338 research outputs found
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We
analysed a sample of 20 Swift observations, taken between 2013 April and
August, and two Chandra observations taken in 2001 September and 2004 June. We
found only marginal hints of a limited number of dips in the light curve,
previously reported to occur in this source, although the uncertainties due to
the low counting statistics of the data are large. The Chandra and Swift
spectra showed clearly spectral variability which resembles those observed in
other ULXs. We can account for this spectral variability in terms of changes in
both the normalization and intrinsic column density of a two-components model
consisting of a blackbody (for the soft component) and a multicolour accretion
disc (for the hard component). We discuss the possibility that strong outflows
ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA
Spectral analysis of SXP59.0 during its 2017 outburst and properties of the soft excess in X-ray binary pulsars
We report the results provided by the XMM-Newton observation of the X-ray
binary pulsar SXP59.0 during its most recent outburst in April 2017. The source
was detected at (0.2-12 keV) = 8 erg cm
s, one of its highest flux levels reported to date. The measured pulse
period was = 58.949(1) s, very similar to the periods measured
in most of the previous observations. The pulsed emission was clearly detected
over the whole energy range between 0.2 and 12 keV, but the pulse profile is
energy dependent and the pulsed fraction increases as the energy increases.
Although the time-averaged EPIC spectrum is dominated by a power-law component
(with photon index ), the data show an evident soft
excess, which can be described with the sum of a black-body and a hot thermal
plasma component (with temperatures eV and
keV, respectively). Moreover, the EPIC
and RGS spectra show narrow emission lines due to N, O, Ne, Mg, and Fe. The
phase-resolved spectral analysis of the EPIC data shows that the flux of the
black-body component varies with the pulse phase, while the plasma component is
almost constant. We show that the black-body component can be attributed to the
reprocessing of the primary emission by the optically thick material at the
inner edge of the accretion disc, while the hot plasma component is due to a
diffuse gas far from the accretion region and the narrow emission lines of the
RGS spectrum are most probably due to photoionized matter around the accreting
source.Comment: 11 pages, 9 figures, 5 tables. Accepted for publication by Astronomy
and Astrophysic
Spectral analysis of IGR J01572-7259 during its 2016 outburst
We report on the results of the observation of IGR J01572-7259
during its most recent outburst in 2016 May, the first since 2008. The source
reached a flux erg cm s, which allowed us to
perform a detailed analysis of its timing and spectral properties. We obtained
a pulse period = 11.58208(2) s. The pulse profile is double
peaked and strongly energy dependent, as the second peak is prominent only at
low energies and the pulsed fraction increases with energy. The main spectral
component is a power-law model, but at low energies we also detected a soft
thermal component, which can be described with either a blackbody or a hot
plasma model. Both the EPIC and RGS spectra show several emission lines, which
can be identified with the transition lines of ionized N, O, Ne, and Fe and
cannot be described with a thermal emission model. The phase-resolved spectral
analysis showed that the flux of both the soft excess and the emission lines
vary with the pulse phase: the soft excess disappears in the first pulse and
becomes significant only in the second, where also the Fe line is stronger.
This variability is difficult to explain with emission from a hot plasma, while
the reprocessing of the primary X-ray emission at the inner edge of the
accretion disk provides a realiable scenario. On the other hand, the narrow
emission lines can be due to the presence of photoionized matter around the
accreting source.Comment: 10 pages, 7 figures, 5 tables. Accepted for publication by Monthly
Notices of the Royal Astronomical Societ
Spectral analysis of SMC X-2 during its 2015 outburst
We report on the results of Swift and XMM-Newton observations of SMC X-2
during its last outburst in 2015 October, the first one since 2000. The source
reached a very high luminosity ( erg s), which allowed
us to perform a detailed analysis of its timing and spectral properties. We
obtained a pulse period = 2.372267(5) s and a characterization
of the pulse profile also at low energies. The main spectral component is a
hard () power-law model with an exponential cut-off, but at
low energies we detected also a soft (with kT 0.15 keV) thermal
component. Several emission lines can be observed at various energies. The
identification of these features with the transition lines of highly ionized N,
O, Ne, Si, and Fe suggests the presence of photoionized matter around the
accreting source.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Letter
Updating the orbital ephemeris of the dipping source XB 1254-690 and the distance to the source
XB 1254-690 is a dipping low mass X-ray binary system hosting a neutron star
and showing type I X-ray bursts. We aim at obtaining more accurate orbital
ephemeris and at constraining the orbital period derivative of the system for
the first time. In addition, we want to better constrain the distance to the
source in order to locate the system in a well defined evolutive scenario. We
apply for the first time an orbital timing technique to XB 1254-690, using the
arrival times of the dips present in the light curves that have been collected
during 26 years of X-ray pointed observations performed from different space
missions. We estimate the dip arrival times using a statistical method that
weights the count-rate inside the dip with respect to the level of the
persistent emission outside the dip. We fit the obtained delays as a function
of the orbital cycles both with a linear and a quadratic function. We infer the
orbital ephemeris of XB 1254-690 improving the accuracy of the orbital period
with respect to previous estimates. We infer a mass of M
M for the donor star, in agreement with the estimations already
present in literature, assuming that the star is in thermal equilibrium while
it transfers part of its mass via the inner Lagrangian point, and assuming a
neutron star mass of 1.4 M. Using these assumptions, we also
constrain the distance to the source, finding a value of 7.6 kpc.
Finally, we discuss the evolution of the system suggesting that it is
compatible with a conservative mass transfer driven by magnetic braking.Comment: 13 pages, 5 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
New orbital ephemerides for the dipping source 4U 1323-619: constraining the distance to the source
4U 1323-619 is a low mass X-ray binary system that shows type I X-ray bursts
and dips. The most accurate estimation of the orbital period is 2.941923(36)
hrs and a distance from the source that is lower than 11 kpc has been proposed.
We aim to obtain the orbital ephemeris, the orbital period of the system, as
well as its derivative to compare the observed luminosity with that predicted
by the theory of secular evolution. We took the advantage of about 26 years of
X-ray data and grouped the selected observations when close in time. We folded
the light curves and used the timing technique, obtaining 12 dip arrival times.
We fit the delays of the dip arrival times both with a linear and a quadratic
function. We locate 4U 1323-619 within a circular area centred at RA (J2000)=
201.6543\degree and DEC (J2000)= -62.1358\degree with an associated error of
0.0002\degree, and confirm the detection of the IR counterpart already
discussed in literature. We estimate an orbital period of P=2.9419156(6) hrs
compatible with the estimations that are present in the literature, but with an
accuracy ten times higher. We also obtain a constraint on the orbital period
derivative for the first time, estimating
s/s. Assuming that the companion star is in thermal equilibrium in the lower
main sequence, and is a neutron star of 1.4 M, we infer a mass of
0.280.03 M for the companion star. Assuming a distance of 10
kpc, we obtained a luminosity of (4.30.5) erg s,
which is not in agreement with what is predicted by the theory of secular
evolution. Using a 3D extinction map of the K radiation in our Galaxy, we
obtain a distance of 4.2 kpc at 68\% confidence level.
(Abridged)Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Behind the dust curtain: the spectacular case of GRB 160623A
We report on the X-ray dust-scattering features observed around the afterglow
of the gamma ray burst GRB 160623A. With an XMM-Newton observation carried out
~2 days after the burst, we found evidence of at least six rings, with angular
size expanding between ~2 and 9 arcmin, as expected for X-ray scattering of the
prompt GRB emission by dust clouds in our Galaxy. From the expansion rate of
the rings, we measured the distances of the dust layers with extraordinary
precision: 528.1 +\- 1.2 pc, 679.2 +\- 1.9 pc, 789.0 +\- 2.8 pc, 952 +\- 5 pc,
1539 +\- 20 pc and 5079 +\- 64 pc. A spectral analysis of the ring spectra,
based on an appropriate dust-scattering model (BARE-GR-B from Zubko et al.
2004}) and the estimated burst fluence, allowed us to derive the column density
of the individual dust layers, which are in the range 7x10^20-1.5x10^22 cm^-2.
The farthest dust-layer (i.e. the one responsible for the smallest ring) is
also the one with the lowest column density and it is possibly very extended,
indicating a diffuse dust region. The properties derived for the six
dust-layers (distance, thickness, and optical depth) are generally in good
agreement with independent information on the reddening along this line of
sight and on the distribution of molecular and atomic gas.Comment: 9 pages, 10 figures, 1 table; accepted for publication in MNRA
Study of the reflection spectrum of the LMXB 4U 1702-429
The source 4U 1702-429 (Ara X-1) is a low-mass X-ray binary system hosting a
neutron star. Albeit the source is quite bright ( erg s)
its broadband spectrum has never been studied. Neither dips nor eclipses have
been observed in the light curve suggesting that its inclination angle is
smaller than 60.We analysed the broadband spectrum of 4U 1702-429 in
the 0.3-60 keV energy range, using XMM-Newton and INTEGRAL data, to constrain
its Compton reflection component if it is present. After excluding the three
time intervals in which three type-I X-ray bursts occurred, we fitted the joint
XMM-Newton and INTEGRAL spectra obtained from simultaneous observations. A
broad emission line at 6.7 keV and two absorption edges at 0.87 and 8.82 keV
were detected. We found that a self-consistent reflection model fits the 0.3-60
keV spectrum well. The broadband continuum is composed of an emission component
originating from the inner region of the accretion disc, a Comptonised direct
emission coming from a corona with an electron temperature of
keV and an optical depth , and, finally, a reflection
component. The best-fit indicates that the broad emission line and the
absorption edge at 8.82 keV, both associated with the presence of \ion{Fe}{xxv}
ions, are produced by reflection in the region above the disc with a ionisation
parameter of . We have inferred that the inner radius,
where the broad emission line originates, is km, and the inner
radius of the accretion disc is km. (Abridged)Comment: 9 pages, 9 figures, accepted for publication by A&
Efficient computation of bifurcation diagrams with a deflated approach to reduced basis spectral element method
The majority of the most common physical phenomena can be described using partial differential equations (PDEs). However, they are very often characterized by strong nonlinearities. Such features lead to the coexistence of multiple solutions studied by the bifurcation theory. Unfortunately, in practical scenarios, one has to exploit numerical methods to compute the solutions of systems of PDEs, even if the classical techniques are usually able to compute only a single solution for any value of a parameter when more branches exist. In this work, we implemented an elaborated deflated continuation method that relies on the spectral element method (SEM) and on the reduced basis (RB) one to efficiently compute bifurcation diagrams with more parameters and more bifurcation points. The deflated continuation method can be obtained combining the classical continuation method and the deflation one: the former is used to entirely track each known branch of the diagram, while the latter is exploited to discover the new ones. Finally, when more than one parameter is considered, the efficiency of the computation is ensured by the fact that the diagrams can be computed during the online phase while, during the offline one, one only has to compute one-dimensional diagrams. In this work, after a more detailed description of the method, we will show the results that can be obtained using it to compute a bifurcation diagram associated with a problem governed by the Navier-Stokes equations
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