Search results for " accretion"
showing 10 items of 122 documents
Flaring activity on the disk of Classical T Tauri Stars: effects on disk stability
2018
Classical T Tauri Stars (CTTSs) are young stellar objects surrounded by a circumstellar disk with which they exchange mass and angular momentum through accretion. Despite this process is a crucial aspect of star formation, some issues are still not clear; in particular how the material loses angular momentum and falls into the star. CTTSs are also characterized by strong X-ray emission. Part of this X-ray emission comes from the heated plasma in the external regions of the stellar corona with temperature between 1 and 100 MK. The plasma heating is presumably due to the strong magnetic field (Feigelson and Montmerle, 1999) in the form of high energetic flares in proximity of the stellar surf…
New insights from recently migrated CROP multichannel seismic data at the outermost Calabrian arc accretionary wedge (Ionian sea)
2011
This study addresses selected aspects of the stratigraphic-structural setting of the outermost Calabrian Arc accretionary wedge and underlines relationships between structural development and Messinian evaporite stratigraphy through the analysis of re-processed CROP multichannel seismic reflection profiles. A detailed seismostratigraphic analysis, calibrated with P-velocities compiled by previous works, images a general bipartition of the Messinian evaporite deposits: the transparent «lower subunit» appears to have undergone ductile-flow deformation», with the development of salt-cored thrusting structures, and the layered «upper subunit» appears to be characterized by brittle deformation. …
Dense matter with eXTP
2019
In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics o…
The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
2017
We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Despite the source being in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). Once the flares were subtracted, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even if a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars is detected. We conclude that the observed flaring could be a manifestation of the presence of an acc…
On the peculiar long-term orbital evolution of the eclipsing accreting millisecond X-ray pulsar SWIFT J1749.4-2807
2022
We present the pulsar timing analysis of the accreting millisecond X-ray pulsar SWIFT J1749.4-2807 monitored by NICER and XMM-Newton during its latest outburst after almost eleven years of quiescence. From the coherent timing analysis of the pulse profiles, we updated the orbital ephemerides of the system. Large phase jumps of the fundamental frequency phase of the signal are visible during the outburst, consistent with what was observed during the previous outburst. Moreover, we report on the marginally significant evidence for non-zero eccentricity ($e\simeq 4\times 10^{-5}$) obtained independently from the analysis of both the 2021 and 2010 outbursts and we discuss possible compatible sc…
Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4–2807 with NICER, XMM-Newton, and NuSTAR
2022
The neutron star low-mass X-ray binary SWIFT J1749.4–2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this manuscript, we perform a spectral characterization of the system throughout its 2021, 2-week-long outburst, analysing 11 NICER observations and quasi-simultaneous XMM-Newton and NuSTAR single observations at the outburst peak. The broad-band spectrum is well-modelled with a blackbody component with a temperature of ∼0.6 keV, most likely consistent with a hotspot on the neutron star surface, and a Comptonization spectrum with power-law index Γ ∼ 1.9, arising from a hot corona at ∼12 keV. No direct emission from the disc was found, possibly due to it being too cool…
Spectral analysis of the low-mass X-ray pulsar 4U 1822-371: Reflection component in a high-inclination system
2021
Context. The X-ray source 4U 1822-371 is an eclipsing low-mass X-ray binary and X-ray pulsar, hosting a NS that shows periodic pulsations in the X-ray band with a period of 0.59 s. The inclination angle of the system is so high (80–85°) that in principle, it should be hard to observe both the direct thermal emission of the central object and the reflection component of the spectrum because they are hidden by the outer edge of the accretion disc. Despite the number of studies carried out on this source, many aspects such as the geometry of the system, its luminosity, and its spectral features are still debated. Aims. Assuming that the source accretes at the Eddington limit, the analysis perf…
A broadband spectral analysis of 4U 1702-429 using XMM-Newton and BeppoSAX data
2018
Most of the X-ray binary systems containing neutron stars classified as Atoll sources show two different spectral states, called soft and hard. Moreover, a large number of these systems show a reflection component relativistically smeared in their spectra, which gives information on the innermost region of the system. Our aim is to investigate the poorly studied broadband spectrum of the low mass X-ray binary system 4U 1702-429, which was recently analysed combining XMM-Newton and INTEGRAL data. The peculiar value of the reflection fraction brought us to analyse further broadband spectra of 4U 1702-429. We re-analysed the spectrum of the XMM-Newton/INTEGRAL observation of 4U 1702-429 in the…
QPO emission from moving hot spots on the surface of neutron stars: a model
2009
We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in lightcurves produced by regular movements of the hot spots during accretion onto the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high frequency Quasi Periodic Oscillations with frequencies associated with the inner zone of the disk. Previously reported simulations showed that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles relatively large. Otherwise, two phenomena challenge the assumption: one …
On the observability of T Tauri accretion shocks in the X-ray band
2010
Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (n_e=10^11-10^13 cm^-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface. Aims. We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-beta<<1 in the post-shock zone. Methods. We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chromosphere, including the effects of radiative cooling, gravity and thermal conduction. We expl…