1.7820 +/- 0.0008 day, and a marginal detection of periodic behavior close to either 5 day or 6 day period. A BRITE-Constellation nanosatellite optical photometric monitoring, using near-contemporaneous observations to the Chandra data, confirms a 1.78060 +/- 0.00088 day period for this star. The optical period coincides with the new Chandra period within their error ranges, demonstrating a link between these two wavebands and providing a powerful lever for probing the photosphere/wind connection in this star. The phase lag of the X-ray maximum relative to the optical maximum is approximately phi=0.45, but consideration of secondary maxima in both datasets indicates possibly two hot spots on the star with an X-ray phase lag of phi=0.1 each. The details of this periodic variation of the X-rays are probed by displaying a phased and trailed X-ray spectrum and by constructing phased light curves for wavelength bands within the HETG spectral coverage, ranging down to bands encompassing groups of emission lines. We propose that the 1.78 day period is the stellar rotation period and explore how stellar bright spots and associated co-rotating interacting regions or CIRs could explain the modulation of the optical and X-ray output for this star and their phase difference.
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Previously, the X-ray emission of zeta Puppis was found to be variable with light curves harbouring trends with a typical timescale longer than the exposure length. The origin of these changes was proposed to be linked to large-scale structures in the wind, but further characterization of the variability at high energies was needed. Since then, a number of new X-ray observations have become available. Furthermore, a cyclic behaviour with a 1.78d period was identified in long optical photometric runs, which is thought to be associated with the launching mechanism of large-scale wind structures. We analysed these new X-ray data, revisited the old data, and compared X-ray with optical data, including when simultaneous. We found that the behaviour in X-rays cannot be explained in terms of a perfect clock because the amplitude and shape of its variations change with time. For example, zeta Puppis was much more strongly variable between 2007 and 2011 than before and after this interval. Comparing the X-ray spectra of the star at maximum and minimum brightness yields no compelling difference beyond the overall flux change: the temperatures, absorptions, and line shapes seem to remain constant, well within errors. The only common feature between X-ray datasets is that the variation amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no clear and coherent correlation can be found between simultaneous X-ray and optical data. Only a subgroup of observations may be combined coherently with the optical period of 1.78d, although the simultaneous optical behaviour is unknown. The currently available data do not reveal any obvious, permanent, and direct correlation between X-ray and optical variations. The origin of the X-ray variability therefore still needs to be ascertained, highlighting the need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.
SAX J2103.5+4545 is the Be/X-ray binary with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work is to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. That is to say, when the energy spectrum is soft the power at high frequencies is suppressed. We also present the results of our monitoring of the Halpha line of the optical counterpart since its discovery in 2003. There is a correlation between the strength and shape of the Halpha line, originated in the circumstellar envelope of the massive companion and the X-ray emission from the vicinity of the neutron star. Halpha emission, indicative of an equatorial disc around the B-type star, is detected whenever the source is bright in X-rays. When the disc is absent, the X-ray emission decreases significantly. The long-term variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss and subsequent reformation. The time scales for the loss and reformation of the disc (about 2 years) are the fastest among Be/X-ray binaries.
A close companion of Zeta Orionis A was found in 2000 with the Navy Precision Optical Interferometer (NPOI), and shown to be a physical companion. Because the primary is a supergiant of type O, for which dynamical mass measurements are very rare, the companion was observed with NPOI over the full 7-year orbit. Our aim was to determine the dynamical mass of a supergiant that, due to the physical separation of more than 10 AU between the components, cannot have undergone mass exchange with the companion. The interferometric observations allow measuring the relative positions of the binary components and their relative brightness. The data collected over the full orbital period allows all seven orbital elements to be determined. In addition to the interferometric observations, we analyzed archival spectra obtained at the Calar Alto, Haute Provence, Cerro Armazones, and La Silla observatories, as well as new spectra obtained at the VLT on Cerro Paranal. In the high-resolution spectra we identified a few lines that can be associated exclusively to one or the other component for the measurement of the radial velocities of both. The combination of astrometry and spectroscopy then yields the stellar masses and the distance to the binary star. The resulting masses for components Aa of 14.0 solar masses and Ab of 7.4 solar masses are low compared to theoretical expectations, with a distance of 294 pc which is smaller than a photometric distance estimate of 387 pc based on the spectral type B0III of the B component. If the latter (because it is also consistent with the distance to the Orion OB1 association) is adopted, the mass of the secondary component Ab of 14 solar masses would agree with classifying a star of type B0.5IV. It is fainter than the primary by about 2.2 magnitudes in the visual. The primary mass is then determined to be 33 solar masses.
We have conducted a 3-month program of simultaneous optical, soft and hard X-ray monitoring of the LMXB 4U1636-536/V801 Ara using the SMARTS 1.3m telescope and archival RXTE/ASM and Swift/XRT data. 4U1636-536 has been exhibiting a large amplitude, quasi-periodic variability since 2002 when its X-ray flux dramatically declined by roughly an order of magnitude. We confirmed that the anti-correlation between soft (2-12 keV) and hard (> 20 keV) X-rays, first investigated by Shih et al. (2005), is not an isolated event but a fundamental characteristic of this sources variability properties. However, the variability itself is neither strictly stable nor changing on an even longer characteristic timescale. We also demonstrate that the optical counterpart varies on the same timescale, and is correlated with the soft, and not the hard, X-rays. This clearly shows that X-ray reprocessing in LMXB discs is mainly driven by soft X-rays. The X-ray spectra in different epochs of the variability revealed a change of spectral characteristics which resemble the state change of black hole X-ray binaries. All the evidence suggests that 4U1636-536 is frequently (~monthly) undergoing X-ray state transitions, a characteristic feature of X-ray novae with their wide range of luminosities associated with outburst events. In its current behavioural mode, this makes 4U1636-536 an ideal target for investigating the details of state changes in luminous X-ray binaries.
We have obtained high time resolution (seconds) photometry of LMC X-2 in December 1997, simultaneously with the Rossi X-ray Timing Explorer (RXTE), in order to search for correlated X-ray and optical variability on timescales from seconds to hours. We find that the optical and X-ray data are correlated only when the source is in a high, active X-ray state. Our analysis shows evidence for the X-ray emission leading the optical with a mean delay of <20s. The timescale for the lag can be reconciled with disc reprocessing, driven by the higher energy X-rays, only by considering the lower limit for the delay. The results are compared with a similar analysis of archival data of Sco X-1.
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