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EnglishWe explain why it is possible to detect directly X-ray emission from near the surface of the neutron star (NS) in SAX J1808.4-3658 but not in most other low-mass X-ray binaries (LMXBs), with the exception that emission from the surface can be seen during burst events. We show that the X-ray emission from SAX J1808.4-3658 mostly originates in the Comptonization process in a relatively optically thin hot region (with an optical depth τ0 around 4 and temperature around 20 keV). Such a transparent region does not prevent us from detecting coherent X-ray pulsation due to hot spots on the NS surface. We give a precise model for the loss of modulation: such suppression of the quasi-periodic oscillation (QPO) amplitude due to scattering can explain the disappearance of kilohertz QPOs with increasing QPO frequency. We also formulate general conditions under which the millisecond X-ray pulsation can be detected in LMXBs. We demonstrate that the observed soft phase lag of the pulsed emission is a result of the downscattering of the hard X-ray photons in the relatively cold material near the NS surface. In the framework of this downscattering model, we propose a method to determine the atmosphere density in that region from soft-lag measurements.
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| Data di pubblicazione: | 2002 | |
| Titolo: | Why Is It Difficult to Detect a Millisecond Pulsar in Neutron Star X-Ray Binaries? | |
| Autori: | L. Titarchuk; Cui W.; Wood K. | |
| Rivista: | THE ASTROPHYSICAL JOURNAL LETTERS | |
| Parole Chiave: | Accretion; Accretion Disks; Stars: Pulsars: Individual: Alphanumeric: SAX J1808.4-3658; Stars: Neutron; X-Rays: Stars | |
| Abstract: | We explain why it is possible to detect directly X-ray emission from near the surface of the neutron star (NS) in SAX J1808.4-3658 but not in most other low-mass X-ray binaries (LMXBs), with the exception that emission from the surface can be seen during burst events. We show that the X-ray emission from SAX J1808.4-3658 mostly originates in the Comptonization process in a relatively optically thin hot region (with an optical depth τ0 around 4 and temperature around 20 keV). Such a transparent region does not prevent us from detecting coherent X-ray pulsation due to hot spots on the NS surface. We give a precise model for the loss of modulation: such suppression of the quasi-periodic oscillation (QPO) amplitude due to scattering can explain the disappearance of kilohertz QPOs with increasing QPO frequency. We also formulate general conditions under which the millisecond X-ray pulsation can be detected in LMXBs. We demonstrate that the observed soft phase lag of the pulsed emission is a result of the downscattering of the hard X-ray photons in the relatively cold material near the NS surface. In the framework of this downscattering model, we propose a method to determine the atmosphere density in that region from soft-lag measurements. | |
| Digital Object Identifier (DOI): | 10.1086/343099 | |
| Handle: | http://hdl.handle.net/11392/532943 | |
| Appare nelle tipologie: | 03.1 Articolo su rivista |
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