Dr. Dmytro Iakubovskyi

Affiliation
Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine
Position
scientific researcher
Homepage
http://virgo.org.ua/index.php?mfc=staff&lang=en&p_id=3&p_cat=researchers


Papers bookmarked by Dmytro Iakubovskyi


arXiv.org papers


  • We investigate the possibility of boiling instability of nuclear liquid in the inner core of the proto-neutron star formed in the core collapse of a type II supernova. We derive a simple criterion for boiling to occur. Using this criterion for one of best described equations of state of supernova matter, we find that boiling is quite possible under the conditions realized inside the proto-neutron star. We discuss consequences of this process such as the increase of heat transfer rate and pressure in the boiling region. We expect that taking this effect into account in the conventional neutrino-driven delayed-shock mechanism of type II supernova explosions can increase the explosion energy and reduce the mass of the neutron-star remnant.
  • We derive constraints on parameters of the radiatively decaying Dark Matter (DM) particles, using XMM-Newton EPIC spectra of the Andromeda galaxy (M31). Using the observations of the outer (5'-13') parts of M31 we improve the existing constraints. For the case of sterile neutrino DM, combining our constraints with the latest computation of abundances of sterile neutrino in the Dodelson-Widrow (DW) scenario, we obtain the lower mass limit m_s < 4 keV, which is stronger than the previous one m_s < 6 kev, obtained recently by Asaka et al. (2007) [hep-ph/0612182]. Comparing this limit with the most recent results on Lyman-alpha forest analysis of Viel et al. (2007) [arXiv:0709.0131] (m_s > 5.6 kev), we argue that the scenario in which all the DM is produced via DW mechanism is ruled out. We discuss however other production mechanisms and note that the sterile neutrino remains a viable candidate of Dark Matter, either warm or cold.
  • We discuss the bounds on the mass of Dark Matter (DM) particles, coming from the analysis of DM phase-space distribution in dwarf spheroidal galaxies (dSphs). After reviewing the existing approaches, we choose two methods to derive such a bound. The first one depends on the information about the current phase space distribution of DM particles only, while the second one uses both the initial and final distributions. We discuss the recent data on dSphs as well as astronomical uncertainties in relevant parameters. As an application, we present lower bounds on the mass of DM particles, coming from various dSphs, using both methods. The model-independent bound holds for any type of fermionic DM. Stronger, model-dependent bounds are quoted for several DM models (thermal relics, non-resonantly and resonantly produced sterile neutrinos, etc.). The latter bounds rely on the assumption that baryonic feedback cannot significantly increase the maximum of a distribution function of DM particles. For the scenario in which all the DM is made of sterile neutrinos produced via non-resonant mixing with the active neutrinos (NRP) this gives m_nrp > 1.7 keV. Combining these results in their most conservative form with the X-ray bounds of DM decay lines, we conclude that the NRP scenario remains allowed in a very narrow parameter window only. This conclusion is independent of the results of the Lyman-alpha analysis. The DM model in which sterile neutrinos are resonantly produced in the presence of lepton asymmetry remains viable. Within the minimal neutrino extension of the Standard Model (the nuMSM), both mass and the mixing angle of the DM sterile neutrino are bounded from above and below, which suggests the possibility for its experimental search.
  • A number of important processes taking place around strong shocks in supernova remnants (SNRs) depend on the shock obliquity. The measured synchrotron flux is a function of the aspect angle between interstellar magnetic field (ISMF) and the line of sight. Thus a model of non-thermal emission from SNRs should account for the orientation of the ambient magnetic field. We develop a new method for the estimation of the aspect angle, based on the comparison between observed and synthesized radio maps of SNRs, making different assumptions about the dependence of electron injection efficiency on the shock obliquity. The method uses the azimuthal profile of radio surface brightness as a probe for orientation of ambient magnetic field because it is almost insensitive to the downstream distribution of magnetic field and emitting electrons. We apply our method to a new radio image of SN 1006 produced on the basis of archival VLA and Parkes data. The image recovers emission from all spatial structures with angular scales from few arcseconds to 15 arcmin. We explore different models of injection efficiency and find the following best-fitting values for the aspect angle of SN 1006: phi=70 +/- 4.2 deg if the injection is isotropic, phi=64 +/- 2.8 deg for quasi-perpendicular injection (SNR has an equatorial belt in both cases) and phi=11 +/- 0.8 deg for quasi-parallel injection (polar-cap model of SNR). In the last case, SN 1006 is expected to have a centrally-peaked morphology contrary to what is observed. Therefore, our analysis provides some indication against the quasi-parallel injection model.
  • Efficient particle acceleration can modify the structure of supernova remnants. In this context we present the results of the combined analysis of the XMM-Newton EPIC archive observations of SN 1006. We aim at describing the spatial distribution of the physical and chemical properties of the X-ray emitting plasma at the shock front. We investigate the contribution of thermal and non-thermal emission to the X-ray spectrum at the rim of the remnant to study how the acceleration processes affect the X-ray emitting plasma. We perform a spatially resolved spectral analysis on a set of regions covering the whole rim of the shell and we exploit the results of the spectral analysis to produce a count-rate image of the "pure" thermal emission of SN 1006 in the 0.5-0.8 keV energy band (subtracting the non-thermal contribution). This image significantly differs from the total image in the same band, specially near the bright limbs. We find that thermal X-ray emission can be associated with the ejecta and we study the azimuthal variation of their physical and chemical properties finding anisotropies in temperature and chemical composition. Thanks to our thermal image we trace the position of the contact discontinuity over the whole shell and we compare it with that expected from 3-D MHD models of SNRs with unmodified shock. We conclude that the shock is modified everywhere in the rim and that the aspect angle between the interstellar magnetic field and the line of sight is significantly lower than 90 degrees
  • We propose a method to synthesize the inverse Compton (IC) gamma-ray image of a supernova remnant starting from the radio (or hard X-ray) map and using results of the spatially resolved X-ray spectral analysis. The method is successfully applied to SN 1006. We found that synthesized IC gamma-ray images of SN 1006 show morphology in nice agreement with that reported by the H.E.S.S. collaboration. The good correlation found between the observed very-high energy gamma-ray and X-ray/radio appearance can be considered as an evidence that the gamma-ray emission of SN 1006 observed by H.E.S.S. is leptonic in origin, though the hadronic origin may not be excluded.
  • We present a new universal relation, satisfied by matter distributions at all observed scales, and show its amazingly good and detailed agreement with the predictions of the most up-to-date pure dark matter simulations of structure formation in the Universe. This work extends the previous analysis [0904.4054; 0909.5203] to a larger range of masses, demonstrates a different scaling law, and compares it with numerical simulations. This behaviour seems to be insensitive to the complicated feedback of baryons on dark matter. Therefore, it potentially allows to compare theoretical predictions directly with observations, thus providing a new tool to constrain the properties of dark matter. Such a universal property, observed in structures of all sizes (from dwarf spheroidal galaxies to galaxy clusters), is difficult to explain without dark matter, thus providing new evidence for its existence.
  • A signal from decaying dark matter (DM) can be unambiguously distinguished from spectral features of astrophysical or instrumental origin by studying its spatial distribution. We demonstrate this approach by examining the recent claim of 0912.0552 regarding the possible DM origin of the 2.5 keV line in Chandra observations of the Milky Way satellite known as Willman 1. Our conservative strategy is to adopt a relatively large dark mass for Willman 1 and relatively small dark masses for the comparison objects. We analyze archival observations by XMM-Newton of M31 and Fornax dwarf spheroidal galaxy (dSph) and Chandra observations of Sculptor dSph. By performing a conservative analysis of X-ray spectra, we show the absence of a DM decay line with parameters consistent with those of 0912.0552. For M31, the observations of the regions between 10 and 20 kpc from the center, where the uncertainties in the DM distribution are minimal, make a strong exclusion at the level above 10sigma. The minimal estimate for the amount of DM in the central 40 kpc of M31 is provided by the model of 0912.4133, assuming the stellar disk's mass to light ratio ~8 and almost constant DM density within a core of 28 kpc. Even in this case one gets an exclusion at 5.7sigma from central region of M31 whereas modeling all processed data from M31 and Fornax produces more than 14sigma exclusion. Therefore, despite possible systematic uncertainties, we exclude the possibility that the spectral feature at ~2.5 keV found in 0912.0552 is a DM decay line. We conclude, however, that the search for DM decay line, although demanding prolonged observations of well-studied dSphs, M31 outskirts and other similar objects, is rather promising, as the nature of a possible signal can be checked. An (expected) non-observation of a DM decay signal in the planned observations of Willman 1 should not discourage further dedicated observations.
  • In this work, we perform the detailed analysis of absorption features in spectra of magnetar candidates observed by XMM-Newton satellite. No significant line-like feature has been found. This negative result may indicate the possible presence of smoothing out the absorption features mechanisms.
  • We review the status of sterile neutrino dark matter and discuss astrophysical and cosmological bounds on its properties as well as future prospects for its experimental searches. We argue that if sterile neutrinos are the dominant fraction of dark matter, detecting an astrophysical signal from their decay (the so-called 'indirect detection') may be the only way to identify these particles experimentally. However, it may be possible to check the dark matter origin of the observed signal unambiguously using its characteristic properties and/or using synergy with accelerator experiments, searching for other sterile neutrinos, responsible for neutrino flavor oscillations. We argue that to fully explore this possibility a dedicated cosmic mission - an X-ray spectrometer - is needed.
  • We consider cosmological solutions and their stability with respect to homogeneous and isotropic perturbations in the braneworld model with the scalar-curvature term in the action for the brane. Part of the results are similar to those obtained by Campos and Sopuerta for the Randall-Sundrum braneworld model. Specifically, the expanding de Sitter solution is an attractor, while the expanding Friedmann solution is a repeller. In the braneworld theory with the scalar-curvature term in the action for the brane, static solutions with matter satisfying the strong energy condition exist not only with closed spatial geometry but also with open and flat ones even in the case where the dark-radiation contribution is absent. In a certain range of parameters, static solutions are stable with respect to homogeneous and isotropic perturbations.