We study the distributions of $\gamma$-ray properties of a sample of $\gamma$-ray loud blazars taken from the third catalogue of blazars detected by Fermi-Large Area Telescope (Fermi-LAT).We compute the $\gamma$-ray dominance ($D_{\rm g}$) of the sample which includes 415 flat spectrum radio quasars (FSRQs) and 535 BL Lacertae objects (BL Lacs). We find that BL Lacs and FSRQs are highly dominated by $\gamma$-ray emission, which is consistent with diffuse high-energy neutrino flux associated with $\gamma$-ray loud blazars. The $\gamma$-ray dominance fairly scales with $\gamma$-ray luminosity ($r \sim +0.5$) in both BL Lacs and FSRQs, but shows little or no correlation ($r \leq 0.2$) with radio luminosity in either sample. BL Lacs and FSRQs occupy separate and parallel regions on the $D_{\rm g}$-luminosity plane. There is a fairly significant correlation ($r \sim 0.5$) between $\gamma$-ray dominance and frequency at synchrotron peak ($\nu_{\rm pk}$) in BL Lacs, which disappears in FSRQs. On the other hand, there is a tight correlation ($r \geq +0.8$) between $\gamma$-ray and radio luminosity with a smooth transition from BL Lacs at low luminosities to FSRQs at high luminosities. Nevertheless, the presence of few BL Lac-like FSRQs is noted. These results suggest that while there may be intrinsic differences between BL Lacs and FSRQs, some form of a unified scheme can also be relevant.

Ongoing research on pulsar timing array (PTA) to detect gravitational radiation is reviewed. Here, we discuss the use of millisecond pulsars as a gravitational wave detector, the sources of gravitational radiation detectable by PTAs and the current status of PTA experiments pointing out the future possibilities.

The study of the solar energetic particle events (SEPs) and their association with solar flares and other activities are very crucial to understand the space weather. Keeping this in view, here we present the study of SEPs (intensity $\geq$10 pfu) during the solar cycles 21–24 (1976–2017) in >10 MeV energy channels associated with solar flares. For our analysis, we have used the data from different instruments onboard SOHO satellite. We have examined the flare size, source location, coronal mass ejections (CMEs) characteristics of associated SEPs. About 31% and 69% of the SEPs were originated from the eastern and western solar hemisphere, respectively. The average CME speed and width were 1238 km s$^{−1}$ and 253$^{\circ}$, respectively. About58% SEPs were associated with halo CMEs and 42% of SEPs associated with CMEs width varying from 10$^{\circ}$ to 250$^{\circ}$, respectively.

Archival Vilnius CCD photometric observations are presented for the heavily reddened star cluster Melotte 105, resulting in colour-magnitude diagrams and spectral class estimates. There is considerable lack of agreement between studies for reddening, age and distance for this cluster explaining why the archival data are being made available by this paper. The derived reddening $E(B − V) = 0.34 \pm 0.04$ mag and the distance $V − M_V = 12.9 \pm 0.3$ mag are directly from the Vilnius photometry. The Gaia Data Release 2(DR2) and Vilnius photometric data of the cluster were used to estimate the structural parameters of the cluster, probability of stellar membership in the cluster, the distance modulus and the cluster age. Lack of $Y$ bandobservations prevented determination of metal abundance. The values of the colour excess and distance module are determined by two different methods (i.e., Q and Zero Age Main Sequence or ZAMS methods). A distance modulus of $12.85 \pm 0.07$ mag was derived by ZAMS fitting, in good agreement with the above estimate. ZAMS fitting indicates a reddening of $0.403\pm 0.02$ mag, within two sigma of the estimate above. The cluster’s metallicity and age are estimated to be 0.24 dex and $240 \pm 25$ Myr, respectively. The derived mass function is in good agreement with the Salpeter slope. The cluster space velocity components (U, V,W) were determined as ($−3.90 \pm 3.34$, $−13.76 \pm 5.69$, $+3.45 \pm 0.41$) km/s. Perigalactic and apogalactic distances were obtained as $R_ p = 6.85$ and $R_a = 7.44$ kpc respectively. The maximum vertical distance from the Galactic plane was calculated as $Z_{\rm max} = 84$ pc and the eccentricity of the orbit was determined as $e = 0.042$.

A simulation of the radiation field inside a habitable module (a diameter of 6 m and length of 12 m) of a spacecraft generated by isotropic Galactic Cosmic Radiation (GCR) in deep interplanetary space is carried out for minimum and maximum solar activity using the FLUKA code. Protons, alpha-particles,deuterons, ${}^3$He, and nuclei with $Z$ > 2 are considered as primary GCR irradiating the spacecraft isotropically. The following particles are included in FLUKA radiation transport through the module shell (15 g/cm$^2$ of Al): protons, neutrons, $\gamma$-rays, electrons, $\pi^{\pm}$-mesons, $\mu^{\pm}$-mesons d, t, and nuclei from He to Ni. The inner particle spectra are needed to assess the astronaut’s radiation risk in a long-term interplanetary mission.

In view of the negative results from various dark matter detection experiments, we propose alternate models by postulating a minimal field strength (minimal curvature) and also a minimal acceleration. These postulates lead to modified Newtonian dynamics, modified Newtonian gravity, and modified relativistic gravity. Through these postulates, we show that the observed flat rotation curves of galaxies can be accounted for. This can also set constraints on the size of large scale structures such as galaxies and clusters, which are consistent with observations. This minimum curvature in general relativity, and consequently the modification of the Hilbert action leads naturally to a term equivalent to cosmological constant. This model thus neatly ties up, with these two postulates: the observed flat rotation curves and the accelerated expansion of the universe.

Solar flare activity is characterized by different classification systems, both in optical and X-ray ranges. The most generally accepted classifications of solar flares describe important parameters of flares such as the maximum of brightness of the flare in the optical range – $H_{\alpha}$ flare class (change from F to B), area of the flare in $H_{\alpha}$ (change from S for areas less than 2 square degrees to 4 for areas more than 24.7 square degrees) and the maximum amplitude of the soft X-ray (SXR)-flux in the band 0.1–0.8 nm ($F^{\rm max}_{0.1−0.8}$) – X-ray flares of classes from C to X. A new classification parameter of solar flares is proposed here – the X-ray index of flare XI, based on GOES measurements of solar radiation in the SXR-range. The XI-index has a clear physical interpretation associated with the total flare energy in the SXR-range. XI is easily calculated for each flare with the use of available GOES data. The XI-index can be used along with other geoeffective parameters of Solaractivity to assess both flares and coronal mass ejections that are connected with them.

MHD shock waves and discontinuities are often observed in the solar wind. We studied a strong shock wave event on 7 June 2014, during a solar maximum. The properties of forward shock are investigated and all physical parameters of interplanetary shock are analyzed. The upstream parameters, such as Alfvenvelocity and sound speed are calculated, and the angle ($\theta$) between the upstream magnetic field direction and the shock normal is estimated. To determine the propagation speed of the shock, a full numerical solution ofshock adiabatic equation is carried out. The thermal pressure for electrons and ions are calculated and also the pressure ratio and the entropy change across the shock are estimated. The results show that the shock strengthis about 3.88 with $\theta \approx 72^{\circ}$ (a quasi-perpendicular case), and shock propagation velocity is about 678 km/s. The increase in the specific entropy is also evaluated.

The paper deals with the dynamics of particles which are on the surface of the cometary nucleus. The key point is to research the behavior of the particles as a result of local sublimation. Generally, we can describe three mechanisms related to the particles behavior. Relatively small particles (of the order ofmicrometers) are ejected into the atmosphere of the comet due to sublimation. Slightly larger particles (of the order of centimeters) can migrate across the comet’s surface towards the equator, while much larger rubbles ofcometary matter remain at rest. The angular width of the particles is the main factor influencing the migration time towards the equator of the comet. The measure of angular width and particles’ migration time is its sizeand coefficient of friction. The numerical calculations presented in the paper refer to a hypothetical comet X/P and the comet 67P/Churyumov–Gerasimenko.