Search results for " Astronomia"
showing 10 items of 486 documents
Observational diagnostics of accretion on young stars and brown dwarfs
2010
I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of…
Time-resolved Spectroscopy of DROXO X-ray Sources: Flares and Fe Kα emission
2011
We present a systematic search for Fe Kα emission from young stellar objects of the ρ Ophiuchi star forming region observed in the Deep Rho Ophiuchi XMM-Newton Observation.
Tracking the evolution of the accretion flow in MAXI J1820+070 during its hard state with the JED-SAD model
2021
X-ray binaries in outburst typically show two canonical X-ray spectral states, i.e. hard and soft states, in which the physical properties of the accretion flow and of the jet are known to change. Recently, the JED-SAD paradigm has been proposed for black hole X-ray binaries, aimed to address the accretion-ejection interplay in these systems. According to this model, the accretion flow is composed by an outer standard Shakura-Sunyaev disk (SAD) and an inner hot Jet Emitting Disk (JED). The JED produces both the hard X-ray emission, effectively playing the role of the hot corona, and the radio jets. In this paper, we use the JED-SAD model to describe the evolution of the accretion flow in th…
Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
2016
We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $\sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $\sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{\odot}$, which assumes a neutron star mass of 1.4 M$_{\odot}$ and a maximum inclination angle of $75^{\…
A Spectral Insight into the Physics of Accreting ms Pulsars
2010
The broadened iron lines observed from accreting compact objects are most easily interpreted in terms of reflection onto the accretion disc of the hard X-ray photons emitted by the central source. In this context, such a broadness is due to the relativistic motion of the reflecting plasma, in the deep gravitational well of the compact object, and can thus serve as a probe of the inner radius of the disc. Here we report about the discovery of such features from a couple of accreting millisecond pulsars, and discuss the constraints which can be derived on the magnetospheric radius.
The reflection component in NS LMXBs
2014
Thanks to the good spectral resolution and large effective area of the EPIC/PN instrument on board of XMM-Newton, we have at hand a large number of observations of accreting low-mass X-ray binaries, that allow for the fist time a comprehensive view on the characteristics of the reflection component at different accretion regimes and to probe the effects of a magnetosphere on its formation. We focus here on a comparative analysis of the reflection component from a series of spectroscopic studies on selected sources: 4U 1705-44, observed both in the soft and hard state, the pulsating ms pulsars SAX J1808.4-3658 and IGR J17511-3057, and the intermittent pulsar HETE J1900-2455. Although the sou…
A detailed study of the rise phase of a long duration X-ray flare in the young star TWA 11B
2010
We analyzed a long duration flare observed in a serendipitous XMM-Newton detection of the M star CD-39 7717B (TWA 11B), member of the young stellar association TW Hya (~ 8 Myr). Only the rise phase (with a duration of ~ 35 ks) and possibly the flare peak were observed. We took advantage of the high count-rate of the X-ray source to carry out a detailed analysis of its spectrum during the whole exposure. After a careful analysis, we interpreted the rise phase as resulting from the ignition of a first group of loops (event A) which triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a single-loop model, while a two-ribbon model was applied for event B. Loop semi-leng…
New view of the corona of classical T Tauri stars: Effects of flaring activity in circumstellar disks
2019
Classical T Tauri stars (CTTSs) are young low-mass stellar objects accreting mass from their circumstellar disks. They are characterized by high levels of coronal activity as revealed by X-ray observations. This activity may affect the disk stability and the circumstellar environment. Here we investigate if an intense coronal activity due to flares occurring close to the accretion disk may perturb the inner disk stability, disrupt the inner part of the disk and, possibly, trigger accretion phenomena with rates comparable with those observed. We model a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodinamic simulations. We explore cases characterized by a dipole …
A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with NuStar
2017
We present $\emph{NuSTAR}$ observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, $F_{\mathrm{Edd}}$, of 0.01; GX 17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K$_{\alpha}$ line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be $R_{in}=1.03\pm0.03$ ISCO (innermost stable circular orbit) assuming a dimensionless spin parameter $a_{*}=cJ/GM^{2}=0.0$, and $R_{in}=1.08\pm0.06$ ISCO for $a_{*}=0.3$ (errors quoted at 1 $\sig…
Muons in air showers at the Pierre Auger Observatory
2015
We present the first hybrid measurement of the average muon number in air showers at ultrahigh energies, initiated by cosmic rays with zenith angles between 62° and 80°. The measurement is based on 174 hybrid events recorded simultaneously with the surface detector array and the fluorescence detector of the Pierre Auger Observatory. The muon number for each shower is derived by scaling a simulated reference profile of the lateral muon density distribution at the ground until it fits the data. A 1019eV shower with a zenith angle of 67°, which arrives at the surface detector array at an altitude of 1450 m above sea level, contains on average (2.68±0.04±0.48(sys))×107 muons with energies large…