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Register a new userContinued cooling of the accretion-heated neutron star crust in the X-ray transient IGR J17480-2446 located in the globular cluster Terzan 5
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We present a new Chandra observation (performed in July 2016) of the neutron star X-ray transient IGR J17480-2446, located in the globular cluster Terzan 5. We study the continued cooling of the neutron star crust in this system that was heated during the 2010 outburst of the source. This new observation was performed two years after the last observation of IGR J17480-2446, hence, significantly extending the cooling baseline. We reanalysed all available Chandra observations of the source (but excluding observations during which one of the known transients in Terzan 5 was in outburst) and fitted the obtained cooling curve with our cooling code NSCool, which allows for much improved modelling than what was previously performed for the source. The data and our fit models indicate that the crust was still cooling ~5.5 years after the outburst ended. The neutron star crust has likely not reached crust-core thermal equilibrium yet, and further cooling is predicted (which can be confirmed with additional Chandra observations in >5 years). Intriguingly, we find indications that the thermal conductivity might be relatively low in part of the crust compared to what has been inferred for other crust-cooling sources and tentatively suggest that this layer might be located around the neutron drip. The reason for this difference is unclear, but might be related to the fact that IGR J17480-2446 harbours a relatively slowly rotating neutron star (with a spin of 11 Hz) that has a relatively strong inferred surface magnetic field ($10^{9-10}$ Gauss) compared to what is known or typically assumed for other cooling sources.
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Accretion disk winds are revealed in Chandra gratings spectra of black holes. The winds are hot and highly ionized (typically composed of He-like and H-like charge states), and show modest blue-shifts. Similar line spectra are sometimes seen in dipping low-mass X-ray binaries, which are likely viewed edge-on; however, that absorption is tied to structures in the outer disk, and blue-shifts are not typically observed. Here we report the detection of blue-shifted He-like Fe XXV (3100 +/- 400 km/s) and H-like Fe XXVI (1000 +/- 200 km/s) absorption lines in a Chandra/HETG spectrum of the transient pulsar and low-mass X-ray binary IGR J17480-2446 in Terzan 5. These features indicate a disk wind with at least superficial similarities to those observed in stellar-mass black holes. The wind does not vary strongly with numerous weak X-ray bursts or flares. A broad Fe K emission line is detected in the spectrum, and fits with different line models suggest that the inner accretion disk in this system may be truncated. If the stellar magnetic field truncates the disk, a field strength of B = 0.7-4.0 E+9 Gauss is implied, which is in line with estimates based on X-ray timing techniques. We discuss our findings in the context of accretion flows onto neutron stars and stellar-mass black holes.
Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray binaries. Terzan 5 has the richest population among globular clusters of X- and radio-pulsars and low-mass X-ray binaries. The detection and study of optical/IR counterparts of low-mass X-ray binaries is fundamental to characterizing both the low-mass donor in the binary system and investigating the mechanisms of the formation and evolution of this class of objects. We aim at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446 discovered in Terzan 5. Adaptive optics (AO) systems represent the only possibility for studying the very dense environment of GC cores from the ground. We carried out observations of the core of Terzan 5 in the near-IR bands with the ESO-VLT NAOS-CONICA instrument. We present the discovery of the likely counterpart in the Ks band and discuss its properties both in outburst and in quiescence. Archival HST observations are used to extend our discussion to the optical bands. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increase from quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of the double stellar generation population of Terzan 5 by using recent stellar evolution models.
We study the spectral state evolution of the Terzan 5 transient neutron star low-mass X-ray binary IGR J17480-2446, and how the best-fit spectral parameters and burst properties evolved with these states, using the Rossi X-ray Timing Explorer data. As reported by other authors, this is the second source which showed transitions between atoll state and `Z state. We find large scale hysteresis in the almost `C-like hardness-intensity track of the source in the atoll state. This discovery is likely to provide a missing piece of the jigsaw puzzle involving various types of hardness-intensity tracks from `q-shaped for Aquila X-1, 4U 1608-52, and many black holes to `C-shaped for many atoll sources. Furthermore, the regular pulsations, a diagonal transition between soft and hard states, and the large scale hysteresis observed from IGR J17480-2446 argue against some of the previous suggestions involving magnetic field about atolls and millisecond pulsars. Our results also suggest that the nature of spectral evolution throughout an outburst does not, at least entirely, depend on the peak luminosity of the outburst. Besides, the source took at least a month to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. In addition, while the soft colour usually increases with intensity in the softer portion of an atoll source, IGR J17480-2446 showed an opposite behaviour. From the detailed spectral fitting we conclude that a blackbody+powerlaw model is the simplest one, which describes the source continuum spectra well throughout the outburst. We find that these two spectral components were plausibly connected with each other, and they worked together to cause the source state evolution. (Truncated).
The neutron star transient and 11 Hz X-ray pulsar IGR J17480-2446, recently discovered in the globular cluster Terzan 5, showed unprecedented bursting activity during its 2010 October-November outburst. We analyzed all X-ray bursts detected with the Rossi X-ray Timing Explorer and find strong evidence that they all have a thermonuclear origin, despite the fact that many do not show the canonical spectral softening along the decay imprinted on type I X-ray bursts by the cooling of the neutron star photosphere. We show that the persistent-to-burst power ratio is fully consistent with the accretion-to-thermonuclear efficiency ratio along the whole outburst, as is typical for type I X-ray bursts. The burst energy, peak luminosity and daily-averaged spectral profiles all evolve smoothly throughout the outburst, in parallel with the persistent (non-burst) luminosity. We also find that the peak burst to persistent luminosity ratio determines whether or not cooling is present in the bursts from IGR J17480-2446, and argue that the apparent lack of cooling is due to the non-cooling bursts having both a lower peak temperature and a higher non-burst (persistent) emission. We conclude that the detection of cooling along the decay is a sufficient, but not a necessary condition to identify an X-ray burst as thermonuclear. Finally, we compare these findings with X-ray bursts from other rapidly accreting neutron stars.
The low mass X-ray binary (LMXB) IGR J17480-2446 is an 11 Hz accreting pulsar located in the core of the globular cluster Terzan 5. This is a mildly recycled accreting pulsar with a peculiar evolutionary history since its total age has been suggested to be less than a few hundred Myr, despite the very old age of Terzan 5 (~12 Gyr). Solving the origin of this age discrepancy might be very valuable because it can reveal why systems like IGR J17480-2446 are so rare in our Galaxy. We have performed numerical simulations (dynamical and binary evolution) to constrain the evolutionary history of IGR J17480-2446 . We find that the binary has a high probability to be the result of close encounters, with a formation mechanism compatible with the tidal capture of the donor star. The result reinforces the hypothesis that IGR J17480-2446 is a binary that started mass transfer in an exceptionally recent time. We also show that primordial interacting binaries in the core of Terzan 5 are strongly affected by a few hundred close encounters (fly-by) during their lifetime. This effect might delay, accelerate or even interrupt the Roche lobe overflow (RLOF) phase. Our calculations show that systems of this kind can form exclusively in dense environments like globular clusters.
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