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Register a new userProbing the MSP prenatal stage: the optical identification of the X-ray burster EXO 1745-248 in Terzan 5
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Authors
F.R. Ferraro
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We report on the optical identification of the neutron star burster EXO 1745-248 in Terzan 5. The identification was performed by exploiting HST/ACS images acquired in Directors Discretionary Time shortly after (approximately 1 month) the Swift detection of the X-ray burst. The comparison between these images and previous archival data revealed the presence of a star that currently brightened by ~3 magnitudes, consistent with expectations during an X-ray outburst. The centroid of this object well agrees with the position, in the archival images, of a star located in the Turn-Off/Sub Giant Branch region of Terzan 5. This supports the scenario that the companion should has recently filled its Roche Lobe. Such a system represents the pre-natal stage of a millisecond pulsar, an evolutionary phase during which heavy mass accretion on the compact object occurs, thus producing X-ray outbursts and re-accelerating the neutron star.
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We study the low-frequency timing properties and the spectral state evolution of the transient neutron star low-mass X-ray binary EXO 1745-248 using the entire Rossi X-ray Timing Explorer Proportional Counter Array data. We tentatively conclude that EXO 1745-248 is an atoll source, and report the discovery of a ~ 0.45 Hz low-frequency quasi-periodic oscillation and ~ 10 Hz peaked noises. If it is an atoll, this source is unusual because (1) instead of a `C-like curve, it traced a clear overall clockwise hysteresis curve in each of the colour-colour diagram and the hardness-intensity diagram; and (2) the source took at least 2.5 months to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. The shape of the hysteresis track was intermediate between the characteristic `q-like curves of several black hole systems and `C-like curves of atolls, implying that EXO 1745-248 is an important source for the unification of the black hole and neutron star accretion processes.
We report the discovery ($20sigma$) of kilohertz quasi-periodic oscillations (kHz QPOs) at ~ 690 Hz from the transient neutron star low-mass X-ray binary EXO 1745-248. We find that this is a lower kHz QPO, and systematically study the time variation of its properties using smaller data segments with and without the shift-and-add technique. The quality (Q) factor occasionally significantly varies within short ranges of frequency and time. A high Q-factor (264.5 +- 38.5) of the QPO is found for a 200 s time segment, which might be the largest value reported in the literature. We argue that an effective way to rule out kHz QPO models is to observationally find such high Q-factors, even for a short duration, as many models cannot explain a high coherence. However, as we demonstrate, the shift-and-add technique cannot find a very high Q-factor which appears for a short period of time. This shows that the coherences of kHz QPOs can be higher than the already high values reported using this technique, implying further constraints on models. We also discuss the energy dependence of fractional rms amplitude and Q-factor of the kHz QPO.
We have obtained exposures of the field of X0512-401 in the globular cluster NGC1851, in X-rays with the Chandra X-ray Observatory, and in the far-UV with the Hubble Space Telescope. We derive an accurate new X-ray position within ~1 for X0512-401, which enables us to confirm that the only plausible candidate for the optical/UV counterpart is the Star A, which we previously identified from WFPC2 imaging. We find no evidence for X-ray or UV flux modulation on the ultra-short (<1 hr) expected binary period, which implies a low system inclination. In addition, we have detected and spatially resolved an X-ray burst event, confirming the association of the burster, quiescent X-ray source, and optical object. The very large Lx/Lopt of this object implies an extraordinarily compact system, similar to the sources in NGC6624 and NGC6712.
We find convincing observational evidence to confirm the optical identification of the X-ray burster X1746-370 located in the globular cluster NGC6441. Chandra/HRC-I imaging yields a much improved X-ray position for the source, which we show to be fully consistent with our rederived position of a UV-excess star, U1, in the same astrometric reference frame. In addition, the smaller Chandra X-ray error circle excludes the only other blue stars previously identified in the old Einstein circle. We have also obtained Hubble Space Telescope/STIS time-resolved optical spectra of star U1. Although there are no strong line features, the flux distribution demonstrates U1 to be unusually bright in the blue and faint in the red, consistent with earlier WFPC2 photometry. More notably, the flux level of the continuum is seen to vary significantly compared to stars of similar brightness. Indeed, the lightcurve can plausibly be fit by a 5.73 hr period sinusoid, which is the period of the recurring X-ray dips seen in this source. The presence of modulations in both wavelengths strengthens the case for an orbital origin, and therefore deepens the puzzle of the unusual energy independent X-ray dips. Lastly, we note that X1746-370 remains the longest period confirmed X-ray burster in a globular cluster, and the only one with a period typical of the galactic population as a whole.
We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745$-$248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 $times$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$. The 3-20 keV spectrum was reproduced successfully by a power law continuum with narrow and broad (2.7 keV in FWHM) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick materials, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 h before the RXTE observation with a line flux upper limit of 1.5 $times$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall back ionized burst wind and an accretion disk, at a distance of $sim$60 km from the neutron star. If this interpretation is correct, the results provide new information on the understanding of nuclear burning processes during thermonuclear X-ray bursts.
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