186 research outputs found
Probing the stellar wind environment of Vela X-1 with MAXI
Vela X-1 is among the best studied and most luminous accreting X-ray pulsars.
The supergiant optical companion produces a strong radiatively-driven stellar
wind, which is accreted onto the neutron star producing highly variable X-ray
emission. A complex phenomenology, due to both gravitational and radiative
effects, needs to be taken into account in order to reproduce orbital spectral
variations. We have investigated the spectral and light curve properties of the
X-ray emission from Vela X-1 along the binary orbit. These studies allow to
constrain the stellar wind properties and its perturbations induced by the
compact object. We took advantage of the All Sky Monitor MAXI/GSC data to
analyze Vela X-1 spectra and light curves. By studying the orbital profiles in
the and keV energy bands, we extracted a sample of orbital light
curves (% of the total) showing a dip around the inferior
conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged
and phase-resolved spectra of both the double-peaked and the standard sample.
The dip in the double-peaked sample needs cm to
be explained by absorption solely, which is not observed in our analysis. We
show how Thomson scattering from an extended and ionized accretion wake can
contribute to the observed dip. Fitted by a cutoff power-law model, the two
analyzed samples show orbital modulation of the photon index, hardening by
around the inferior conjunction, compared to earlier and later
phases, hinting a likely inadequacy of this model. On the contrary, including a
partial covering component at certain orbital phase bins allows a constant
photon index along the orbital phases, indicating a highly inhomogeneous
environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&
In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4-7323
AX J0049.4-7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a
~750 s pulsar which has been observed over the last ~17 years by several X-ray
telescopes. Despite numerous observations, little is known about its X-ray
behaviour. Therefore, we coherently analysed archival Swift, Chandra,
XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already
published ASCA data, to study its X-ray long-term spectral and flux
variability. AX J0049.4-7323 shows a high X-ray variability, spanning more than
three orders of magnitudes, from L ~ 1.6E37 erg/s (0.3-8 keV, d=62 kpc) down to
L ~ 8E33 erg/s. RXTE, Chandra, Swift, and ASCA observed, in addition to the
expected enhancement of X-ray luminosity at periastron, flux variations by a
factor of ~ 270 with peak luminosities of ~2.1E36 erg/s far from periastron.
These properties are difficult to reconcile with the typical long-term
variability of Be/XRBs, traditionally interpreted in terms of type I and type
II outbursts. The study of AX J0049.4-7323 is complemented with a spectral
analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend
when the emission becomes fainter, and an analysis of optical/UV data collected
by the UVOT telescope on board Swift. In addition, we measured a secular
spin-up rate of s day, which
suggests that the pulsar has not yet achieved its equilibrium period. Assuming
spherical accretion, we estimated an upper limit for the magnetic field
strength of the pulsar of ~3E12 G.Comment: Accepted for publication in Astronomy & Astrophysic
X-ray and optical monitoring of the December 2017 outburst of the Be/X-ray binary AX J0049.4-7323
AX J0049.4-7323 is a Be/X-ray binary that shows an unusual and poorly
understood optical variability that consists of periodic and bright optical
outbursts, simultaneous with X-ray outbursts, characterised by a highly
asymmetric profile. The periodicity of the outbursts is thought to correspond
to the orbital period of the neutron star. To understand the behaviour shown by
this source, we performed the first multi-wavelength monitoring campaign during
the periastron passage of December 2017. The monitoring lasted for ~37 days and
consisted of X-ray, near-ultraviolet, and optical data from the Neil Gehrels
Swift Observatory, the optical I band from the OGLE survey, and spectroscopic
observations of the H-alpha line performed with the 3.9 m Anglo-Australian
Telescope. These observations revealed AX J0049.4-7323 during an anomalous
outburst having remarkably different properties compared to the previous ones.
In the I band, it showed a longer rise timescale (~60 days instead of 1-5 days)
and a longer decay timescale. At the peak of the outburst, it showed a sudden
increase in luminosity in the I band, corresponding to the onset of the X-ray
outburst. The monitoring of the H-alpha emission line showed a fast and highly
variable profile composed of three peaks with variable reciprocal brightness.
We interpreted these results as a circumstellar disc warped by tidal
interactions with the neutron star in a high eccentricity orbit during its
periastron passage. The fast jump in optical luminosity at the peak of the
outburst and the previous asymmetric outbursts might be caused by the
reprocessing of the X-ray photons in the circumstellar disc or the tidal
displacement of a large amount of material from the circumstellar disc or the
outer layers of the donor star during the periastron passage of the neutron
star, which led to an increase in size of the region emitting in the I band.Comment: Accepted for publication in Astronomy & Astrophysic
Quantitative Information Flow and Applications to Differential Privacy
International audienceSecure information flow is the problem of ensuring that the information made publicly available by a computational system does not leak information that should be kept secret. Since it is practically impossible to avoid leakage entirely, in recent years there has been a growing interest in considering the quantitative aspects of information flow, in order to measure and compare the amount of leakage. Information theory is widely regarded as a natural framework to provide firm foundations to quantitative information flow. In this notes we review the two main information-theoretic approaches that have been investigated: the one based on Shannon entropy, and the one based on Rényi min-entropy. Furthermore, we discuss some applications in the area of privacy. In particular, we consider statistical databases and the recently-proposed notion of differential privacy. Using the information-theoretic view, we discuss the bound that differential privacy induces on leakage, and the trade-off between utility and privac
Trust in Crowds: probabilistic behaviour in anonymity protocols
The existing analysis of the Crowds anonymity protocol assumes that a participating member is either ‘honest’ or ‘corrupted’. This paper generalises this analysis so that each member is assumed to maliciously disclose the identity of other nodes with a probability determined by her vulnerability to corruption. Within this model, the trust in a principal is defined to be the probability that she behaves honestly. We investigate the effect of such a probabilistic behaviour on the anonymity of the principals participating in the protocol, and formulate the necessary conditions to achieve ‘probable innocence’. Using these conditions, we propose a generalised Crowds-Trust protocol which uses trust information to achieves ‘probable innocence’ for principals exhibiting probabilistic behaviour
Efficient Numerical Frameworks for Multi-objective Cyber Security Planning
We consider the problem of optimal investment in cyber-security by an enterprise. Optimality is measured with respect to the overall (1) monetary cost of implementation, (2) negative side-effects of cyber-security controls (indirect costs), and (3) mitigation of the cyber-security risk. We consider “passive” and “reactive” threats, the former representing the case where attack attempts are independent of the defender’s plan, the latter, where attackers can adapt and react to an implemented cyber-security defense. Moreover, we model in three different ways the combined effect of multiple cyber-security controls, depending on their degree of complementarity and correlation. We also consider multi-stage attacks and the potential correlations in the success of different stages. First, we formalize the problem as a non-linear multi-objective integer programming. We then convert them into Mixed Integer Linear Programs (MILP) that very efficiently solve for the exact Pareto-optimal solutions even when the number of available controls is large. In our case study, we consider 27 of the most typical security controls, each with multiple intensity levels of implementation, and 37 common vulnerabilities facing a typical SME. We compare our findings against expert-recommended critical controls. We then investigate the effect of the security models on the resulting optimal plan and contrast the merits of different security metrics. In particular, we show the superior robustness of the security measures based on the “reactive” threat model, and the significance of the hitherto overlooked role of correlations
Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023
The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and
reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the
evolution of different spectral and timing properties of the source using NICER
observations. The pulse period is found to decrease from 208 s to 205 s, and
the pulse profile evolves significantly with energy and luminosity. The
hardness ratio and hardness intensity diagram (HID) show remarkable evolution
during the outburst. The HID turns towards the diagonal branch from the
horizontal branch above a transition (critical) luminosity, suggesting the
presence of two accretion modes. Each NICER spectrum can be described using a
cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At
higher luminosities, an additional Gaussian at 6.67 keV is used. The observed
photon index shows negative and positive correlations with X-ray flux below and
above the critical luminosity, respectively. The evolution of spectral and
timing parameters suggests a possible change in the emission mechanism and
beaming pattern of the pulsar depending on the spectral transition to sub- and
super-critical accretion regimes. Based on the critical luminosity, the
magnetic field of the neutron star can be estimated in the order of 10
or 10 G, assuming different theoretical models. Moreover, the observed
iron emission line evolves from a narrow to a broad feature with luminosity.
Two emission lines originating from neutral and highly ionized Fe atoms were
evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).Comment: Published in Monthly Notices of the Royal Astronomical Societ
Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52
We report the temporal and spectral analysis of three thermonuclear X-ray
bursts from 4U 1608-52, observed by the Neutron Star Interior Composition
Explorer (NICER) during and just after the outburst observed from the source in
2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds
after the initial peaks. The secondary peaks show a fast rise exponential
decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray
spectral analysis reveals that the peak flux, blackbody temperature, and
apparent emitting radius values of the initial peaks are in agreement with
X-ray bursts previously observed from 4U 1608-52, while the same values for the
secondary peaks tend toward the lower end of the distribution of bursts
observed from this source. The third X-ray burst, which happened during much
lower accretion rates did not show any evidence for a deviation from an
exponential decay and was significantly brighter than the previous bursts. We
present the properties of the secondary peaks and discuss the events within the
framework of short recurrence time bursts or bursts with secondary peaks. We
find that the current observations do not fit in standard scenarios and
challenge our understanding of flame spreading.Comment: Accepted for publication in the Astrophysical Journa
- …