Coronal vertical structure variations in normal branch of GX 17+2: AstroSat’s SXT and LAXPC perspective

2020 ◽  
Vol 499 (2) ◽  
pp. 2214-2228
Author(s):  
S Malu ◽  
K Sriram ◽  
V K Agrawal
Keyword(s):  
Cross Correlation ◽  
Temporal Analysis ◽  
Light Curves ◽  
Power Density Spectrum ◽  
Large Area ◽  
Density Spectrum ◽  
X Ray ◽  
Correlation Studies ◽  
Reflection Model ◽  
Normal Branch

ABSTRACT We performed spectro-temporal analysis in the 0.8–50 keV energy band of the neutron star Z source GX 17+2 using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data. The source was found to vary in the normal branch (NB) of the hardness–intensity diagram. Cross-correlation studies of LAXPC light curves in soft and hard X-ray band unveiled anticorrelated lags of the order of few hundred seconds. For the first time, cross-correlation studies were performed using SXT soft and LAXPC hard light curves and they exhibited correlated and anticorrelated lags of the order of a hundred seconds. Power density spectrum displayed normal branch oscillations (NBOs) of 6.7–7.8 Hz (quality factor 1.5–4.0). Spectral modelling resulted in inner disc radius of ∼12–16 km with Γ ∼ 2.31–2.44 indicating that disc is close to the innermost stable circular orbit and a similar value of disc radius was noticed based on the reflection model. Different methods were used to constrain the corona size in GX 17+2. Using the detected lags, corona size was found to be 27–46 km (β = 0.1, β = vcorona/vdisc) and 138–231 km (β = 0.5). Assuming the X-ray emission to be arising from the boundary layer (BL), its size was determined to be 57–71 km. Assuming that BL is ionizing the disc’s inner region, its size was constrained to ∼19–86 km. Using NBO frequency, the transition shell radius was found to be around 32 km. Observed lags and no movement of the inner disc front strongly indicate that the varying corona structure is causing the X-ray variation in the NB of Z source GX 17+2.

scholarly journals Properties of Faint X-ray Activity of XTE J1908+094 in 2019

Galaxies ◽  
2021 ◽  
Vol 9 (2) ◽  
pp. 25
Author(s):  
Debjit Chatterjee ◽  
Arghajit Jana ◽  
Kaushik Chatterjee ◽  
Riya Bhowmick ◽  
Sujoy Kumar Nath ◽  
...  
Keyword(s):  
Black Hole ◽  
Temporal Analysis ◽  
Power Density Spectrum ◽  
Periodic Oscillations ◽  
Telescope Array ◽  
Density Spectrum ◽  
X Ray ◽  
Black Hole Candidate ◽  
Short Period ◽  
Spectral State

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR). We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely, in the soft–intermediate) spectral state during this short period of X-ray activity. We notice a faint but broad Fe Kα emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be 6.5−0.7+0.5M⊙, with 90% confidence.

scholarly journals RECOGNIZING DIFFERENT TYPES OF STOCHASTIC PROCESSES

2006 ◽  
Vol 06 (01) ◽  
pp. L1-L6
Author(s):  
JONG U. KIM ◽  
LASZLO B. KISH
Keyword(s):  
Image Processing ◽  
Stochastic Processes ◽  
Cross Correlation ◽  
Correlation Method ◽  
Correlation Coefficients ◽  
Power Density Spectrum ◽  
Density Spectrum ◽  
Random Telegraph Signals ◽  
Different Types ◽  
Coefficient Method

We propose a new cross-correlation method that can recognize independent realizations of the same type of stochastic processes and can be used as a new kind of pattern recognition tool in biometrics, sensing, forensic, security and image processing applications. The method, which we call bispectrum correlation coefficient method, makes use of the cross-correlation of the bispectra. Three kinds of cross-correlation coefficients are introduced. To demonstrate the new method, six different random telegraph signals are tested, where four of them have the same power density spectrum. It is shown that the three coefficients can map the different stochastic processes to specific sub-volumes in a cube.

scholarly journals Variability of the Ultraviolet Continuum and Emission Lines of NGC 3783

1994 ◽  
Vol 159 ◽  
pp. 403-403
Author(s):  
G.A. Reichert
Keyword(s):  
Seyfert Galaxy ◽  
Light Curves ◽  
Emission Lines ◽  
Power Density Spectrum ◽  
Ultraviolet Continuum ◽  
Density Spectrum ◽  
International Ultraviolet Explorer ◽  
Monitoring Effort ◽  
Ngc 5548 ◽  
The Continuum

On behalf of the International AGN Watch, I report on the results of intensive ultraviolet spectral monitoring of the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular basis for a total of seven months, once every 4 days for the first 172 days and once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light curves for the continuum fluxes exhibited two well-defined local minima or “dips,” the first lasting ≲ 20 days and the second ≲ 4 days, with additional episodes of relatively rapid flickering of approximately the same amplitude. As in the case of NGC 5548 (the only other Seyfert galaxy that has been the subject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that the continuum became “harder” when brighter. The variations in the continuum occurred simultaneously at all wavelengths (Δt < 2 days). Generally, the amplitude of variability of the emission lines was lower than (or comparable to) that of the continuum. Apart from Mg II (which varied little) and N V (which is relatively weak and badly blended with Lyα) the light curves of the emission lines are very similar to the continuum light curves, in each case with a small systematic delay or “lag.” As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags found for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of (formally) ∼ 0 days for He II+O III], and ∼ 4 days for Lyα, and C IV. The data further suggest lags of ∼ 4 days for Si IV+O IV], and 8–30 days for Si III]+C III]. Uncertainties in these quantities are likely to be of order 2–3 days for the stronger features (Lyα, C IV), and 3–4 days for the weaker ones (He II+O III], Si IV+O IV], Si III]+C III]). Mg II lagged the 1460 Å continuum by ∼ 9 days, although this result depends on the method of measuring the line flux, and may in fact be due to variability of the underlying Fe II lines. Correlation analysis further shows that the power density spectrum contains substantial unresolved power over time scales of ≲ 2 days, and that the character of the continuum variability may change with time.

scholarly journals Inertial oscillation modes of an inclined dipolar magnetosphere as a source of band-limited noise in X-ray pulsars

2020 ◽  
Vol 496 (1) ◽  
pp. 13-18
Author(s):  
Pavel Abolmasov ◽  
Anton Biryukov
Keyword(s):  
Equilibrium Points ◽  
Broad Band ◽  
Periodic Oscillation ◽  
Power Density Spectrum ◽  
Inertial Oscillation ◽  
Noise Component ◽  
Density Spectrum ◽  
X Ray ◽  
Field Lines ◽  
Band Limited

ABSTRACT Magnetic fields of strongly magnetized stars can trap conducting matter due to frozen-in condition. In the force-free regime, the motion of the matter along the field lines may be considered in the ‘bead on a wire’ approximation. Such a motion, if gravity and centrifugal forces are taken into account, has equilibrium points, some of which are stable. In most cases, stability is possible in about several per cent of the possible locations. Corresponding oscillation frequencies span the range from zero to $\sqrt{3}$ of the spin frequency. We suggest that this variability mode may be excited in some X-ray pulsars during the outbursts and create the peaked broad-band noise component near the break frequency in the power density spectrum, as well as produce some of the quasi-periodic oscillation features in this frequency range. Existence of this variability does not require any changes in mass accretion rate and involves only a small amount of matter infiltrating from the disc and magnetic flow due to interchange instabilities.

scholarly journals A non-linear mathematical model for the X-ray variability of the microquasar GRS 1915+105 – III. Low-frequency quasi-periodic oscillations

2020 ◽  
Vol 497 (1) ◽  
pp. 405-415
Author(s):  
E Massaro ◽  
F Capitanio ◽  
M Feroci ◽  
T Mineo
Keyword(s):  
Mathematical Model ◽  
Equilibrium Points ◽  
Low Frequency ◽  
Input Function ◽  
Power Density Spectrum ◽  
Periodic Oscillations ◽  
Density Spectrum ◽  
X Ray ◽  
Almost All ◽  
The Mathematical Model

ABSTRACT The X-ray emission from the microquasar GRS 1915+105 shows, together with a very complex variability on different time-scales, the presence of low-frequency quasi-periodic oscillations (LFQPOs) at frequencies lower than ∼30 Hz. In this paper, we demonstrate that these oscillations can be consistently and naturally obtained as solutions of a system of two ordinary differential equations, which is able to reproduce almost all variability classes of GRS 1915+105. We modified the Hindmarsh–Rose model and obtained a system with two dynamical variables x(t), y(t), where the first one represents the X-ray flux from the source, and an input function J(t), whose mean level J0 and its time evolution is responsible of the variability class. We found that for values of J0 around the boundary between the unstable and the stable interval, where the equilibrium points are of spiral type, one obtains an oscillating behaviour in the model light curve similar to the observed ones with a broad Lorentzian feature in the power density spectrum and, occasionally, with one or two harmonics. Rapid fluctuations of J(t), as those originating from turbulence, stabilize the LFQPOs, resulting in a slowly amplitude modulated pattern. To validate the model, we compared the results with real RXTE data, which resulted remarkably similar to those obtained from the mathematical model. Our results allow us to favour an intrinsic hypothesis on the origin of LFQPOs in accretion discs ultimately related to the same mechanism responsible for the spiking limit cycle.

scholarly journals Timing and spectral analysis of 2S 1417−624 during its 2018 outburst

2021 ◽  
Author(s):  
M M Serim ◽  
Ö C Özüdoğru ◽  
Ç K Dönmez ◽  
Ş Şahiner ◽  
D Serim ◽  
...  
Keyword(s):  
Spectral Characteristics ◽  
Noise Spectrum ◽  
High Energy ◽  
Power Density Spectrum ◽  
Mode Switching ◽  
Model Parameters ◽  
Noise Component ◽  
Density Spectrum ◽  
X Ray ◽  
Spectral Changes

Abstract We investigate timing and spectral characteristics of the transient X-ray pulsar 2S 1417−624 during its 2018 outburst with NICER follow up observations. We describe the spectra with high-energy cut-off and partial covering fraction absorption (PCFA) model and present flux-dependent spectral changes of the source during the 2018 outburst. Utilizing the correlation-mode switching of the spectral model parameters, we confirm the previously reported sub-critical to critical regime transitions and we argue that secondary transition from the gas-dominated to the radiation pressure-dominated disc do not lead to significant spectral changes below 12 keV. Using the existing accretion theories, we model the spin frequency evolution of 2S 1417−624 and investigate the noise processes of a transient X-ray pulsar for the first time using both polynomial and luminosity-dependent models for the spin frequency evolution. For the first model, the power density spectrum of the torque fluctuations indicate that the source exhibits red noise component (Γ ∼ −2) within the timescales of outburst duration which is typical for disc-fed systems. On the other hand, the noise spectrum tends to be white on longer timescales with high timing noise level that indicates an ongoing accretion process in between outburst episodes. For the second model, most of the red noise component is eliminated and the noise spectrum is found to be consistent with a white noise structure observed in wind-fed systems.

scholarly journals Optical/X-ray correlations during the V404 Cygni June 2015 outburst

2018 ◽  
Vol 620 ◽  
pp. A110 ◽  
Author(s):  
J. Alfonso-Garzón ◽  
C. Sánchez-Fernández ◽  
P. A. Charles ◽  
J. M. Mas-Hesse ◽  
P. Gandhi ◽  
...  
Keyword(s):  
Time Delays ◽  
Cross Correlation ◽  
Light Curves ◽  
Synchrotron Emission ◽  
X Rays ◽  
X Ray ◽  
Physical Mechanisms ◽  
Cross Correlation Analysis ◽  
All Optical ◽  
Negative Time

Context. We present a multiwavelength analysis of the simultaneous optical and X-ray light curves of the microquasar V404 Cyg during the June 2015 outburst. Aims. We have performed a comprehensive analysis of all the INTEGRAL/IBIS, JEM–X, and OMC observations during the brightest epoch of the outburst, along with complementary NuSTAR, AAVSO, and VSNET data, to examine the timing relationship between the simultaneous optical and X-ray light curves, in order to understand the emission mechanisms and physical locations. Methods. We have identified all optical flares that have simultaneous X-ray observations, and performed a cross-correlation analysis to estimate the time delays between the optical and soft and hard X-ray emission. We also compared the evolution of the optical and X-ray emission with the hardness ratios. Results. We have identified several types of behaviour during the outburst. On many occasions, the optical flares occur simultaneously with X-ray flares, but at other times, positive and negative time delays between the optical and X-ray emission are measured. Conclusions. We conclude that the observed optical variability is driven by different physical mechanisms, including reprocessing of X-rays in the accretion disc and/or the companion star, interaction of the jet ejections with surrounding material or with previously ejected blobs, and synchrotron emission from the jet.

Mechanical behaviour of jute fibre-reinforced polyester composite: Characterization of damage mechanisms using acoustic emission and microstructural observations

2019 ◽  
Vol 53 (24) ◽  
pp. 3377-3394 ◽  
Author(s):  
Adem Alia ◽  
Gilbert Fantozzi ◽  
Nathalie Godin ◽  
Hocine Osmani ◽  
Pascal Reynaud
Keyword(s):  
Acoustic Emission ◽  
Matrix Cracking ◽  
Power Density Spectrum ◽  
Damage Mechanisms ◽  
Density Spectrum ◽  
X Ray ◽  
Time Frequency ◽  
Pull Out ◽  
Fibre Matrix

The present work deals with the mechanical characterization of a woven jute fabrics reinforced polyester resin composite. Two stacking sequences were considered: [0]8 and [+45/−45]2S. In-situ acoustic emission technique, X-ray computed tomography (X-ray CT) and post-mortem microscopic observations were used to characterise the damage mechanisms and to follow their evolutions during uniaxial tension. The microstructural analysis and X-ray CT revealed the following damage modes for the two lay-ups: fibre-matrix debondings which constitute the dominant mechanism, matrix crackings and fibre breakages and pull-outs. The acoustic emission data were processed using an unsupervised pattern recognition technique which combines principal components analysis and k-means optimized by a genetic algorithm. Both temporal and frequential features of acoustic emission signals were considered. The Laplacian score and dendrogram were used to determine the relevant and uncorrelated descriptors for clustering. Three clusters of events were obtained and the waveforms of each one were examined. Furthermore, the frequency contents of signals of each cluster were accurately investigated using power density spectrum and smoothed pseudo Wigner-Ville time-frequency distribution and the discrepancies between clusters are highlighted. Cluster 1 (Cl 1) characterized by signals of low frequency and intermediate amplitude, cluster 2 (Cl 2) characterized by signals of higher frequency and an amplitude similar to that of Cl 1 and cluster 3 (Cl 3) characterized by signals of higher amplitude. A correlation between the clusters and the damage mechanisms was established by means of interrupted tensile tests: Cl 1 is assigned to the matrix cracking, Cl 2 to the fibre-matrix debonding and Cl 3 to the fibre breakage and pull-out. The kinetic of evolution of each damage mode was monitored for the two stacking sequences.

scholarly journals A broad-band look of the accreting millisecond X-ray pulsar SAX J1748.9−2021 using AstroSat and XMM–Newton

2020 ◽  
Vol 492 (3) ◽  
pp. 4361-4368 ◽  
Author(s):  
Rahul Sharma ◽  
Aru Beri ◽  
Andrea Sanna ◽  
Anjan Dutta
Keyword(s):  
Timing Analysis ◽  
Broad Band ◽  
Energy Bands ◽  
Large Area ◽  
Pulse Profile ◽  
Time Resolved ◽  
X Ray ◽  
Reflection Model ◽  
Single Temperature ◽  
Energy Pulse

ABSTRACT SAX J1748.9−2021 is a transient accretion powered millisecond X-ray pulsar located in the globular cluster NGC 6440. We report on the spectral and timing analysis of SAX J1748.9−2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average local spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3 to 7 and 7 to 20 keV energy bands suggest constant fractional amplitude ∼0.5 per cent for fundamental component, contrary to previously observed energy pulse profile dependence. Our AstroSat observations revealed the source to be in a hard spectral state. The 1–50 keV spectrum from SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) on-board AstroSat can be well described with a single temperature blackbody and thermal Comptonization. Moreover, we found that the combined spectra from XMM–Newton (EPIC-PN) and AstroSat (SXT + LAXPC) indicated the presence of reflection features in the form of iron (Fe Kα) line that we modelled with the reflection model xillvercp. One of the two X-ray burst observed during the AstroSat/LAXPC observation showed hard X-ray emission (&gt;30 keV) due to Compton up-scattering of thermal photons by the hot corona. Time-resolved analysis performed on the bursts revealed complex evolution in emission radius of blackbody for second burst suggestive of mild photospheric radius expansion.

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