1,189 research outputs found
The accretion environment of Supergiant Fast X-ray Transients probed with XMM-Newton
Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable
variability in the X-ray domain, widely ascribed to the accretion from a clumpy
stellar wind. In this paper we performed a systematic and homogeneous analysis
of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to
probe spectral variations on timescales as short as a few hundred of seconds.
Our ultimate goal is to investigate if SFXT flares and outbursts are triggered
by the presence of clumps and eventually reveal whether strongly or mildly
dense clumps are required. For all sources, we employ a technique developed by
our group, making use of an adaptive rebinned hardness ratio to optimally
select the time intervals for the spectral extraction. A total of twelve
observations performed in the direction of five SFXTs are reported. We show
that both strongly and mildly dense clumps can trigger these events. In the
former case, the local absorption column density may increase by a factor of
>>3, while in the latter case, the increase is only by a factor of 2-3 (or
lower). Overall, there seems to be no obvious correlation between the dynamic
ranges in the X-ray fluxes and absorption column densities in SFXTs, with an
indication that lower densities are recorded at the highest fluxes. This can be
explained by the presence of accretion inhibition mechanism(s). We propose a
classification of the flares/outbursts from these sources to drive future
observational investigations. We suggest that the difference between the
classes of flares/outbursts is related to the fact that the mechanism(s)
inhibiting accretion can be overcome more easily in some sources compared to
others. We also investigate the possibility that different stellar wind
structures, rather than clumps, could provide the means to temporarily overcome
the inhibition of accretion in SFXTs.Comment: Accepted for publication on A&
The transitional millisecond pulsar IGR J18245-2452 during its 2013 outburst at X-rays and soft gamma-rays
IGR~J18245--2452/PSR J1824--2452I is one of the rare transitional accreting
millisecond X-ray pulsars, showing direct evidence of switches between states
of rotation powered radio pulsations and accretion powered X-ray pulsations,
dubbed transitional pulsars. IGR~J18245--2452 is the only transitional pulsar
so far to have shown a full accretion episode, reaching an X-ray luminosity of
~erg~s permitting its discovery with INTEGRAL in 2013. In
this paper, we report on a detailed analysis of the data collected with the
IBIS/ISGRI and the two JEM-X monitors on-board INTEGRAL at the time of the 2013
outburst. We make use of some complementary data obtained with the instruments
on-board XMM-Newton and Swift in order to perform the averaged broad-band
spectral analysis of the source in the energy range 0.4 -- 250~keV. We have
found that this spectrum is the hardest among the accreting millisecond X-ray
pulsars. We improved the ephemeris, now valid across its full outburst, and
report the detection of pulsed emission up to keV in both the ISGRI
() and Fermi/GBM () bandpass. The alignment of the
ISGRI and Fermi GBM 20 -- 60 keV pulse profiles are consistent at a $\sim25\
\mu$s level. We compared the pulse profiles obtained at soft X-rays with \xmm\
with the soft \gr-ray ones, and derived the pulsed fractions of the fundamental
and first harmonic, as well as the time lag of the fundamental harmonic, up to
s, as a function of energy. We report on a thermonuclear X-ray burst
detected with \Integ, and using the properties of the previously type-I X-ray
burst, we show that all these events are powered primarily by helium ignited at
a depth of g cm. For such a helium
burst the estimated recurrence time of d is in
agreement with the observations.Comment: 10 pages, 6 Figures, 3 Tables Astronomy and Astrophysics Journal,
accepted for publication on the 13th of April 201
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
Glancing through the accretion column of EXO 2030+375
We took advantage of the large collecting area and good timing capabilities
of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry
onto the magnetized neutron star hosted in the high mass X-ray binary EXO
2030+375 during the rise of a source Type-I outburst in 2014. We carried out a
timing and spectral analysis of the XMM-Newton observation as function of the
neutron star spin phase. We used a phenomenological spectral continuum model
comprising the required fluorescence emission lines. Two neutral absorption
components are present: one covering fully the source and one only partially.
The same analysis was also carried out on two Suzaku observations of the source
performed during outbursts in 2007 and 2012, to search for possible spectral
variations at different luminosities. The XMM-Newton data caught the source at
an X-ray luminosity of erg s and revealed the presence
of a narrow dip-like feature in its pulse profile that was never reported
before. The width of this feature corresponds to about one hundredth of the
neutron star spin period. From the results of the phase-resolved spectral
analysis we suggest that this feature can be ascribed to the self-obscuration
of the accretion stream passing in front of the observer line of sight. We
inferred from the Suzaku observation carried out in 2007 that the
self-obscuration of the accretion stream might produce a significantly wider
feature in the neutron star pulsed profile at higher luminosities
( erg s).Comment: Accepted for publication on A&
Low-Cost Visual Sensor Node for BlueTooth-Based Measurement Networks
This paper proposes a wireless visual sensor comprising of a low-cost grayscale camera as the sensing hardware and a BlueTooth (BT) 100-m slave module as the transmission hardware. The latter enables the sensor to be inserted as a node in a medium-range BT measurement network. The proposed sensor, which is designed to satisfy precise specifications in terms of image-transfer rate and power consumption, represents a good image-acquisition solution in a wide range of applications where hardwiring between camera and image processor is undesired. This paper describes the hardware used and the overall operating principle behind the sensor, discusses the sensor-node performance, and reports the results of reliability tests
Measurement of Passive R, L, and C Components Under Nonsinusoidal Conditions: The Solution of Some Case Studies
This paper deals with the measurement of the R, L, and C parameters of passive components in nonsinusoidal conditions. Since these components usually work with voltage and current waveforms that are different from sinusoidal ones, nonsinusoidal characterization has to be made. The importance of nonsinusoidal characterization of passive components is highlighted through the analysis of two case studies: (1) the influence of distorted waveforms on the line impedance stabilizer network (LISN) passive component behaviors and (2) the influence of voltage and current harmonics on hybrid filter responses. In this paper, the authors propose and describe a measurement method based on linear system identification and model parameter estimation techniques. Then, the two case studies are analyzed and described with the help of some test results
Performance Characterization of a Wireless Instrumentation Bus
The architecture and operation of a wireless instrumentation bus based on the Bluetooth standard are presented together with a detailed characterization. The bus, via suitable wireless interface boards, allows measurement instruments equipped with standard RS232, IEEE488, universal serial bus, and local area network interface ports to be simply connected within the wireless area without any firmware modification. In addition, a comparison with traditional wired interface buses is carried out in terms of communication reliability and throughput with reference to some basic operating modes. Finally, the results of distance tests and a detailed evaluation of power consumption for each wireless interface module are reported. This will enable measurement system designers to evaluate the convenience of the solution for their application requirements
"Hiccup" accretion in the swinging pulsar IGR J18245-2452
IGR J18245-2452 is the fifteenth discovered accreting millisecond X-ray
pulsar and the first source of this class showing direct evidence for
transition between accretion and rotational powered emission states. These
swing provided the strongest confirmation of the pulsar recycling scenario
available so far. During the two XMM-Newton observations that were carried out
while the source was in outburst in April 2013, IGR J18245-2452 displayed a
unique and peculiar variability of its X-ray emission. In this work, we report
on a detailed analysis of the XMM- Newton data and focus in particular on the
timing and spectral variability of the source. IGR J18245-2452 continuously
switches between lower and higher intensity states, with typical variations in
flux up to a factor of about 500 in time scales as short as few seconds. These
variations in the source intensity are sometimes associated to a dramatic
spectral hardening, during which the power-law photon index of the source
changes from Gamma=1.7 to Gamma=0.9. The pulse profiles extracted at different
count rates and energies show a complex variability. These phenomena are not
usually observed in accreting millisecond X-ray pulsars, at least not on such a
short time scale. Fast variability was also found in the ATCA radio
observations carried out for about 6 hours during the outburst at a frequency
of 5.5 and 9 GHz. We interpret the variability observed from IGR J18245-2452 in
terms of a "hiccup" accretion phase, during which the accretion of material
from the inner boundary of the Keplerian disk is reduced by the onset of
centrifugal inhibition of accretion, possibly causing the launch of strong
outflows. Changes across accretion and propeller regimes have been long
predicted and reproduced by MHD simulations of accreting millisecond X-ray
pulsars but never observed to produce an extreme variability as that shown by
IGR J18245-2452.Comment: A&A in press. Revised versio
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