319 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
Il progetto TN-1
Descrione hardware e software della scheda Transmission Node 1 (modulo della stazione sismica GAIA1), sua installazione ed utilizzo
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
The two Ultraluminous X-ray sources in the galaxy NGC 925
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC
925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality,
simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first
glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because
of its high X-ray luminosity ( erg s) and its
spectral/temporal features, a closer inspection shows that its properties are
more similar to those of a typical super-Eddington accreting stellar black hole
and we classify it as a `broadened disc' ultraluminous X-ray source. Based on
the physical interpretation of this spectral state, we suggest that ULX-1 is
seen at small inclination angles, possibly through the evacuated cone of a
powerful wind originating in the accretion disc. The spectral classification of
ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington
rates as none of its spectral/temporal properties can be associated to either
the soft or hard state of Galactic accreting black hole binaries.Comment: Accepted on MNRAS with very minor comments, 7 pages, 5 figures, 1
tabl
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
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