Abstracts of Interest
Selected by:
Jarryd Day
Abstract: 1911.12398
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Title:Modeling the accretion disk around the high-mass protostar GGD 27-MM1
(Submitted on 27 Nov 2019)
Abstract: Recent high-angular resolution (40 mas) ALMA observations at 1.14 mm resolve a compact (R~200 au) flattened dust structure perpendicular to the HH 80-81 jet emanating from the GGD 27-MM1 high-mass protostar, making it a robust candidate for a true accretion disk. The jet/disk system (HH 80-81 / GGD 27-MM1) resemble those found in association with low- and intermediate-mass protostars. We present radiative transfer models that fit the 1.14 mm ALMA dust image of this disk which allow us to obtain its physical parameters and predict its density and temperature structure. Our results indicate that this accretion disk is compact (Rdisk~170 au) and massive (5Msun), about 20% of the stellar mass of 20 Msun. We estimate the total dynamical mass of the star-disk system from the molecular line emission finding a range between 21 and 30 Msun, which is consistent with our model. We fit the density and temperature structures found by our model with power law functions. These results suggest that accretion disks around massive stars are more massive and hotter than their low-mass siblings, but they still are quite stable. We also compare the temperature distribution in the GGD 27-MM1 disk with that found in low- and intermediate-mass stars and discuss possible implications on the water snow line. We have also carried about a study of the distance based on Gaia DR2 data and the population of young stellar objects (YSOs) in this region, and from the extinction maps. We conclude that the source distance is in within 1.2 and 1.4 kpc, closer than what was derived in previous studies (1.7 kpc).
Abstract: 1911.12455
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Full Text: [ PostScript, PDF]
Title:Structural analysis of massive galaxies using HST deep imaging at z < 0.5
(Submitted on 27 Nov 2019)
Abstract: Taking advantage of HST CANDELS data, we analyze the lowest redshift (z<0.5) massive galaxies in order to disentangle their structural constituents and study possible faint non-axis-symmetric features. Due to the excellent HST spatial resolution for intermediate-z objects, they are hard to model by purely automatic parametric fitting algorithms. We performed careful single and double Sérsic fits to their galaxy surface brightness profiles. We also compare the model color profiles with the observed ones and also derive multi-component global effective radii attempting to obtain a better interpretation of the mass-size relation. Additionally, we test the robustness of our measured structural parameters via simulations. We find that the Sérsic index does not offer a good proxy for the visual morphological type for our sample of massive galaxies. Our derived multi-component effective radii give a better description of the size of our sample galaxies than those inferred from single Sérsic models with GALFIT. Our galaxy population lays on the scatter of the local mass-size relation, indicating that these massive galaxies do not experience a significant growth in size since z~0.5. Interestingly the few outliers are late-type galaxies, indicating that spheroids must reach the local mass-size relation earlier. For most of our sample galaxies, both single and multi-component Sérsic models with GALFIT show substantial systematic deviations from the observed SBPs in the outskirts. These residuals may be partly due to several factors, namely a non-optimal data reduction for low surface brightness features, the existence of prominent stellar haloes for massive galaxies and could also arise from conceptual shortcomings of parametric 2D image decomposition tools. They consequently propagate into galaxy color profiles.
Abstract: 1911.12577
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Full Text: [ PostScript, PDF]
Title:Hunting ghosts: the iconic stellar stream(s) around NG5907 under scrutiny
(Submitted on 28 Nov 2019)
Abstract: Stellar streams are regarded as crucial objects to test galaxy formation models, with their morphology tracing the underlying potentials and their occurrence tracking the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC5907, has recently been questioned by new observations with the Dragonfly telescope. This new work only finds parts of the stream, even though they reach a 1 sigma surface brightness limit of 30.3 mag per sq. arcsec in the g-band. Using 7.2 hours of Luminance L-band imaging with the Milanković 1.4 meter telescope, we have re-observed the putative double loop part to confirm or reject this assessment. We do not find signs of the double loop, but see only a single, knee-shaped stellar stream. Comparing our to the data by the Dragonfly team, we find the same features. Our observations reach a 1 sigma surface brightness limit of 29.7 mag per sq. arcsec in the g-band. These findings emphasize the need for independent confirmation of detections of very low-surface brightness features.
Abstract: 1911.12828
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Full Text: [ PostScript, PDF]
Title:Discovery of a Powerful >10^61 erg AGN Outburst in Distant Galaxy Cluster SPT-CLJ0528-5300
(Submitted on 28 Nov 2019)
Abstract: We present ~103 ks of Chandra observations of the galaxy cluster SPT-CLJ0528-5300 (SPT0528, z=0.768). This cluster harbors the most radio-loud (L_1.4GHz = 1.01 x 10^33 erg/s/Hz) central AGN of any cluster in the South Pole Telescope (SPT) SZ survey with available X-ray data. We find evidence of AGN-inflated cavities in the X-ray emission, which are consistent with the orientation of the jet direction revealed by ATCA radio data. The combined probability that two such depressions -- each at ~1.4-1.8sigma significance, oriented ~180 degrees apart and aligned with the jet axis -- would occur by chance is 0.1%. At >10^61 erg, the outburst in SPT0528 is among the most energetic known in the universe, and certainly the most powerful known at z>0.25. This work demonstrates that such powerful outbursts can be detected even in shallow X-ray exposures out to relatively high redshifts (z~0.8), providing an avenue for studying the evolution of extreme AGN feedback. The ratio of the cavity power (P_cav = 9.4+/-5.8 x 10^45 erg/s) to the cooling luminosity (L_cool = 1.5+/-0.5 x 10^44 erg/s) for SPT0528 is among the highest measured to date. If, in the future, additional systems are discovered at similar redshifts with equally high P_cav/L_cool ratios, it would imply that the feedback/cooling cycle was not as gentle at high redshifts as in the low-redshift universe.
Abstract: 1911.12833
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Full Text: [ PostScript, PDF]
Title:Emission from a Pulsar Wind Nebula: Application to the Persistent Radio Counterpart of FRB 121102
(Submitted on 28 Nov 2019)
Abstract: The properties of fast radio bursts (FRBs) indicate that the physical origin of this type of astrophysical phenomenon is related to neutron stars. The first detected repeating source, FRB 121102, is associated with a persistent radio counterpart. In this paper, we propose that this radio counterpart could arise from a pulsar wind nebula powered by a magnetar without surrounding supernova ejecta. Its medium is a stratified structure produced by a progenitor wind. The model parameters are constrained by the spectrum of the counterpart emission, the size of the nebula, and the large but decreasing rotation measure (RM) of the repeating bursts. In addition, the observed dispersion measure is consistent with the assumption that all of the RM comes from the shocked medium.
Abstract: 1911.13293
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Full Text: [ PostScript, PDF]
Title:Relativistic capture of dark matter by electrons in neutron stars
(Submitted on 29 Nov 2019)
Abstract: Dark matter can capture in neutron stars and heat them to observable luminosities. We study relativistic scattering of dark matter on highly degenerate electrons. We develop a Lorentz invariant formalism to calculate the capture probability of dark matter that accounts for the relativistic motion of the target particles and Pauli exclusion principle. We find that the actual capture probability can be five orders of magnitude larger than the one estimated using a nonrelativistic approach. For dark matter masses $10~{\rm eV}\textup{--}10~{\rm PeV}$, neutron star heating complements and can be more sensitive than terrestrial direct detection searches. Our results show that old neutron stars could be the most promising target for discovering leptophilic dark matter.
Abstract: 1912.00026
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Full Text: [ PostScript, PDF]
Title:Synchrotron spectra of GRB prompt emission and pulsar wind nebulae
(Submitted on 29 Nov 2019)
Abstract: Particle acceleration is a fundamental process in many high-energy astrophysical environments and determines the spectral features of their synchrotron emission. We have studied the adiabatic stochastic acceleration (ASA) of electrons arising from the basic dynamics of magnetohydrodynamic (MHD) turbulence and found that the ASA acts to efficiently harden the injected electron energy spectrum. The dominance of the ASA at low energies and the dominance of synchrotron cooling at high energies result in a broken power-law shape of both electron energy spectrum and photon synchrotron spectrum. Furthermore, we have applied the ASA to studying the synchrotron spectra of the prompt emission of gamma-ray bursts (GRBs) and pulsar wind nebulae (PWNe). The good agreement between our theories and observations confirms that the stochastic particle acceleration is indispensable in explaining their synchrotron emission.
Abstract: 1912.00084
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Full Text: [ PostScript, PDF]
Title:Stellar population properties of individual massive early-type galaxies at 1.4 < z < 2
(Submitted on 29 Nov 2019)
Abstract: We analyse publicly available, individual spectra of four, massive ($M>10^{11}M_{\odot}$) early-type galaxies with redshifts in the range 1.4 < z < 2 to determine their stellar content, extending our previous work up to z~2. The wide wavelength range of the VLT/X-Shooter spectroscopic data in the UV-Optical-NIR arms along with the availability of spectro-photometry allows us to explore different techniques to obtain the stellar population properties, namely through age/metallicity sensitive spectral indices, full spectral fitting and broad-band photometric fitting. Moreover, together with the widely used optical Lick indices we consider further indices in the UV rest-frame, and demonstrate that UV indices significantly help the accuracy of the resulting population parameters.
We find galaxy ages ranging from 0.2 to 4 Gyr, where the oldest galaxy is found at the lowest redshift, with an excellent agreement between ages determined via indices, full spectral fitting or broad-band colours. These ages are in perfect agreement with ages of local galaxies at the same velocity dispersion when we assume pure passive evolution. Total metallicities derived from indices show some scatter (between less than half-solar to very high values, ([Z/H]~0.6]). We speculate on possible mechanisms explaining these values, but given the sample size and low S/N of the spectra no conclusion can be made.
Indices in the UV-rest frame generally lead to similar conclusions as optical indices. For the oldest galaxy (4 Gyr) we show that its UV-indices can only be explained by stellar population models including a UV contribution from old stellar populations, suggesting that old, UV bright populations start to inhabit mature galaxies of a few Gyr of age. This is the highest redshift (z~1.4) detection of the UV-upturn up to date.
Abstract: 1912.00179
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Full Text: [ PostScript, PDF]
Title:Search for Fast Radio Bursts in the Direction of the Galaxies M31 and M33
(Submitted on 30 Nov 2019)
Abstract: The results of a search for individual fast radio bursts with the Large Phased Array of the Lebedev Physical Institute at 111 MHz during July 2012 through August 2018 are presented. The signals were distinguished by convolving the data with a template with a fixed form, followed by convolution with test dispersion measures. Areas of sky containing the galaxies M31 and M33 were chosen for the search. Three radio bursts were detected in the vicinity of M33, five in the vicinity of M31, and one in a region offset from the center of M31 by an hour in right ascension. The dispersion measures of the detected bursts range from 203 to 1262 $ pc \cdot cm^{-3}$.
Abstract: 1912.00526
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Full Text: [ PostScript, PDF]
Title:Fast Radio Bursts Not Made By Neutron Stars
(Submitted on 2 Dec 2019)
Abstract: Popular models of repeating Fast Radio Bursts (and perhaps of all Fast Radio Bursts) involve neutron stars because they may have high rotational or magnetostatic energy densities available to power energetic bursts. These models take two forms: giant but rare pulsar-like pulses like those of Rotating RAdio Transients and outbursts like those of Soft Gamma Repeaters. Here I collate the evidence, recently strengthened, against these models.
Abstract: 1912.00625
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Full Text: [ PostScript, PDF]
Title:Identifying nearby sources of ultra-high-energy cosmic rays with deep learning
(Submitted on 2 Dec 2019)
Abstract: We present a method to analyse arrival directions of ultra-high-energy cosmic rays (UHECRs) using a classifier defined by a deep convolutional neural network trained on a HEALPix grid. To illustrate the efficacy of the method, we employ it to estimate prospects of detecting a large-scale anisotropy of UHECRs induced by a nearby source with an (orbital) detector having a uniform exposure of the celestial sphere and compare the results with our earlier calculations based on the angular power spectrum. A minimal model for extragalactic cosmic rays and neutrinos by Kachelrieß, Kalashev, Ostapchenko and Semikoz (2017) is assumed for definiteness and nearby active galactic nuclei Centaurus A, M82, NGC253, M87 and Fornax A are considered as possible sources of UHECRs. We demonstrate that the proposed method drastically improves sensitivity of an experiment by decreasing the minimal required amount of detected UHECRs or the minimal detectable fraction of from-source events several times compared to the approach based on the angular power spectrum. The method can be readily applied to the analysis of data of the Telescope Array, the Pierre Auger Observatory and other cosmic ray experiments.
Abstract: 1912.01258
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Full Text: [ PostScript, PDF]
Title:Multi-messenger hunts for heavy WIMPs
(Submitted on 3 Dec 2019)
Abstract: Heavy neutrinos have a long history of consideration in the literature, in particular related to their role as solutions to the problems of neutrino mass, baryon asymmetry, and possibly dark matter. Interestingly, recent developments in the Madala hypothesis, a standard model extension designed to explain persistent LHC lepton anomalies, may also necessitate a heavy neutrino. This prospect is exciting as a dark matter model consisting of a TeV-scale leptophilic fermionic particle is also invoked to explain the electron-positron excess observed by the DAMPE experiment. The tantalising similarities between these new fermions may allow indirect dark matter detection methods to probe empirically compelling standard model extensions, like the Madala hypothesis. However, the leptophilic nature and large mass mean the expected gamma-ray signatures of annihilation or decay are weaker than those in the traditionally considered heavy quark and tau lepton channels. In this work we explore whether the KM3NeT neutrino detector could take advantage of the leptophilic nature of the added particle to provide an alternative means of exploring such interesting connections between cosmology and collider physics. We demonstrate that dwarf galaxies, in particular highly dense ultra-faint dwarf galaxies like Triangulum II, provide very strong prospects for KM3NeT searches.
Abstract: 1912.01653
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Full Text: [ PostScript, PDF]
Title:Primordial black holes and gravitational waves in non-standard cosmologies
(Submitted on 3 Dec 2019)
Abstract: For primordial black holes (PBH) to form a considerable fraction of cold dark matter, the required amplitude of primordial scalar perturbations is quite large ($P_{\zeta}(k) \sim 10^{-2}$) if PBH is formed in radiation epoch. In alternate cosmological histories, where additional epochs of arbitrary equation of state precede radiation epoch, the dynamics of PBH formation and relevant mass ranges can be different leading to requirement of lower primordial power at smaller scales of inflation. Moreover, this alternate history can modify the predictions for the gravitational wave (GW) spectrum, which can be probed by upcoming GW observations. In this paper we show that an early kination epoch can lead to percent level abundance of PBH for a lower amplitude of $P_{\zeta}(k)$ as compared to PBH formation in a standard radiation epoch. Moreover, we calculate the effect of early kination epoch on the GW spectrum for first and second orders in perturbation which show enhancement in the amplitude of the GW spectrum in a kination epoch with respect to that in a standard radiation epoch.
Abstract: 1912.01783
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Full Text: [ PostScript, PDF]
Title:Searching for a standard Drake equation
(Submitted on 2 Dec 2019)
Abstract: In the 20th century the scientific search for extraterrestrial intelligence began, and the Drake equation was proposed to estimate the number of extraterrestrial species humanity could attempt to detect, N. This paper discusses the need to update and standardize this equation. A new and more accurate expression is deduced which contains the classic equation as a particular case, and its advantages are discussed. A necessary condition is also stated for its use in scientific contexts: if N is defined as the total number of civilizations like ours then N = 1, given that we exist, and consequently the working hypothesis of the SETI project can be expressed as N>1. In this case, the Drake equation is being applied in a scientific way, because it is trying to test a hypothesis based on evidence.
Abstract: 1912.01821
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Full Text: [ PostScript, PDF]
Title:Cool Jupiters greatly outnumber their toasty siblings: Occurrence rates from the Anglo-Australian Planet Search
(Submitted on 4 Dec 2019)
Abstract: Our understanding of planetary systems different to our own has grown dramatically in the past 30 years. However, our efforts to ascertain the degree to which the Solar system is abnormal or unique have been hindered by the observational biases inherent to the methods that have yielded the greatest exoplanet hauls. On the basis of such surveys, one might consider our planetary system highly unusual - but the reality is that we are only now beginning to uncover the true picture. In this work, we use the full eighteen-year archive of data from the Anglo-Australian Planet Search to examine the abundance of 'Cool Jupiters' - analogs to the Solar system's giant planets, Jupiter and Saturn. We find that such planets are intrinsically far more common through the cosmos than their siblings, the hot Jupiters. We find that the occurrence rate of such 'Cool Jupiters' is $6.73^{+2.09}_{-1.13}$\%, almost an order of magnitude higher than the occurrence of hot Jupiters (at $0.84^{+0.70}_{-0.20}$\%). We also find that the occurrence rate of giant planets is essentially constant beyond orbital distances of $\sim$1\,au. Our results reinforce the importance of legacy radial velocity surveys for the understanding of the Solar system's place in the cosmos.
Abstract: 1912.01880
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Full Text: [ PostScript, PDF]
Title:Unraveling The Complex Nature Of The Very High-Energy $γ$-Ray Blazar PKS 2155-304
(Submitted on 4 Dec 2019)
Abstract: PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, E>100 GeV) $\gamma$ rays every year since 2002. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet (UV), X-ray and high energy $\gamma$-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling. In this paper, we discuss the complex spectral and temporal variability observed in PKS 2155-304. The data discussed include VHE $\gamma$-ray data collected with H.E.S.S. between 2013 and 2016, complemented with multiwavelength (MWL) observations from Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, and the ATOM telescope. During the period of monitoring, PKS 2155-304 was transitioning from its lower state to the flaring states, and exhibiting different flavors of outbursts. For the first time, orphan optical flare lasting a few months was observed. Correlation studies show an indication of correlation between the X-ray and VHE $\gamma$-ray fluxes. Interestingly, a comparison of optical and X-ray or VHE $\gamma$-ray fluxes does not show global correlation. However, two distinct tracks in the diagram were found, which correspond to the different flaring activity states of PKS 2155-304.
Abstract: 1912.01895
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Full Text: [ PostScript, PDF]
Title:Where Did They Come From, Where Did They Go. Grazing Fireballs
(Submitted on 4 Dec 2019 (v1), last revised 5 Dec 2019 (this version, v2))
Abstract: For centuries extremely-long grazing fireball displays have fascinated observers and inspired people to ponder about their origins. The Desert Fireball Network (DFN) is the largest single fireball network in the world, covering about one third of Australian skies. This expansive size has enabled us to capture a majority of the atmospheric trajectory of a spectacular grazing event that lasted over90 seconds, penetrated as deep as ~58.5km, and traveled over 1,300 km through the atmosphere before exiting back into interplanetary space. Based on our triangulation and dynamic analyses of the event, we have estimated the initial mass to be at least 60 kg, which would correspond to a30 cm object given a chondritic density (3500 kg m-3). However, this initial mass estimate is likely a lower bound, considering the minimal deceleration observed in the luminous phase. The most intriguing quality of this close encounter is that the meteoroid originated from an Apollo-type orbit and was inserted into a Jupiter-family comet (JFC) orbit due to the net energy gained during the close encounter with the Earth. Based on numerical simulations, the meteoroid will likely spend ~200kyrs on a JFC orbit and have numerous encounters with Jupiter, the first of which will occur in January-March 2025. Eventually the meteoroid will likely be ejected from the Solar System or be flung into a trans-Neptunian orbit.
Abstract: 1912.01897
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Full Text: [ PostScript, PDF]
Title:The Large Amplitude X-ray Variability in NGC 7589: Possible Evidence for Accretion Mode Transition
(Submitted on 4 Dec 2019)
Abstract: We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/XRT and XMM-Newton over 17 years. Results of long-term monitoring data in the UV, optical and infrared bands over ~20 years are also presented. The Eddington ratio increased from $10^{-3}$ to $\sim0.13$, suggesting a transition of the accretion flow from an ADAF to a standard thin accretion disc. Further evidence supporting the thin disc in the high luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component ($\sim19^{+15}_{-7}$eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of $\sim10^{7}\,M_{\mathrm{Sun}}$. These results may indicate that NGC 7589 had experienced accretion mode transition over a timescale of a few years, suggesting the idea that similar accretion processes are at work for massive black hole and black hole X-ray binaries.
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