Classification and Spectral Evolution of Outbursts of Aql X-1

We present a broad classification of all outbursts detected with the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) and the Monitor of All Sky X-Ray Image (MAXI) of Aql X-1. We identify three types of outbursts; long-high, medium-low, and short-low, based on the duration and maximum flux. We analyse the trends in the "phase-space" of flux-derivative versus flux to demonstrate the differences in the three identified outburst types. We present a spectral analysis of the observations of Aql X-1 performed by the Proportional Counter Array (PCA) onboard RXTE during the 2000 and 2011 outbursts of the long-high class and the 2010 outburst of the medium-low class. We model the source spectrum with a hybrid thermal/non-thermal hot plasma emission model (EQPAIR in XSPEC, Coppi 2000) together with a Gaussian component to model the Fe K_alpha emission line. We construct time histories of the source flux, the optical depth of the corona (tau), the seed photon temperature (kT_bb) and the hard state compactness (l_h) for these three outbursts. We show that the physical parameters of either classes reach the same values throughout the outbursts, the only difference being the maximum flux. We discuss our results in the terms of modes of interaction of the star with the disc and size of the disc kept hot by irradiation. We conclude that irradiation is the dominant physical process leading to the different classes of outbursts.Comment: MNRAS accepted. 12 pages, 9 figures, 3 table

What does a measurement of mass and/or radius of a neutron star constrain: Equation of state or gravity?

Neutron stars (NSs) are thought to be excellent laboratories for determining the equation of state (EoS) of cold dense matter. Their strong gravity suggests that they can also be used to constrain gravity models. The mass and radius (M-R) of a NS both depend on the choice of EoS and gravity, meaning that NSs cannot be simultaneously good laboratories for both of these questions. A measurement of M-R would constrain the less well known physics input. The assumption that M-R measurements can be used to constrain EoS-presumes general relativity (GR) is the ultimate model of gravity in the classical regime. We calculate the radial profile of compactness and curvature (square root of the full contraction of the Weyl tensor) within a NS and determine the domain not probed by the Solar System tests of GR. We find that, except for a tiny sphere of radius less than a millimeter at the center, the curvature is several orders of magnitude above the values present in Solar System tests. The compactness is beyond the solar surface value for r>10 m, and increases by 5 orders of magnitude towards the surface. With the density being only an order of magnitude higher than that probed by nuclear scattering experiments, our results suggest that the employment of GR as the theory of gravity describing the hydrostatic equilibrium of NSs is a rather remarkable extrapolation from the regime of tested validity, as opposed to that of EoS models. Our larger ignorance of gravity within NSs suggests that a measurement of M-R constrains gravity rather than EoS, and given that EoS has yet to be determined by nucleon scattering experiments, M-R measurements cannot tightly constrain the gravity models either. Near the surface the curvature and compactness attain their largest values, while EoS in this region is fairly well known. This renders the crust as the best site to look for deviations from GR.Comment: Phys.Rev. D published, typos corrected to match the published versio

Partial accretion in the propeller stage of low mass X-ray binary Aql X--1

Aql X--1 is one of the most prolific low mass X-ray binary transients (LMXBTs) showing outbursts almost annually. We present the results of our spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts. We investigate the spectral changes related to the changing disk-magnetosphere interaction modes of Aql X--1. The X-ray light curves of the outbursts of LMXBTs typically show phases of fast rise and exponential decay. The decay phase shows a "knee" where the flux goes from the slow decay to the rapid decay stage. We assume that the rapid decay corresponds to a weak propeller stage at which a fraction of the inflowing matter in the disk accretes onto the star. We introduce a novel method for inferring, from the light curve, the fraction of the inflowing matter in the disk that accretes onto the NS depending on the fastness parameter. We determine the fastness parameter range within which the transition from the accretion to the partial propeller stage is realized. This fastness parameter range is a measure of the scale-height of the disk in units of the inner disk radius. We applied the method to a sample of outbursts of Aql X--1 with different maximum flux and duration times. We show that different outbursts with different maximum luminosity and duration follow a similar path in the parameter space of accreted/inflowing mass flux fraction versus fastness parameter.Comment: 16 pages, 5 figures, 5 tables, accepted for publication in Ap

Neutron stars in a perturbative f(R)f(R) gravity model with strong magnetic fields

We investigate the effect of a strong magnetic field on the structure of neutron stars in a model with perturbative $f(R)$ gravity. The effect of an interior strong magnetic field of about $10^{17 \sim 18}$ G on the equation of state is derived in the context of a quantum hadrodynamics (QHD) model. We solve the modified spherically symmetric hydrostatic equilibrium equations derived for a gravity model with $f(R)=R+\alpha R^2$. Effects of both the finite magnetic field and the modified gravity are detailed for various values of the magnetic field and the perturbation parameter $\alpha$ along with a discussion of their physical implications. We show that there exists a parameter space of the modified gravity and the magnetic field strength, in which even a soft equation of state can accommodate a large ($> 2$ M$_\odot$) maximum neutron star mass through the modified mass-radius relation

TranSMS: Transformers for Super-Resolution Calibration in Magnetic Particle Imaging

Magnetic particle imaging (MPI) offers exceptional contrast for magnetic nanoparticles (MNP) at high spatio-temporal resolution. A common procedure in MPI starts with a calibration scan to measure the system matrix (SM), which is then used to set up an inverse problem to reconstruct images of the MNP distribution during subsequent scans. This calibration enables the reconstruction to sensitively account for various system imperfections. Yet time-consuming SM measurements have to be repeated under notable changes in system properties. Here, we introduce a novel deep learning approach for accelerated MPI calibration based on Transformers for SM super-resolution (TranSMS). Low-resolution SM measurements are performed using large MNP samples for improved signal-to-noise ratio efficiency, and the high-resolution SM is super-resolved via model-based deep learning. TranSMS leverages a vision transformer module to capture contextual relationships in low-resolution input images, a dense convolutional module for localizing high-resolution image features, and a data-consistency module to ensure measurement fidelity. Demonstrations on simulated and experimental data indicate that TranSMS significantly improves SM recovery and MPI reconstruction for up to 64-fold acceleration in two-dimensional imaging

DEQ-MPI: A Deep Equilibrium Reconstruction with Learned Consistency for Magnetic Particle Imaging

Magnetic particle imaging (MPI) offers unparalleled contrast and resolution for tracing magnetic nanoparticles. A common imaging procedure calibrates a system matrix (SM) that is used to reconstruct data from subsequent scans. The ill-posed reconstruction problem can be solved by simultaneously enforcing data consistency based on the SM and regularizing the solution based on an image prior. Traditional hand-crafted priors cannot capture the complex attributes of MPI images, whereas recent MPI methods based on learned priors can suffer from extensive inference times or limited generalization performance. Here, we introduce a novel physics-driven method for MPI reconstruction based on a deep equilibrium model with learned data consistency (DEQ-MPI). DEQ-MPI reconstructs images by augmenting neural networks into an iterative optimization, as inspired by unrolling methods in deep learning. Yet, conventional unrolling methods are computationally restricted to few iterations resulting in non-convergent solutions, and they use hand-crafted consistency measures that can yield suboptimal capture of the data distribution. DEQ-MPI instead trains an implicit mapping to maximize the quality of a convergent solution, and it incorporates a learned consistency measure to better account for the data distribution. Demonstrations on simulated and experimental data indicate that DEQ-MPI achieves superior image quality and competitive inference time to state-of-the-art MPI reconstruction methods

Timing analysis of 2S 1417-624 observed with NICER and insight-HXMT

We present a study of timing properties of the accreting pulsar 2S 1417-624 observed during its 2018 outburst, based on Swift/BAT, Fermi/GBM, Insight-HXMT and NICER observations. We report a dramatic change of the pulse profiles with luminosity. The morphology of the profile in the range 0.2-10.0 keV switches from double to triple peaks at ∼ 2.5×10^(37) D^{2}_{10} erg s−1 and from triple to quadruple peaks at ∼ 7×10^{37} D^2{2}_{10} erg s−1. The profile at high energies (25-100 keV) shows significant evolutions as well. We explain this phenomenon according to existing theoretical models. We argue that the first change is related to the transition from the sub to the super-critical accretion regime, while the second to the transition of the accretion disc from the gas-dominated to the radiation pressuredominated state. Considering the spin-up as well due to the accretion torque, this interpretation allows to estimate the magnetic field self-consistently at ∼ 7 × 1012 G

Effective early termination techniques for text similarity join operator

Bu çalışma, 26-28 Ekim 2005 tarihleri arasında İstanbul[Türkiye]'da düzenlenen 20. International Symposium on Computer and Information Sciences'da bildiri olarak sunulmuştur.Text similarity join operator joins two relations if their join attributes are textually similar to each other, and it has a variety of application domains including integration and querying of data from heterogeneous resources; cleansing of data; and mining of data. Although, the text similarity join operator is widely used, its processing is expensive due to the huge number of similarity computations performed. In this paper, we incorporate some short cut evaluation techniques from the Information Retrieval domain, namely Harman, quit, continue, and maximal similarity filter heuristics, into the previously proposed text similarity join algorithms to reduce the amount of similarity computations needed during the join operation. We experimentally evaluate the original and the heuristic based similarity join algorithms using real data obtained from the DBLP Bibliography database, and observe performance improvements with continue and maximal similarity filter heuristics.Inst Elec & Elect Engineers, Turkey SectBoğaziçi Üniversites

Constant cyclotron line energy in hercules X–1 - joint insight-HXMT and NuSTAR observations

The long-term evolution of the centroid energy of the CRSF in Her X-1 is still a mystery. We report a new measurement from a campaign between Insight-HXMT and NuSTAR performed in February 2018. Generally, the two satellites show well consistent results of timing and spectral properties. The joint spectral analysis confirms that the previously observed long decay phase has ended, and that the line energy instead keeps constant around 37.5 keV after flux correction