4,513 research outputs found
Measuring the spin up of the Accreting Millisecond Pulsar XTE J1751-305
We perform a timing analysis on RXTE data of the accreting millisecond pulsar
XTE J1751-305 observed during the April 2002 outburst. After having corrected
for Doppler effects on the pulse phases due to the orbital motion of the
source, we performed a timing analysis on the phase delays, which gives, for
the first time for this source, an estimate of the average spin frequency
derivative = (3.7 +/- 1.0)E-13 Hz/s. We discuss the torque resulting
from the spin-up of the neutron star deriving a dynamical estimate of the mass
accretion rate and comparing it with the one obtained from X-ray flux.
Constraints on the distance to the source are discussed, leading to a lower
limit of \sim 6.7 kpc.Comment: 7 pages, 3 figures, Accepted for publication by MNRA
The Broad Band Spectrum of MXB 1728-34 Observed by BeppoSAX
We report on the results of a broad band (0.1-100 keV) spectral analysis of
the bursting atoll source MXB 1728-34 observed by the BeppoSAX satellite. Three
bursts were present during this observation. The spectrum during the bursts can
be fitted by a blackbody with a temperature of 2 keV. From the bursts we also
estimate a distance to the source of 5.1 kpc. MXB 1728-34 was in a rather soft
state during the BeppoSAX observation. The persistent spectrum is well fitted
by a continuum consisting of a soft blackbody emission and a comptonized
spectrum. We interpreted the soft component as the emission from the accretion
disk. Taking into account a spectral hardening factor of 1.7, we estimated that
the inner disk radius is km, where i is the
inclination angle. The comptonized component could originate in a spherical
corona, with temperature of 10 keV and optical depth of 5, surrounding the
neutron star. A broad gaussian emission line at 6.7 keV is observed in the
spectrum, probably emitted in the ionized corona or in the inner part of the
disk. Another emission line is present at 1.66 keV.Comment: 12 pages, accepted by Ap
High Energy Radiation from Neutron Star Binaries
This paper surveys our current knowledge of the hard X-ray emission
properties of old accreting neutron stars in low mass X-ray binaries. Hard
X-ray components extending up to energies of a few hundred keV have been
clearly detected in sources of both the Atoll and Z classes. The presence and
characteristics of these hard components are discussed in relation to source
properties and state. An overall anticorrelation between the fraction
luminosity in hard X-rays and mass accretion rate is apparent over different
sources spanning a large range of luminosities as well as individual source
undergoing state changes. Evidence for a second, yet unknown, parameter
controlling the hard X-ray emission is emerging. We draw a parallel with the
spectral properties of X-ray binaries hosting a stellar mass accreting black
hole, and conclude that, at a merely phenomenological level, there appears to
be a close analogy between the spectral properties of black hole candidates in
their high and intermediate states and Z-sources. We briefly mention models
that have been proposed for the hard X-ray emission of neutron star low mass
X-ray binaries and comment on perspectives in the INTEGRAL era.Comment: 11 pages, including 4 figures. To appear in the proceedings of the
XXII Moriond Astrophysics Meeting "The Gamma-Ray Universe" (Les Arcs, March
9-16, 2002), eds. A. Goldwurm, D. Neumann, and J. Tran Thanh Van, The Gioi
Publishers (Vietnam
Timing of the Accreting Millisecond Pulsar XTE J1814-338
We present a precise timing analysis of the accreting millisecond pulsar XTE
J1814-338 during its 2003 outburst, observed by RXTE. A full orbital solution
is given for the first time; Doppler effects induced by the motion of the
source in the binary system were corrected, leading to a refined estimate of
the orbital period, P_orb=15388.7229(2)s, and of the projected semimajor axis,
a sini/c= 390.633(9) lt-ms. We could then investigate the spin behaviour of the
accreting compact object during the outburst. We report here a refined value of
the spin frequency (nu=314.35610879(1) Hz) and the first estimate of the spin
frequency derivative of this source while accreting (nu^dot=(-6.7 +/- 0.7)
10^(-14) Hz/s). This spin down behaviour arises when both the fundamental
frequency and the second harmonic are taken into consideration. We discuss this
in the context of the interaction between the disc and the quickly rotating
magnetosphere, at accretion rates sufficiently low to allow a threading of the
accretion disc in regions where the Keplerian velocity is slower than the
magnetosphere velocity. We also present indications of a jitter of the pulse
phases around the mean trend, which we argue results from movements of the
accreting hotspots in response to variations of the accretion rate.Comment: 7 pages, 4 figures, Accepted for publication by MNRA
Chandra Observation of the Persistent Emission from the Dipping Source XB 1916-053
We present the results of a 50 ks long Chandra observation of the dipping
source XB 1916-053. During the observation two X-ray bursts occurred and the
dips were not present at each orbital period. From the zero-order image we
estimate the precise X-ray coordinates of the source with a 90% uncertainty of
0.6''. In this work we focus on the spectral study of discrete absorption
features, during the persistent emission, using the High Energy Transmission
Grating Spectrometer on board the Chandra satellite. We detect, for the first
time in the 1st-order spectra of XB 1916-053, absorption lines associated to Ne
X, Mg XII, Si XIV, and S XVI, and confirm the presence of the Fe XXV and Fe
XXVI absorption lines with a larger accuracy with respect to the previous XMM
EPIC pn observation. Assuming that the line widths are due to a bulk motion or
a turbulence associated to the coronal activity, we estimate that the lines are
produced in a photoionized absorber distant from the neutron star 4 x 10^{10}
cm, near the disk edge.Comment: 20 pages, 10 figures, submitted to ApJ on 2005-09-22, accepted by ApJ
on 2006-05-0
XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658
We report on a 63-ks long XMM-Newton observation of the accreting millisecond
pulsar SAX J1808.4-3658 during the latest X-ray outburst which started on
September 21st 2008. The pn spectrum shows a highly significant emission line
in the energy band where the iron K-alpha line is expected, and which we
identify as emission from neutral (or mildly ionized) iron. The line profile
appears to be quite broad (more than 1 keV FWHM) and asymmetric; the most
probable explanation for this profile is Doppler and relativistic broadening
from the inner accretion disc. From a fit with a diskline profile we find an
inner radius of the disc of 8.7^(+3.7)_(-2.7) R_g, corresponding to
18.0^(+7.6)_(-5.6) km for a 1.4 Msun neutron star. The disc therefore appears
truncated inside the corotation radius (31 km for SAX J1808.4-3658) in
agreement with the fact that the source was still showing pulsations during the
XMM-Newton observation.Comment: 5 pages, 3 figures, accepted for publication in A&A Letters, typos
corrected, references adde
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Educational strategies in data journalism: A comparative study of six European countries
The article explores training programs in higher education with regard to data journalism from a multi-national perspective. By carrying out a comparative analysis in six European countries (Germany, Switzerland, the Netherlands, Italy, Poland, and the United Kingdom), it covers different models of media systems and journalistic cultures envisaged by Hallin and Mancini. Based on a desk review and in-depth interviews with instructors of data journalism in each country, the article identifies different approaches to the way data journalism is taught. In Europe, such programs are offered by four types of organizations: academic, vocational, professional, and civic. The role played by those organizations can be explained as a result of the peculiarities of national media systems. But there are also commonalities, for example, non-academic institutions (such as the European Journalism Center or the Center for Investigative Journalism) and major international news outlets (such as The Guardian and The New York Times) seem to take over a leading role in all of the analyzed countries. Generally speaking, data journalism education appears to be a very young discipline that frequently neglects fundamental journalistic topics such as ethical issues, transparency, accountability, and responsiveness although they are crucial in a journalistic field as sophisticated tools to reveal hidden aspects of reality
Searching for pulsed emission from XTE J0929-314 at high radio frequencies
The aim of this work is to search for radio signals in the quiescent phase of
accreting millisecond X-ray pulsars, in this way giving an ultimate proof of
the recycling model, thereby unambiguously establishing that accreting
millisecond X-ray pulsars are the progenitors of radio millisecond pulsars.
To overcome the possible free-free absorption caused by matter surrounding
accreting millisecond X-ray pulsars in their quiescence phase, we performed the
observations at high frequencies. Making use of particularly precise orbital
and spin parameters obtained from X-ray observations, we carried out a deep
search for radio-pulsed emission from the accreting millisecond X-ray pulsar
XTE J0929-314 in three steps, correcting for the effect of the dispersion due
to the interstellar medium, eliminating the orbital motions effects, and
finally folding the time series.
No radio pulsation is present in the analyzed data down to a limit of 68
microJy at 6.4 GHz and 26 microJy at 8.5 GHz.
We discuss several mechanisms that could prevent the detection, concluding
that beaming factor and intrinsic low luminosity are the most likely
explanations.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
We analyze a large collection of RXTE archive data from April 1997 to August
2003 of the bright X-ray source Scorpius X-1 in order to study the broadband
spectral evolution of the source for different values of the inferred mass
accretion rate by studying energy spectra from selected regions in the Z-track
of its Color-Color Diagram. A two-component model, consisting of a soft thermal
component interpreted as thermal emission from an accretion disk and a thermal
Comptonization component, is unable to fit the whole 3--200 keV energy spectrum
at low accretion rates. Strong residuals in the highest energy band of the
spectrum require the addition of a third component that can be fitted with a
power-law component, that could represent a second thermal Comptonization from
a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We
discuss the physical implications derived from the results of our analysis,
with a particular emphasis on the hardest part of the X-ray emission and its
possible origins.Comment: 18 pages. Accepted for publication in Ap
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