258 research outputs found
An Excursion-Theoretic Approach to Stability ofDiscrete-Time Stochastic Hybrid Systems
We address stability of a class of Markovian discrete-time stochastic hybrid systems. This class of systems is characterized by the state-space of the system being partitioned into a safe or target set and its exterior, and the dynamics of the system being different in each domain. We give conditions for L 1-boundedness of Lyapunov functions based on certain negative drift conditions outside the target set, together with some more minor assumptions. We then apply our results to a wide class of randomly switched systems (or iterated function systems), for which we give conditions for global asymptotic stability almost surely and in L 1. The systems need not be time-homogeneous, and our results apply to certain systems for which functional-analytic or martingale-based estimates are difficult or impossible to ge
Swift J1728.9-3613 is a black hole X-ray binary: spectral and timing study using NICER
We study different timing and spectral properties of the new Galactic X-ray
transient Swift J1728.9-3613 using NICER and Swift, discovered by the Burst
Alert Telescope (BAT) on the Neil Gehrels Swift Observatory. The source went
through multiple transitions to different spectral states during the outburst,
and the complete evolution created a q-shaped track in the hardness intensity
diagram. A partial hysteresis is also observed in the RMS-intensity diagram,
which is another well-defined phenomenon of black hole transients. In SIMS,
power density spectra were dominated by broadband noise components, and two
type B QPOs were detected. We have fitted 1-10 keV energy spectra obtained from
NICER observations that were performed during the outburst, and the temporal
evolution of spectral parameters were studied. On MJD 58584.69, a small-scale
reflare happened, and we observed that the spectral index decreased to a much
lower value associated with finite changes in other spectral parameters also,
and the 1-10 keV averaged flux also increased. We observed that the innermost
radius of the accretion disc was almost constant during the soft state, which
corresponds to the Innermost Stable Circular Orbit (ISCO). We have measured the
lower limit of mass of the compact object to be approximately 4.6 M,
considering a non-spinning black hole binary system, by fitting 1-10 keV NICER
spectra with the diskbb component. The soft-to-hard transition occurred when
the bolometric luminosity was 0.01 times the Eddington luminosity. Based on our
combined study of the evolution of the timing and spectral properties, we
conclude that the new source Swift J1728.9-3613 is a black hole X-ray binary.Comment: 6 figures, 2 tables, submitted in MNRA
Rate enhancement of gated drift-diffusion process by optimal resetting
`Gating' is a widely observed phenomenon in biochemistry that describes the
transition between the activated (or open) and deactivated (or closed) states
of an ion-channel, which makes transport through that channel highly selective.
In general, gating is a mechanism that imposes an additional restriction on a
transport, as the process ends only when the `gate' is open and continues
otherwise. When diffusion occurs in presence of a constant bias to a {\it
gated} target, i.e., to a target that switches between an open and a closed
state, the dynamics essentially slows down compared to {\it ungated}
drift-diffusion, resulting in an increase in the mean completion time. In this
work, we utilize stochastic resetting as an external protocol to counterbalance
the delay due to gating. We consider a particle that undergoes drift-diffusion
in the presence of a stochastically gated target and is moreover subjected to a
rate-limiting resetting dynamics. Calculating the minimal mean completion time
rendered by an optimal resetting for this exactly-solvable system, we construct
a phase diagram that owns three distinct phases: (i) where resetting can make
gated drift-diffusion faster even compared to the original ungated process,
(ii) where resetting still expedites gated drift-diffusion, but not beyond the
original ungated process, and (iii) where resetting fails to expedite gated
drift-diffusion. Gated drift-diffusion aptly models various stochastic
processes such as chemical reactions that exclusively take place for certain
activated state of the reactants. Our work predicts the conditions where
stochastic resetting can act as a useful strategy to enhance the rate of such
processes without compromising on their selectivity.Comment: 12 Pages, 8 Figure
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