arXiv subjects

Denys Malyshev

Publications and source records attributed to Denys Malyshev.

At least 37 records · Page 2Linked to original sources

arXiv2021

The Post-impact Evolution of the X-ray Emitting Gas in SNR 1987A Viewed by XMM-Newton

Since 1996 the blast wave driven by SN 1987A has been interacting with the dense circumstellar material, which provides us with a unique opportunity to study the early evolution of a newborn supernova remnant (SNR). Based on the XMM-Newton RGS and EPIC-pn X-ray observations from 2007 to 2019, we investigated the post-impact evolution of the X-ray emitting gas in SNR 1987A. The hot plasma is represented by two non-equilibrium ionization components with temperature of $\sim0.6$ keV and $\sim2.5$ keV. The low-temperature plasma has a density $\sim2400$ cm$^{-3}$, which is likely dominated by the lower density gas inside the equatorial ring (ER). The high-temperature plasma with a density $\sim550$ cm$^{-3}$ could be dominated by the H II region and the high-latitude material beyond the ring. In the last few years, the emission measure of the low-temperature plasma has been decreasing, indicating that the blast wave has left the main ER. But the blast wave is still propagating into the high-latitude gas, resulting in the steadily increase of the high-temperature emission measure. In the meantime, the average abundances of N, O, Ne, and Mg are found to be declining, which may reflect the different chemical compositions between two plasma components. We also detected the Fe K lines in most of the observations, showing increasing flux and centroid energy. We interpret the Fe K lines as from a third hot component, which may come from the reflected shock-heated gas or originate from Fe-rich ejecta clumps, shocked by the reverse shock.

astro-ph.HE↗

arXiv2021

Technical comment on the paper of Dessert et al. "The dark matter interpretation of the 3.5 keV line is inconsistent with blank-sky observations"

An unidentified line at energy around 3.5 keV was detected in the spectra of dark matter-dominated objects. Recent work of Dessert et al. [1812.06976] used 30 Msec of XMM-Newton blank-sky observations to constrain the admissible line flux, challenging its dark matter decay origin. We demonstrate that these bounds are overestimated by more than an order of magnitude due to improper background modeling. Therefore the dark matter interpretation of the 3.5 keV signal remains viable.

astro-ph.CO↗

arXiv2021

Annihilating Dark Matter Search with 12 Years of Fermi LAT Data in Nearby Galaxy Clusters

Galaxy clusters are the largest virialised objects in the Universe, and as such, have a high dark matter (DM) concentration. This abundance of dark matter makes them promising targets for indirect DM searches. Here we report the details of a search, utilising almost 12~years of Fermi/LAT data, for gamma ray signatures from the pair annihilation of WIMP dark matter in the GeV energy band. From this, we present the constraints on the annihilation cross section for the bb-bar, W+W- and gamma-gamma channels, derived from the non-detection of a characteristic signal from five nearby high galactic latitude galaxy clusters (Centaurus, Coma, Virgo, Perseus and Fornax). We discuss the potential of a boost to the signal due the presence of substructures in the DM halos of selected objects, as well as the impact of uncertainties in DM profiles on the presented results. We assert that the obtained limits are, within a small factor, comparable to the best available limits of those based on Fermi/LAT observations of dwarf spheroidal galaxies.

astro-ph.HE↗

arXiv2020

THESEUS Insights into ALP, Dark Photon and Sterile Neutrino Dark Matter

Through a series of simulated observations, we investigate the capability of the instruments aboard the forthcoming THESEUS mission for the detection of a characteristic signal from decaying dark matter (DM) in the keV-MeV energy range. We focus our studies on three well studied Standard Model extensions hosting axion-like particle, dark photon, and sterile neutrino DM candidates. We show that, due to the sensitivity of THESEUS' X and Gamma Imaging Spectrometer (XGIS) instrument, existing constraints on dark matter parameters can be improved by a factor of up to ~300, depending on the considered DM model and assuming a zero level of systematic uncertainty. We also show that even a minimal level of systematic uncertainty of 1% can impair potential constraints by one to two orders of magnitude. We argue that nonetheless, the constraints imposed by THESEUS will be substantially better than existing ones and will well complement the constraints of upcoming missions such as eXTP and Athena. Ultimately, the limits imposed by THESEUS and future missions will ensure a robust and thorough coverage of the parameter space for decaying DM models, enabling either a detection of dark matter or a significant improvement of relevant limits.

astro-ph.HE↗

arXiv2020

Orbital variability of the optical linear polarization of the $γ$-ray binary LS I +61 303 and new constraints on the orbital parameters

We studied the variability of the linear polarization and brightness of the $γ$-ray binary LS I +61 303. High-precision BVR photopolarimetric observations were carried out with the Dipol-2 polarimeter on the 2.2 m remotely controlled UH88 telescope at Mauna Kea Observatory and the 60 cm Tohoku telescope at Haleakala Observatory (Hawaii) over 140 nights in 2016--2019. We determined the position angle of the intrinsic polarization $θ\simeq 11^\circ$, which can either be associated with the projection of the Be star's decretion disk axis on the plane of sky, or can differ from it by $90^\circ$. Using the Lomb-Scargle method, we performed timing analyses and period searches of our polarimetric and photometric data. We found statistically significant periodic variability of the normalized Stokes parameters $q$ and $u$ in all passbands. The most significant period of variability, $P_\text{Pol} = 13.244 \pm 0.012$ d, is equal to one half of the orbital period $P_\text{orb} = 26.496$ d. Using a model of Thomson scattering by a cloud that orbits the Be star, we obtained constraints on the orbital parameters, including a small eccentricity $e<0.2$ and periastron phase of $ϕ_\text{p}\approx 0.6$, which coincides with the peaks in the radio, X-ray, and TeV emission. These constraints are independent of the assumption about the orientation of the decretion disk plane on the sky. We also extensively discuss the apparent inconsistency with the previous measurements of the orbital parameters from radial velocities. By folding the photometry data acquired during a three-year time span with the orbital period, we found a linear phase shift of the moments of the brightness maximum, confirming the possible existence of superorbital variability.

astro-ph.SR↗

arXiv2020

A study of Neural networks point source extraction on simulated Fermi/LAT Telescope images

Astrophysical images in the GeV band are challenging to analyze due to the strong contribution of the background and foreground astrophysical diffuse emission and relatively broad point spread function of modern space-based instruments. In certain cases, even finding of point sources on the image becomes a non-trivial task. We present a method for point sources extraction using a convolution neural network (CNN) trained on our own artificial data set which imitates images from the Fermi Large Area Telescope. These images are raw count photon maps of 10x10 degrees covering energies from 1 to 10 GeV. We compare different CNN architectures that demonstrate accuracy increase by ~15% and reduces the inference time by at least the factor of 4 accuracy improvement with respect to a similar state of the art models.

cs.CV↗

arXiv2020

Gamma-ray binaries

Gamma-ray binaries are a subclass of high-mass binary systems whose energy spectrum peaks at high energies (E$\gtrsim$100 MeV) and extends to very high energies (E$\gtrsim$100 GeV) $γ$ rays. In this review we summarize properties of well-studied non-transient $γ$-ray binaries as well as briefly discuss poorly known systems and transient systems hosting a microquasar. We discuss also theoretical models that have been used to describe spectral and timing characteristics of considered systems

astro-ph.HE↗

arXiv2020

eXTP perspectives for the $ν$MSM sterile neutrino dark matter model

We discuss the potential of the eXTP X-ray telescope, in particular its Spectroscopic Focusing Array (SFA), Large Area Detector (LAD) and Wide Field Monitor (WFM) for the detection of a signal from keV-scale decaying dark matter. We show that the sensitivity of the eXTP is sufficient to improve existing constraints on the mixing angle of the neutrino Minimal extension of the Standard Model ($ν$MSM) by a factor of 5-10 within the dark matter mass range 2-50 keV, assuming 1% level of systematic uncertainty. We assert that the eXTP will be able to probe previously inaccessible range of $ν$MSM parameters and serve as a precursor for the Athena mission in decaying dark matter searches.

astro-ph.HE↗

arXiv2020

Multi-wavelength observations of PSRJ 2032+4127 during the 2017 periastron passage

PSRJ 2032+4127 is only the second known gamma-ray binary where it is confirmed that a young radio pulsar is in orbit around a Be-star. The interaction of the pulsar wind with the mass outflow from the companion leads to broad band emission from radio up to TeV energies. In the current paper we present results of optical monitoring of the 2017 periastron passage with the Nordic Optical Telescope. These observations are complemented by X-ray (Swift/XRT, NuSTAR) and GeV (Fermi/LAT) monitoring. Joint analysis of the evolution of the parameters of the Halpha line and the broadband (X-ray to TeV) spectral shape allows us to propose a model linking the observed emission to the interaction of the pulsar and Be-star winds under the assumption of the inclined disc geometry. Our model allows the observed flux and spectral evolution of the system to be explained in a self-consistent way.

astro-ph.HE↗

arXiv2019

Unidentified Fermi Objects in the view of H.E.S.S. -- Possible Dark Matter Clumps

There is strong evidence about the existence of unknown dark matter in the Universe. Many different theories about this dark matter exist, but most probably it is made of a new kind of fundamental particle that has to be massive, stable, electrically neutral, and having only weak interaction with standard matter (weakly interacting massive particles). In principle, those particles could produce gamma rays by their annihilation or decay. Therefore, a Gamma-ray signal from a dark matter origin would provide one of the clearest and most concluding evidences for dark matter. High resolution cosmological N-body simulations have shown that dark matter subhalos in the Milky Way halo may developed in the Universe. Those subhalos could pop-up in gamma-ray surveys as unidentified sources. In this paper we present H.E.S.S. observations of unidentified sources selected from Fermi-LAT catalogs. These sources fulfill main features which would characterize a dark matter subhalo, namely, having no obvious counterpart at other wavelengths and being steady hard sources.

astro-ph.HE↗

arXiv2019

Evidence for recent GeV brightening of the SN 1987A region

We report on a recent (2016-2018) enhancement of the GeV emission from the SN 1987A region as observed with Fermi/LAT. The observed signal is characterised by a power-law spectrum with a slope of 2.1 +/- 0.2 and is detected only at energies >1 GeV. The Fermi/LAT data constrain the position of the signal to within 0.15 degree around SN 1987A. Although a recent increase in the gamma-ray emission from SN 1987A seems to be a natural explanation for the detected emission, given the youth of the source and its rapid evolution, the Fermi/LAT location also overlaps with several other potential gamma-ray sources: 30 Dor C, Honeycomb nebula, RX J0536.9-6913, and a hypothetical, previously unknown transient source. We argue that multiwavelength observations of the region performed during the next few years can clarify the nature of the signal and encourage such observations. We also present upper limits on the time-averaged flux of SN 1987A based on 10 years of Fermi/LAT exposure, which can be used to better constrain the particle acceleration models of this source.

astro-ph.HE↗

arXiv2018

A comprehensive study of high-energy gamma-ray and radio emission from Cyg X-3

We study high-energy $γ$-rays observed from Cyg X-3 by the Fermi Large Area Telescope and the 15-GHz emission observed by the Ryle Telescope and the Arcminute Microkelvin Imager. We measure the $γ$-ray spectrum averaged over strong flares much more accurately than before, and find it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$. We find a weaker spectrum in the soft spectral state, but only upper limits in the hard and intermediate states. We measure strong orbital modulation during the flaring state, well modelled by anisotropic Compton scattering of blackbody photons from the donor by jet relativistic electrons. We discover a weaker orbital modulation of the 15 GHz radio emission, which is well modelled by free-free absorption by the stellar wind. We then study cross-correlations between radio, $γ$-ray and X-ray emissions. We find the cross-correlation between the radio and $γ$-ray emissions peaks at a lag less than 1 d, while we detect a distinct radio lag of $\sim$50 d with respect to the soft X-rays in the soft spectral state.

astro-ph.HE↗

arXiv2017

Cosmic ray spectrum in the local Galaxy

We study the spectral properties of the cosmic-ray spectrum in the interstellar medium within 1 kpc distance from the Sun. We used eight-year exposure data of molecular clouds of the Gould Belt obtained with the Fermi/LAT telescope to precisely measure the cosmic-ray spectrum at different locations in the local Galaxy. We compared this measurement with the direct measurements of the cosmic-ray flux in and around the solar system obtained by Voyager and AMS-02 or PAMELA. We find that the average cosmic-ray spectrum in the local Galaxy in the 1-100 GeV range is well described by a broken power-law in rigidity with a low-energy slope of $2.33^{+0.06}_{-0.08}$ and a break at $18^{+7}_{-4}$ GV, with a slope change by $0.59\pm 0.11$. This result is consistent with an earlier analysis of the gamma-ray signal from the Gould Belt clouds based on a shorter exposure of Fermi/LAT and with a different event selection. The break at 10-20 GV is also consistent with the combined Voyager + AMS-02 measurements in/around the solar system. The slope of the spectrum below the break agrees with the slope of the average cosmic-ray spectrum in the inner part of the disk of the Milky Way that was previously derived from the Fermi/LAT gamma-ray data. We conjecture that it is this slope of 2.33 and not the locally measured softer slope of 2.7-2.8 that is determined by the balance between a steady-state injection of cosmic rays with a power-law slope of 2-2.1 that is due to Fermi acceleration and the energy-dependent propagation of cosmic-ray particles through the turbulent interstellar magnetic field with a Kolmogorov turbulence spectrum. The approximation of a continuous-in-time injection of cosmic rays at a constant rate breaks down, which causes the softening of the spectrum at higher energies.

astro-ph.HE↗

arXiv2017

High-energy gamma-rays from Cyg X-1

We have obtained a firm detection of Cyg X-1 during its hard and intermediate spectral states in the energy range of 40 MeV--60 GeV based on observations by the Fermi Large Area Telescope, confirming the independent results at $\geq$60MeV of a previous work. The detection significance is $\simeq\!8σ$ in the 0.1--10 GeV range. In the soft state, we have found only upper limits on the emission at energies $\gtrsim$0.1 MeV. However, we have found emission with a very soft spectrum in the 40--80 MeV range, not detected previously. This is likely to represent the high-energy cutoff of the high-energy power-law tail observed in the soft state. Similarly, we have detected a $γ$-ray soft excess in the hard state, which appears to be of similar origin. We have also confirmed the presence of an orbital modulation of the detected emission in the hard state, expected if the $γ$-rays are from Compton upscattering of stellar blackbody photons. However, the observed modulation is significantly weaker than that predicted if the blackbody upscattering were the dominant source of $γ$-rays. This argues for a significant contribution from $γ$-rays produced by the synchrotron-self-Compton process. We have found that such strong contribution is possible if the jet is strongly clumped. We reproduce the observed hard-state average broad-band spectrum using a self-consistent jet model, taking into account all the relevant emission processes, e$^\pm$ pair absorption, and clumping. This model also reproduces the amplitude of the observed orbital modulation.

astro-ph.HE↗

arXiv2016

Decaying dark matter search with NuSTAR deep sky observations

We present the results of the search for decaying dark matter with particle mass in the 6-40 keV range with NuSTAR deep observations of COSMOS and ECDFS empty sky fields. We show that main contribution to the decaying dark matter signal from the Milky Way galaxy comes through the aperture of the NuSTAR detector, rather than through the focusing optics. High sensitivity of the NuSTAR detector, combined with the large aperture and large exposure times of the two observation fields allow us to improve previously existing constraints on the dark matter decay time by up to an order of magnitude in the mass range 10-30 keV. In the particular case of the nuMSM sterile neutrino dark matter, our constraints impose an upper bound m<20 keV on the dark matter particle mass. We report detection of four unidentified spectral lines in our data set. These line detections are either due to the systematic effects (uncertainties of calibrations of the NuSTAR detectors) or have an astrophysical origin. We discuss different possibilities for testing the nature of the detected lines.

astro-ph.HE↗

arXiv2016

Searching for decaying dark matter in deep XMM-Newton observation of the Draco dwarf spheroidal

We present results of a search for the 3.5 keV emission line in our recent very long (~ 1.4 Ms) XMM-Newton observation of the Draco dwarf spheroidal galaxy. The astrophysical X-ray emission from such dark matter-dominated galaxies is faint, thus they provide a test for the dark matter origin of the 3.5 keV line previously detected in other massive, but X-ray bright objects, such as galaxies and galaxy clusters. We do not detect a statistically significant emission line from Draco; this constrains the lifetime of a decaying dark matter particle to tau > (7-9) x 10^27 s at 95% CL (combining all three XMM-Newton cameras; the interval corresponds to the uncertainty of the dark matter column density in the direction of Draco). The PN camera, which has the highest sensitivity of the three, does show a positive spectral residual (above the carefully modeled continuum) at E = 3.54 +/- 0.06 keV with a 2.3 sigma significance. The two MOS cameras show less-significant or no positive deviations, consistently within 1 sigma with PN. Our Draco limit on tau is consistent with previous detections in the stacked galaxy clusters, M31 and the Galactic Center within their 1-2 sigma uncertainties, but is inconsistent with the high signal from the core of the Perseus cluster (which has itself been inconsistent with the rest of the detections). We conclude that this Draco observation does not exclude the dark matter interpretation of the 3.5 keV line in those objects.

astro-ph.HE↗

arXiv2015

The high-energy gamma-ray detection of G73.9+0.9, a supernova remnant interacting with a molecular cloud

We have analysed the Fermi LAT data on the SNR G73.9+0.9. We have confirmed a previous detection of high-energy gamma-rays from this source at a high significance of $\simeq 12σ$. The observed spectrum shows a significant curvature, peaking in $E F_E$ at $\sim$1 GeV. We have also calculated the flux upper limits in the mm-wavelength and X-ray ranges from Planck and XMM-Newton, respectively. We have inspected the intensity of the CO (1$\rightarrow $0) emission line and found a large peak at a velocity range corresponding to the previously estimated source distance of $\sim$4 kpc, which may indicate an association between a molecular cloud and the SNR. The gamma-ray emission appears due to interaction of accelerated particles within the SNR with the matter of the cloud. The most likely radiative process responsible for the gamma-ray emission is decay of neutral pions produced in ion-ion collisions. While a dominant leptonic origin of this emission can be ruled out, the relativistic electron population related to the observed radio flux will necessarily lead to a certain level of bremsstrahlung gamma-ray emission. Based on this broad-band modelling, we have developed a method to estimate the magnetic field, yielding $B\geq 80\,μ$G at our best estimate of the molecular cloud density (or less at a lower density). G73.9+0.9 appears similar, though somewhat weaker, to other SNRs interacting with a local dense medium detected by the LAT.

astro-ph.HE↗

arXiv2013

High-energy gamma-ray emission from Cyg X-1 measured by Fermi and its theoretical implications

We have obtained measurements and upper limits on the emission of Cyg X-1 in the photon energy range of 0.03--300 GeV based on observations by Fermi. We present the results separately for the hard and soft spectral states, as well for all of the analysed data. In the hard state, we detect a weak steady emission in the 0.1--10 GeV range with a power-law photon index of Gamma=2.6+-0.2 at a 4 sigma statistical significance. This measurement, even if considered to be an upper limit, strongly constrains Compton emission of the steady radio jet, present in that state. The number of relativistic electrons in the jet has to be low enough for the spectral components due to Compton upscattering of the stellar blackbody and synchrotron radiation to be within the observed fluxes. If optically-thin synchrotron emission of the jet is to account for the MeV tail, as implied by the recently-claimed strong polarization in that energy range, the magnetic field in the jet has to be much above equipartition. The GeV-range measurements also strongly constrain models of hot accretion flows, most likely present in the hard state, in which gamma-rays are produced from decay of neutral pions produced in collisions of energetic ions in an inner part of the flow. In the soft state, the obtained upper limits constrain electron acceleration in a non-thermal corona, most likely present around a blackbody accretion disc. The coronal emission above 30 MeV has to be rather weak, which is most readily explained by absorption of gamma-rays in pair-producing photon-photon collisions. Then, the size of the bulk of the corona is less than a few tens of the gravitational radii.

astro-ph.HE↗