Fourier Power Spectra of Solar Noise Storms

We analyzed three noise storms recorded on 200?–?400 MHz Trieste Callisto radio spectra on 2 July 2012, 8 July 2012, and 16 July 2012 by the Fourier method. We divided intervals of the noise storms into five-minute intervals, and in these intervals we computed the mean Fourier spectra as a function of the wave numbers in the frequency and height-scale spaces. We found that these Fourier spectra, where the spectrum from the quiet-activity interval was subtracted, are power-law spectra. The mean power-law index of these spectra in the range \(\ln(k_{z}) = [1.8, 2.9]\) (where \(k_{z}\) is the wave number in the height-scale space) is \(-1.7\pm0.14\), \(-1.6\pm0.14\), and \(-1.5 \pm0.12\) for the 2 July 2012, the 8 July 2012, and the 16 July 2012 noise storms, respectively. It appears that as the number of Type-I bursts in the studied interval increases, the power-law index becomes closer to \(-5/3\); this is known as the Kolmogorov spectral index. The power-law index of the noise storms is very similar to that of the narrowband dm-spikes found in our previous studies. Furthermore, we found a break in the power spectra at \(\ln(k_{z}) \approx2.9\), and the mean power-law index values above this break are \(-2.9\pm0.46\), \(-3.1\pm0.65\), and \(-3.4\pm0.98\), respectively.     

Sunspot Positions and Areas from Observations by Pierre Gassendi

Solar activity behaviour on the eve of the Maunder minimum may provide important information on the period of further suppression of sunspot population. We analyse sunspot positions and areas in the 1630s extracted from rare drawings published by Pierre Gassendi in Opera Omnia. This work was published in two different editions, the first in Lyon and the second almost 70 years later in Florence. The drawings published in Lyon are found to be slightly different from those published in Florence, which produces a discrepancy in the position of spots of a few degrees, while sunspot group areas may differ by a factor of two. We reveal that the orientation of the drawings in the book is not always the same as might be seen in the telescope. We conjecture that the time of Gassendi’s observations covers the beginning of a new Schwabe cycle in the southern hemisphere. The differential rotation rate in the 1630s is also assessed and discussed.     

Advective accretion flow properties around rotating black holes – application to GRO J1655-40

We examine the properties of the viscous dissipative accretion flow around rotating black holes in the presence of mass loss. Considering the thin disc approximation, we self-consistently calculate the inflow-outflow solutions and observe that the mass outflow rates decrease with the increase in viscosity parameter (\(\alpha \)). Further, we carry out the model calculation of quasi-periodic oscillation frequency (\(\nu _{\mathrm{QPO}}\)) that is frequently observed in black hole sources and observe that \(\nu ^\mathrm{max}_{\mathrm{QPO}}\) increases with the increase of black hole spin (\(a_k\)). Then, we employ our model in order to explain the High Frequency Quasi-Periodic Oscillations (HFQPOs) observed in black hole source GRO J1655-40. While doing this, we attempt to constrain the range of \(a_k\) based on observed HFQPOs (\(\sim \)300 Hz and \(\sim \)450 Hz) for the black hole source GRO J1655-40.     

Template-based classification of SDSS-<Emphasis Type="BoldItalic">GALEX</Emphasis> point sources

We have classified a sample of 37,492 objects from SDSS into QSOs, galaxies and stars using photometric data over five wave bands (u, g, r, i and z) and UV GALEX data over two wave bands (near-UV and far-UV) based on a template fitting method. The advantage of this method of classification is that it does not require any spectroscopic data and hence the objects for which spectroscopic data is not available can also be studied using this technique. In this study, we have found that our method is consistent by spectroscopic methods given that their UV information is available. Our study shows that the UV colours are especially important for separating quasars and stars, as well as spiral and starburst galaxies. Thus it is evident that the UV bands play a crucial role in the classification and characterization of astronomical objects that emit over a wide range of wavelengths, but especially for those that are bright at UV. We have achieved the efficiency of 89% for the QSOs, 63% for the galaxies and 84% for the stars. This classification is also found to be in agreement with the emission line diagnostic diagrams.     

A new technique for understanding magnetosphere–ionosphere coupling using directional derivatives of SuperDARN convection flow

The purpose of this paper is to present and evaluate a new technique to better understand ionospheric convection and it’s magnetospheric drivers using convection maps derived from the Super Dual Auroral Radar Network (SuperDARN). We postulate that the directional derivative of the SuperDARN ionospheric convection flow can be used as a technique for understanding solar wind–magnetosphere–ionosphere coupling by identifying regions of strong acceleration/deceleration of plasma flow associated with drivers of magnetospheric convection such as magnetic reconnection. Thus, the technique may be used to identify the open–closed magnetic field line boundary (OCB) in certain circumstances. In this study, directional derivatives of the SuperDARN ionospheric convection flow over a four and a half hour interval on Nov. 04, 2001, is presented during which the interplanetary magnetic field was predominantly southward. At each one-minute time point in the interval the positive peak in the directional derivative of flow is identified and evaluated via comparison with known indicators of the OCB including the poleward boundary of ultraviolet emissions from three FUV detectors onboard the IMAGE spacecraft as well as the SuperDARN spectral widths. Good comparison is found between the location of the peak in the directional derivative of SuperDARN flow and the poleward boundary of ultraviolet emissions confirming that acceleration of ionospheric plasma flow is associated with magnetic reconnection and the open–closed boundary.     

Observations and physics of prompt emission of gamma ray bursts

Gamma ray bursts (GRBs) are the brightest explosions known to occur in the Universe. For the last several decades, they have been extensively observed and studied using both space as well as ground based observatories. In this review, the observational breakthroughs made till date, the techniques of observation and analyses of obtained data, temporal and spectral properties of the observed prompt emission of GRBs including polarisation, as well as the various theoretical models adopted to explain them are discussed.     

Why chromatic imaging matters

During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.     

The X10 Flare on 29 October 2003: Was It Triggered by Magnetic Reconnection between Counter-Helical Fluxes?

Vector magnetograms taken at Huairou Solar Observing Station (HSOS) and Mees Solar Observatory (MSO) reveal that the super active region (AR) NOAA 10486 was a complex region containing current helicity flux of opposite signs. The main positive sunspots were dominated by negative helicity fields, while positive helicity patches persisted both inside and around the main positive sunspots. Based on a comparison of two days of deduced current helicity density, pronounced changes associated with the occurrence of an X10 flare that peaked at 20:49 UT on 29 October 2003 were noticed. The average current helicity density (negative) of the main sunspots decreased significantly by about 50%. Accordingly, the helicity densities of counter-helical patches (positive) were also found to decay by the same proportion or more. In addition, two hard X-ray (HXR) “footpoints” were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the flare in the 50 – 100 keV energy range. The cores of these two HXR footpoints were adjacent to the positions of two patches with positive current helicity that disappeared after the flare. This strongly suggested that the X10 flare on 29 October 2003 resulted from reconnection between magnetic flux tubes having opposite current helicity. Finally, the global decrease of current helicity in AR 10486 by ∼50% can be understood as the helicity launched away by the halo coronal mass ejection (CME) associated with the X10 flare. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

The intriguing orbital period variability of Y Leonis

The orbital period variation of the oEA system Y Leo is revised by taking into account new times of minimum light covering an extended time base of 101.8 yr. A multiperiodic ephemeris was finally established by carefully approaching the problem of periodicity detection for the considered periodic components. A method relying on Monte Carlo simulations was applied. The problem of the long-term behaviour of the O–C curve was taken into account using parabolic, and parabolic + periodic ephemerides. The physical interpretation of the mathematical models describing both long- and short-term behaviour of the O–C curve was performed by considering different mechanisms: the conservative mass transfer, the light-time effect, and the orbital period modulation through the cyclic magnetic activity of the late spectral type secondary component in the system. The consequences of these interpretations are rather intriguing and emphasize the need of new and detailed observational studies on Y Leo.