Abstracts of Interest
Selected by:
Tiffany Collins
Abstract: 1911.06823
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Title:Effectively using unsupervised machine learning in next generation astronomical surveys
(Submitted on 15 Nov 2019)
Abstract: In recent years many works have shown that unsupervised Machine Learning (ML) can help detect unusual objects and uncover trends in large astronomical datasets, but a few challenges remain. We show here, for example, that different methods, or even small variations of the same method, can produce significantly different outcomes. While intuitively somewhat surprising, this can naturally occur when applying unsupervised ML to highly dimensional data, where there can be many reasonable yet different answers to the same question. In such a case the outcome of any single unsupervised ML method should be considered a sample from a conceivably wide range of possibilities. We therefore suggest an approach that eschews finding an optimal outcome, instead facilitating the production and examination of many valid ones. This can be achieved by incorporating unsupervised ML into data visualisation portals. We present here such a portal that we are developing, applied to the sample of SDSS spectra of galaxies. The main feature of the portal is interactive 2D maps of the data. Different maps are constructed by applying dimensionality reduction to different subspaces of the data, so that each map contains different information that in turn gives a different perspective on the data. The interactive maps are intuitive to use, and we demonstrate how peculiar objects and trends can be detected by means of a few button clicks. We believe that including tools in this spirit in next generation astronomical surveys will be important for making unexpected discoveries, either by professional astronomers or by citizen scientists, and will generally enable the benefits of visual inspection even when dealing with very complex and extensive datasets. Our portal is available online at galaxyportal.space.
Abstract: 1911.06839
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Title:Estimation of Key Sunquake Parameters through Hydrodynamic Modeling and Cross-Correlation Analysis
(Submitted on 15 Nov 2019)
Abstract: Sunquakes are one of the more distinct secondary phenomena related to solar flares, where energy deposition in the lower layers of the Sun's atmosphere excites acoustic waves easily visible in HMI dopplergrams. We explore two possible sources of sunquakes in the context of the electron beam hypothesis, as an instantaneous transfer of momentum and as a gradual applied force. We model the sunquake excitation and compare with five observed sunquake events using a cross-correlation analysis. We find that at least half the events studied are consistent with the electron beam hypothesis and estimate the energy required to excite the sunquakes to be within the range determined by previous studies.
Abstract: 1911.06864
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Full Text: [ PostScript, PDF]
Title:Observation of Acceleration of HI Clouds Within the Fermi Bubbles
(Submitted on 15 Nov 2019)
Abstract: The ~200 HI clouds observed to be entrained in the Fermi Bubble wind show a trend of increasing maximum |VLSR| with Galactic latitude. We analyze previous observations and present new data from the Green Bank Telescope that rule out systematic effects as the source of this phenomenon. Instead, it is likely evidence for acceleration of the clouds. The data suggest that clouds in the lower 2 kpc of the Fermi Bubbles, within the Bubble boundaries established from X-ray studies, have an outflow velocity that rises from ~150 - 200 km/s close to the Galactic Center and reaches ~330 km/s at a distance of 2.5 - 3.5 kpc. These parameters are also consistent with the kinematics of UV absorption lines from highly ionized species observed against two targets behind the Fermi Bubbles at $b = -6.6^{\circ}$, and $b = +11.2^{\circ}$. The implied neutral cloud lifetime is 4 - 10 Myr.
Abstract: 1911.07240
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Full Text: [ PostScript, PDF]
Title:A charming ICECUBE discover?
(Submitted on 17 Nov 2019)
Abstract: Last two years high energy neutrino data are studied. The two recent tau neutrino double bang candidate are discussed within their detectability, noise and expected rate. The neutrino flavor distribution mainly favoring equal electron and muon presence, is reminded. The angular distribution of highest muon neutrino tracks is analyzed. Their horizontal strong anisotropy and their remarkable up-down asymmetry, with the absence of clustering, is noticed. The main persistent missing of astrophysical X,gamma sources (as GRB and AGN flaring source) and all the above signatures led us to suggest a dominance of prompt charmed (atmospheric) events able to pollute, to smear and to hide any minor astronomical presence.
Abstract: 1911.07441
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Full Text: [ PostScript, PDF]
Title:Modelling the interaction between relativistic and non-relativistic winds in binary pulsar systems: strong magnetization of the pulsar wind
(Submitted on 18 Nov 2019)
Abstract: We present a numerical study of the properties of the flow produced by the collision of a magnetized anisotropic pulsar wind with the circumbinary environment. We focus on studying the impact of the high wind magnetization on the geometrical structure of the shocked flow. This work is an extension of our earlier studies that focused on a purely hydrodynamic interaction and weak wind magnetization. We consider the collision in the axisymmetric approximation, that is, the pulsar rotation axis is assumed to be oriented along the line between the pulsar and the optical star. The increase of the magnetization results in the expansion of the opening cone in which the shocked pulsar wind propagates. This effect is explained in the frameworks of the conventional theory of collimation of magnetized winds. This finding has a direct implication for scenarios that involve Doppler boosting as the primary mechanism behind the GeV flares detected with the Fermi/LAT from PSR B1259-63/LS2883. The maximum enhancement of the apparent emission is determined by the ratio of $4\pi$ to the solid in which the shocked pulsar wind propagates. Our simulations suggest that this enhancement factor is decreased by the impact of the magnetic field.
Abstract: 1911.07552
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Title:The newborn black hole in GRB 191014C manifests that is alive
(Submitted on 18 Nov 2019)
Abstract: The popular view that BHs (BH) are dark objects, a sink of energy rather than an energy source, arises from three assumptions, that they are (i) in vacuum, (ii) in an asymptotically flat space-time, and (iii) stationary. As a result, despite theoretical efforts, the search for an efficient mechanism to extract the energy from a BH, able to power the most energetic astrophysical sources, gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), has been unsuccessful for decades. Here we show that an extremely efficient electrodynamical process of BH energy extraction occurs in the "inner engine", violating the assumptions (i)-(iii), composed of a rotating BH in a background of very-low-density ionized plasma and a test, ordered magnetic field, aligned and parallel with the rotation axis. It operates in a sequence of "quantized" steps, each emitting a "blackholic quantum", initially of energy $10^{37}$ erg, over a timescale of $10^{-14}$ s, leading to GeV and TeV photons with luminosities of $10^{51}$ erg s$^{-1}$. Every event takes away only $10^{-16}$ of the extractable energy of the BH, allowing the "inner engine" to operate for thousands of years or more. The blackholic quantum of energy, with such a characteristic short timescale, is emitted in the entire Universe in view of the ubiquitous and homogeneous cosmological presence of GRBs. This suggests the intriguing possibility that, rather than representing the end of life, BHs may have a relevant role in the evolution of life in our Universe.
Abstract: 1911.07825
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Title:Know thy star, know thy planet: Chemo-kinematically characterizing TESS targets
(Submitted on 18 Nov 2019)
Abstract: The Transiting Exoplanet Survey Satellite (TESS) has already begun to discover what will ultimately be thousands of exoplanets around nearby cool bright stars. These potential host stars must be well-understood to accurately characterize exoplanets at the individual and population levels. We present a catalogue of the chemo-kinematic properties of 2,218,434 stars in the TESS Candidate Target List using survey data from Gaia DR2, APOGEE, GALAH, RAVE, LAMOST, and photometrically-derived stellar properties from SkyMapper. We compute kinematic thin disc, thick disc, and halo membership probabilities for these stars and find that though the majority of TESS targets are in the thin disc, 4% of them reside in the thick disc and <1% of them are in the halo. The TESS Objects of Interest in our sample also display similar contributions from the thin disc, thick disc, and halo with a majority of them being in the thin disc. We also explore metallicity and [alpha/Fe] distributions for each Galactic component and show that each cross-matched survey exhibits metallicity and [alpha/Fe] distribution functions that peak from higher to lower metallicity and lower to higher [alpha/Fe] from the thin disc to the halo. This catalogue will be useful to explore planet occurrence rates, among other things, with respect to kinematics, component-membership, metallicity, or [alpha/Fe].
Abstract: 1911.07855
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Full Text: [ PostScript, PDF]
Title:The impact of our local environment on cosmological statistics
(Submitted on 18 Nov 2019)
Abstract: We conduct a thorough investigation into the possibility that residing in an overdense region of the Universe may induce bias in measurements of the large-scale structure. We compute the conditional correlation function and angular power spectrum of density and lensing fluctuations while holding the local spherically-averaged density fixed and show that for Gaussian fields this has no effect on the angular power at $l>0$. We identify a range of scales where a perturbative approach allows analytic progress to be made, and we compute leading-order conditional power spectra using an Edgeworth expansion and second-order perturbation theory. We find no evidence for any significant bias to cosmological power spectra from our local density contrast. We show that when smoothed over a large region around the observer, conditioning on the local density typically affects density power spectra by less than a percent at cosmological distances, below cosmic variance. We find that while typical corrections to the lensing angular power spectrum can be at the 10% level on the largest angular scales and for source redshifts $z_s \lesssim 0.1$, for the typical redshifts targeted by upcoming wide imaging surveys the corrections are sub-percent and negligible, in contrast to previous claims in the literature. Using an estimate of the local spherically-averaged density from a composite galaxy redshift catalogue we find that the corrections from conditioning on our own local density are below cosmic variance and subdominant to other non-linear effects. We discuss the potential implications of our results for cosmology and point out that a measurement of the local density contrast may be used as a consistency test of cosmological models.
Abstract: 1911.08198
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Full Text: [ PostScript, PDF]
Title:Nature of $γ$-ray variability in blazars
(Submitted on 19 Nov 2019)
Abstract: We present an in-depth and systematic analysis of a sample of 20 powerful blazars, including 12 BL Lacs and 8 flat spectrum radio quasars, utilizing Fermi/LAT observations from the period 2008--2018 using various analysis tools such as flux distribution, symmetry analysis, and time series analysis. Results show that blazars with steeper $\gamma$-ray spectral indexes are found to be more variable; and the $\gamma$-ray flux distribution closely resemble both normal and lognormal probability distribution functions. The statistical variability properties of the sources as studied by power spectral density analysis are consistent with flicker noise ($P(\nu)\propto1/\nu$) -- an indication of long-memory processes at work. Statistical analysis of the distribution of flux rise and decay rates in the light curves of the sources, aimed at distinguishing between particle acceleration and energy dissipation timescales, counter-intuitively suggests that both kinds of rates follow a similar distribution and the derived mean variability timescales are in the order of a few weeks. The corresponding emission region size is used to constrain location of $\gamma$-ray production sites in the sources to be a few parsecs. Additionally, using Lomb-Scargle periodogram aided with extensive Monte Carlo simulations, we detected year timescale quasi-periodic oscillations in the sources S5 0716+714, Mrk 421, ON +325, PKS 1424-418 and PKS 2155-304; and the detection significance was computed taking proper account of the red-noise and other artifacts inherent in the observations. We explain our results in the light of current blazar models with relativistic shocks propagating down the jet viewed close to the line of sight.
Abstract: 1911.08321
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Full Text: [ PostScript, PDF]
Title:Evolution of High-energy Particle Distribution in Supernova Remnants
(Submitted on 18 Nov 2019)
Abstract: The spectra fits to a sample of 34 supernova remnants (Zeng et al., 2019) are updated. $\gamma$-ray spectra of 20 supernova remnants (SNRs) with a soft TeV spectrum are further analyzed. We found that 17 of them can be fitted in the hadronic scenario with a single power-law ion distribution with an index of $\sim$ 2.6, which is significantly softer than the ion distribution inferred from $\gamma$-ray observations of star-burst galaxies. If Galactic cosmic rays are mostly produced by SNRs, this result suggests that SNRs in star-burst galaxies may never reach the phase with a soft $\gamma$-ray spectra or escape of high-energy particles from SNRs before they reach this phase with a soft $\gamma$-ray spectrum dominates the contribution of SNRs to Galactic cosmic rays.
Abstract: 1911.08092
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Full Text: [ PostScript, PDF]
Title:Emergence of life in an inflationary universe
(Submitted on 19 Nov 2019)
Abstract: Abiotic emergence of ordered information stored in the form of RNA is an important unresolved problem concerning the origin of life. A polymer longer than 40--100 nucleotides is necessary to expect a self-replicating activity, but the formation of such a long polymer having a correct nucleotide sequence by random reactions seems statistically unlikely. However, our universe, created by a single inflation event, likely includes more than $10^{100}$ Sun-like stars. If life can emerge at least once in such a large volume, it is not in contradiction with our observations of life on Earth, even if the expected number of abiogenesis events is negligibly small within the observable universe that contains only $10^{22}$ stars. Here, a quantitative relation is derived between the minimum RNA length $l_{\min}$ required to be the first biological polymer, and the universe size necessary to expect the formation of such a long and active RNA by randomly adding monomers. It is then shown that an active RNA can indeed be produced somewhere in an inflationary universe. On the other hand, $l_{\min}$ must be shorter than $\sim$20 nucleotides for the abiogenesis probability close to unity on a terrestrial planet, but a self-replicating activity is not expected for such a short RNA. Therefore, if extraterrestrial organisms of a different origin from those on Earth are discovered in the future, it would imply an unknown mechanism at work to polymerize nucleotides much faster than random statistical processes.
Abstract: 1911.08661
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Full Text: [ PostScript, PDF]
Title:Stellar Magnetic Activity and "Butterfly Diagram" of Kepler-63
(Submitted on 20 Nov 2019)
Abstract: Context. The study of young solar type stars is fundamental for a better understanding of the magnetic activity of the Sun. As a planet in transit crosses in front of its host star, a darkspot on the stellar surface may be occulted, causing a detectable variation in the light curve. Kepler-63 is a young solar-like star withan age of only 210 Myr that exhibit photometric variations compatible with spot signatures. Since its orbiting planet is in an almostpolar orbit, different latitudes of the star can be probed by the method of spot transit mapping. Methods. A total of 150 transits of Kepler-63b were observed in the short cadence light curve, corresponding to a total duration of about 4 years. Each transit light curve was fit by a model that simulates planetary transits and allows the inclusion of starspots on the surface of the host star. This enables the physical characterisation of the spots size, intensity, and location. We determine the spot position in a reference frame that rotates with the star, and thus obtain the latitudinal distribution of the spots. Results. A total of 297 spots were fit and their sizes, intensities, and positions determined. The latitude distribution of spots exhibits a bimodality with a lack of spots around 34°. The high latitude spots dominate the magnetic cycle of Kepler-63. For a mean stellar rotation period of 5.400d, 59 spots were found at approximately the same longitude and latitude on a later transit. Conclusions. Due to the geometry of the Kepler-63 system, we were able to build a starspot "butterfly diagram". It was also possible to infer Kepler-63 differential rotation from the presence of spots at different latitudes. This star was found to rotate almost rigidly with a period of 5.400d and relative shear close to 0.01% for latitudes less than 34°, whereas the high latitudes do not follow a well behaved pattern.
Abstract: 1911.08961
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Full Text: [ PostScript, PDF]
Title:A very-high-energy component deep in the Gamma-ray Burst afterglow
(Submitted on 20 Nov 2019)
Abstract: Gamma-ray bursts (GRBs) are brief flashes of gamma rays, considered to be the most energetic explosive phenomena in the Universe. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow -- produced by the interaction between the ejected matter and the circumburst medium -- slows down, and a gradual decrease in brightness is observed. GRBs typically emit most of their energy via gamma-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elussive. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow -ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.
Abstract: 1810.05976
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Full Text: [ PostScript, PDF]
Title:Relativistic Electron Scattering and Big Bang Nucleosynthesis
(Submitted on 14 Oct 2018 (v1), last revised 19 Nov 2019 (this version, v3))
Abstract: This paper is superseded by arXiv:1911.07334. Big-bang nucleosynthesis (BBN) is a valuable tool to constrain the physics of the early universe and is the only probe of the radiation-dominated epoch. A fundamental assumption in BBN is that the nuclear velocity distributions obey Maxwell-Boltzmann statistics as they do in stars. In this letter, however, we point out that there is a fundamental difference between stellar reaction rates and BBN reaction rates. Specifically, the BBN epoch is characterized by a dilute baryon plasma for which the velocity distribution of nuclei is mainly determined by the dominant Coulomb scattering with mildly relativistic electrons. This modifies the nuclear velocity distributions and significantly alters the thermonuclear reaction rates, and hence, the light-element abundances. We show that this novel result alters all previous calculations of light-element abundances from BBN, and indeed exacerbates the discrepancies between BBN and inferred primordial light-element abundances possibly suggesting the need for new physics in the early universe.
Abstract: 1911.09372
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Full Text: [ PostScript, PDF]
Title:Axion-like particles and high energy astrophysics
(Submitted on 21 Nov 2019)
Abstract: Axion-like particles (ALPs) are light, neutral, pseudo-scalar bosons predicted by several extensions of the Standard Model of particle physics -- such as the String Theory -- and are supposed to interact primarily only with two photons. In the presence of an external magnetic field, photon-ALP oscillations occur and can produce sizable astrophysical effects in the very-high energy (VHE) band ($100 \, {\rm GeV} - 100 \, {\rm TeV}$). Photon-ALP oscillations increase the transparency of the Universe to VHE photons partially preventing the gamma-gamma absorption caused by the Extragalactic Background Light (EBL). Moreover, they have important implications for active galactic nuclei (AGN) by modifying their observed spectra both for flat spectrum radio quasars (FSRQs) and BL Lacs. Many attempts have been made in order to constrain the ALP parameter space by studying irregularities in spectra due to photon-ALP conversion in galaxy clusters and the consequences of ALP emission by stars. Upcoming new VHE photon detectors like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE, HERD and ASTRI will settle the issue.
Abstract: 1911.09407
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Full Text: [ PostScript, PDF]
Title:Redshifted 21-cm emission signal from the halos in Dark Ages
(Submitted on 21 Nov 2019)
Abstract: The emission in the hyperfine structure 21 cm line of atomic hydrogen arising in the halos with masses $\sim10^6-10^{10}$ M$_\odot$ from the Dark Ages in the models with Warm Dark Matter (WDM) is analysed. The halos are assumed to be formed from Gaussian density peaks of cosmological density perturbations at $10\lesssim z\lesssim50$. Semi-analytical modelling of the formation of individual spherical halos in multi-component models shows that gas in them has the kinetic temperature in the range of $60-800$ K under adiabatic compression of the collapsing halo, and the temperature of each halo depends on the time of virialization. It is shown that inelastic collisions between neutral hydrogen atoms are the dominant excitation mechanism for hyperfine structure levels, which pulls the spin temperature closer to the kinetic temperature. The brightness temperature of individual halos is in the range of 1-10 K, depending on the mass of the halo and its virialization redshift, and increasing as these two increase. The apparent angular radii of such halos are in the range 0.06-1.2 arcseconds, their surface number density decreasing exponentially from a few per arcmin$^2$ for the lowest mass and redshift to nearly zero for higher values. Assuming a 1 MHz observation bandwidth the surface number density of the halo at various redshifts is evaluated as well as beam-averaged differential antenna temperatures and fluxes of hydrogen emission from halos of different masses. The beam-averaged signal strongly depends on the cut-off scale in the mass function of dark ages halos that may be caused by free-streaming of WDM particles. The finding is compared with the upper limits on the amplitude of the power spectrum of the hydrogen 21-cm line fluctuations derived from the recent observation data obtained with MWA and LOFAR.
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