Abstracts of Interest
Selected by:
Adnaan Thakur
Abstract: 2408.04699
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Title:Artemis-enabled Stellar Imager (AeSI): A Lunar Long-Baseline UV/Optical Imaging Interferometer
View PDFAbstract:NASA's return to the Moon presents unparalleled opportunities to advance high-impact scientific capabilities. At the cutting edge of these possibilities are extremely high-resolution interferometric observations at visible and ultraviolet wavelengths. Such technology can resolve the surfaces of stars, explore the inner accretion disks of nascent stars and black holes, and eventually enable us to observe surface features and weather patterns on nearby exoplanets. We have been awarded Phase 1 support from NASA's Innovative Advanced Concepts (NIAC) program to explore the feasibility of constructing a high-resolution, long-baseline UV/optical imaging interferometer on the lunar surface, in conjunction with the Artemis Program. A 1996 study comparing interferometers on the Moon versus free-flyers in space concluded that, without pre-existing lunar infrastructure, free-flyers were preferable. However, with the advent of the Artemis Program, it is now crucial to revisit the potential of building lunar interferometers. Our objective is to conduct a study with the same level of rigor applied to large baseline, free-flying interferometers during the 2003-2005 NASA Vision Missions Studies. This preparation is essential for timely and effective utilization of the forthcoming lunar infrastructure. In this paper, we highlight the groundbreaking potential of a lunar surface-based interferometer. This concept study will be a huge step forward to larger arrays on both the moon and free-flying in space, over a wide variety of wavelengths and science topics. Our Phase 1 study began in April 2024, and here we present a concise overview of our vision and the progress made so far.
Abstract: 2408.04064
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Title:Are long gamma-ray bursts progenitors to merging binary black holes?
View PDF HTML (experimental)Abstract:The distribution of delay times between the formation of binary black hole (BBH) progenitors and their gravitational-wave (GW) merger provides important clues about their unknown formation histories. When inferring the delay time distribution, it is typically assumed that BBH progenitor formation traces the star formation rate (SFR). In this work, we consider the rate of long gamma-ray bursts (LGRBs) instead of the SFR. LGRBs are thought to correspond to the formation of (possibly spinning) black holes, and may therefore be related to the population of BBH progenitors. By comparing the redshift evolution of the LGRB rate as inferred by Ghirlanda & Salvaterra (2022) and the BBH merger rate inferred by LIGO-Virgo-KAGRA (LVK) observations, we find that the delay time distribution between LGRBs and BBH mergers is well-described by a power law with minimum delay time $10$ Myr and slope $\alpha ={-0.96}^{+0.64}_{-0.85}$ (90\% credibility). This matches theoretical predictions for the BBH delay time distribution, which in turns lends support to the hypothesis that LGRBs trace BBH progenitor formation. However, comparing the absolute rates of these two populations, we find that at most $f = {0.04}^{+0.18}_{-0.03}$ of LGRBs may evolve into merging BBH. We also consider the possibility that LGRBs only produce BBH systems with large aligned spins (with effective inspiral spin $\chi_\mathrm{eff} > 0.2$). In this case, we find $f = {0.003}^{+0.011}_{-0.002}$ and the delay time distribution favors the steepest power law slopes we consider ($\alpha = -2$).
Abstract: 2408.03982
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Title:JWST/NIRSpec WIDE survey: a z=4.6 low-mass star-forming galaxy hosting a jet-driven shock with low ionisation and solar metallicity
View PDFAbstract:We present NIRSpec/MSA observations from the JWST large-area survey WIDE, targeting the rest-frame UV-optical spectrum of Ulema, a radio-AGN host at redshift z=4.6348. The low-resolution prism spectrum displays high equivalent width nebular emission, with remarkably high ratios of low-ionisation species of oxygen, nitrogen and sulphur, relative to hydrogen; auroral O$^+$ emission is clearly detected, possibly also C$^+$. From the high-resolution grating spectrum, we measure a gas velocity dispersion $\sigma$~400 km s$^{-1}$, broad enough to rule out star-forming gas in equilibrium in the gravitational potential of the galaxy. Emission-line ratio diagnostics suggest that the nebular emission is due to a shock which ran out of pre-shock gas. To infer the physical properties of the system, we model simultaneously the galaxy spectral energy distribution (SED) and shock-driven line emission under a Bayesian framework. We find a relatively low-mass, star-forming system (M* = 1.4$\times$10^{10} M$_\odot$, SFR = 70 M$_\odot$ yr$^{-1}$), where shock-driven emission contributes 50 per cent to the total H$\beta$ luminosity. The nebular metallicity is near solar - three times higher than that predicted by the mass-metallicity relation at z=4.6, possibly related to fast-paced chemical evolution near the galaxy nucleus. We find no evidence for a recent decline in the SFR of the galaxy, meaning that, already at this early epoch, fast radio-mode AGN feedback was poorly coupled with the bulk of the star-forming gas; therefore, most of the feedback energy must end up in the galaxy halo, setting the stage for future quenching.
Abstract: 2408.03916
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Title:Building spectral templates and reconstructing parameters for core collapse supernovae with CASTOR
View PDF HTML (experimental)Abstract:The future of time-domain optical astronomy relies on the development of techniques and software capable of handling a rising amount of data and gradually complementing, or replacing if necessary, real observations. Next generation surveys, like the Large Synoptic Survey Telescope (LSST), will open the door to the new era of optical astrophysics, creating, at the same time, a deficiency in spectroscopic data necessary to confirm the nature of each event and to fully recover the parametric space. In this framework, we developed Core collApse Supernovae parameTers estimatOR (CASTOR), a novel software for data analysis. CASTOR combines Gaussian Process and other Machine Learning techniques to build time-series templates of synthetic spectra and to estimate parameters of core collapse supernovae for which only multi-band photometry is available. Techniques to build templates are fully data driven and non-parametric through empirical and robust models, and rely on the direct comparison with a training set of 111 core collapse supernovae from the literature. Furthermore, CASTOR employees the real photometric data and the reconstructed synthetic spectra of an event to estimate parameters that belong to the supernova ejecta, to the stellar progenitor and to the event itself, in a rapid and user-friendly framework. In this work we provide a demonstration of how CASTOR works, studying available data from SN2015ap and comparing our results with those available in literature.
Abstract: 2408.03638
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Title:Superdiffusion of energetic particles at shocks: A Lévy Flight model for acceleration
View PDF HTML (experimental)Abstract:In the Heliosphere, power-law particle distributions are observed e.g. upstream of interplanetary shocks, which can result from superdiffusive transport. This non-Gaussian transport regime may result from intermittent magnetic field structures. Recently, we showed that a Lévy flight model reproduces the observed features at shocks: power-law distributions upstream and enhanced intensities at the shock. We extend the Lévy flight model to study the impact of superdiffusive transport on particle acceleration at shocks. The acceleration time scale and spectral slope are compared to Gaussian diffusion and a Lévy walk model. The fractional transport equation is solved by sampling the number density with the corresponding stochastic differential equation that is driven by an alpha-stable Lévy distribution. For both Gaussian and superdiffusive transport we use a modified version of CRPropa 3.2. We obtain the number density and energy spectra for constant and energy-dependent anomalous diffusion and find, compared to the case of Gaussian diffusion, harder energy spectra at the shock as well as faster acceleration. The spectral slope is even harder than predicted for Lévy walks. Lévy flight models of superdiffusive transport lead to observed features in the Heliosphere. We further show that superdiffusive transport impacts the acceleration process by changing the probability to escape the shock. The flexibility of the Lévy flight model allows for further studies in the future, taking the shock geometry and magnetic field structure into account.
Abstract: 2408.03313
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Title:Ninety percent circular polarization detected in a repeating fast radio burst
View PDF HTML (experimental)Abstract:Fast radio bursts (FRBs) are extra-galactic sources with unknown physical mechanisms. They emit millisecond-duration radio pulses with isotropic equivalent energy of $10^{36}\sim10^{41}$ ergs. This corresponds to a brightness temperature of FRB emission typically reaching the level of $10^{36}$ K, but can be as high as above $10^{40}$ K for sub-microsecond timescale structures, suggesting the presence of underlying coherent relativistic radiation mechanisms. polarization carries the key information to understand the physical origin of FRBs, with linear polarization usually tracing the geometric configuration of magnetic fields and circular polarization probing both intrinsic radiation mechanisms and propagation effects. Here we show that the repeating sources FRB 20201124A emits $90.9\pm 1.1\%$ circularly polarized radio pulses. Such a high degree of circular polarization was unexpected in theory and unprecedented in observation in the case of FRBs, since such a high degree of circular polarization was only common among Solar or Jovian radio activities, attributed to the sub-relativistic electrons. We note that there is no obvious correlation between the degree of circular polarization and burst fluence. Besides the high degree of circular polarization, we also detected rapid swing and orthogonal jump in the position angle of linear polarization. The detection of the high degree circular polarization in FRB 20201124A, together with its linear polarization properties that show orthogonal modes, place strong constraints on FRB physical mechanisms, calling for an interplay between magnetospheric radiation and propagation effects in shaping the observed FRB radiation.
Abstract: 2408.03306
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Title:A search for soft X-ray emission lines in the afterglow spectrum of GRB 221009A
View PDF HTML (experimental)Abstract:GRB 221009A was the Brightest gamma-ray burst Of All Time (BOAT), surpassing in prompt brightness all GRBs discovered in ~50 yr and in afterglow brightness in ~20 yr. We observed the BOAT with XMM-Newton 2.3 d after the prompt. The X-ray afterglow was still very bright and we collected the largest number of photons with the Reflection Grating Spectrometers (RGS) on a GRB. We searched the RGS data for narrow emission or absorption features. We did not detect any bright line feature. A candidate narrow feature is identified at a (rest-frame) energy of 1.455+0.006-0.014 keV, consistent with an Mg XII K{\alpha} emission line, slightly redshifted (0.012) with respect to the host galaxy. We assessed a marginal statistical significance of 3.0sigma for this faint feature based on conservative Monte Carlo simulations, which requires caution for any physical interpretation. If this line feature would be for real, we propose that it might originate from the reflection in the innermost regions of the infalling funnel from low-level late-time activity emission of the central engine.
Abstract: 2408.03048
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Title:Exploring the range of impacts of helium in the spectra of double detonation models for Type Ia supernovae
View PDF HTML (experimental)Abstract:In the double detonation scenario the ignition of a surface He detonation on a sub-Chandrasekhar mass white dwarf leads to a secondary core detonation. Double detonation models have shown promise for explaining Type Ia supernovae (SNe Ia) with a variety of luminosities. A key feature of such models is unburnt He in the ejecta, which can show significant variation in both its mass and velocity distribution. Many previous radiative transfer simulations for double detonation models have neglected treatment of non-thermal ionization and excitation, preventing them from robustly assessing whether He spectral features are expected to form. We present a non local thermodynamic equilibrium radiative transfer simulation, including treatment for non-thermal electrons, for a double detonation model with a modest mass of He (${\sim}$0.04 M${\odot}$) ejected at reasonably low velocities (${\sim}$12000$\,\mathrm{km}\,\mathrm{s}^{-1}$). Despite our simulation predicting no clear optical He features, a strong and persistent He I 10830$\,Å$ absorption feature forms that is significantly blended with the spectral contribution of Mg II 10927$\,Å$. For some normal SNe Ia the Mg feature shows an extended blue wing, previously attributed to C I, however the simulated He feature shows its strongest absorption at wavelengths consistent with this wing. We therefore suggest this extended wing is instead a spectral signature of He. The He feature predicted by this particular model is too strong and persistent to be consistent with normal SNe Ia, however, this motivates further work to use this observable signature to test the parameter space for double detonation models.
Abstract: 2408.02534
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Title:Quantifying the coincidence between gravitational waves and fast radio bursts from neutron star--black hole mergers
View PDF HTML (experimental)Abstract:Fast radio bursts (FRBs) are mysterious astrophysical transients whose origin and mechanism remain unclear. Compact object mergers may be a promising channel to produce some FRBs. Neutron star-black hole (NSBH) mergers could produce FRBs through mechanisms involving neutron star tidal disruption or magnetospheric disturbances. This could present an opportunity for multi-messenger gravitational-wave observations, providing new insight into the nature of FRBs and nuclear matter. However, some of the gravitational-wave signals may be marginal detections with signal-to-noise ratios < 8 or have large sky location and distance uncertainties, making it less straightforward to confidently associate an FRB with the gravitational-wave signal. One must therefore take care to avoid a false positive association. We demonstrate how to do this with simulated data. We calculate the posterior odds -- a measurement of our relative belief for a common versus unrelated origin of a coincident NSBH and FRB. We find that a coincident FRB+NSBH from a common source can yield a statistically significant posterior odds in a network with at least two observatories, but only if we require a coincidence in time and and sky location, rather than time alone. However, we find that for our model, we require a network signal-to-noise ratio greater than 10 to be confident in the common-source detection, when using a threshold of ln odds > 8. We suggest that a coincident NSBH+FRB detection could help distinguish between FRB engines by discriminating between disrupting and non-disrupting models.
Abstract: 2407.07244
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Title:Characterisation of Supernovae Interacting with Dense Circumstellar Matter with a Flat Density Profile
View PDF HTML (experimental)Abstract:Interaction between supernova (SN) ejecta and dense circumstellar medium (CSM) with a flat density structure ($\rho \propto r^{-s}, s < 1.5$) was recently proposed as a possible mechanism behind interacting SNe that exhibit exceptionally long rise times exceeding 100 days. In such a configuration, the interaction luminosity keeps rising until the reverse shock propagates into the inner layers of the SN ejecta. We investigate the light curves of SNe interacting with a flatly distributed CSM in detail, incorporating the effects of photon diffusion inside the CSM into the model. We show that three physical processes - the shock breakout, the propagation of the reverse shock into the inner ejecta, and the departure of the shock from the dense CSM - predominantly determine the qualitative behaviour of the light curves. Based on the presence and precedence of these processes, the light curves of SNe interacting with flatly distributed CSM can be classified into five distinct morphological classes. We also show that our model can qualitatively reproduce doubly peaked SNe whose peaks are a few tens of days apart, such as SN 2005bf and SN 2022xxf. Our results show that the density distribution of the CSM is an important property of CSM that contributes to the diversity in light curves of interacting SNe.
Abstract: 2406.14366
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Title:GRB 211211A: The Case for an Engine Powered over r-Process Powered Blue Kilonova
View PDF HTML (experimental)Abstract:The recent Gamma-Ray Burst (GRB) GRB 211211A provides the earliest ($\sim 5$ h) data of a kilonova (KN) event, displaying bright ($\sim10^{42}$ erg s$^{-1}$) and blue early emission. Previously, this KN has been explained using simplistic multi-component fitting methods. Here, in order to understand the physical origin of the KN emission in GRB 211211A, we employ an analytic multi-zone model for r-process powered KN. We find that r-process powered KN models alone cannot explain the fast temporal evolution and the spectral energy distribution (SED) of the observed emission. Specifically, i) r-process models require high ejecta mass to match early luminosity, which overpredicts late-time emission, while ii) red KN models that reproduce late emission underpredict early luminosity. We propose an alternative scenario involving early contributions from the GRB central engine via a late low-power jet, consistent with plateau emission in short GRBs and GeV emission detected by Fermi-LAT at $\sim10^4$ s after GRB 211211A. Such late central engine activity, with an energy budget of $\sim \text{a few }\%$ of that of the prompt jet, combined with a single red-KN ejecta component, can naturally explain the light curve and SED of the observed emission; with the late-jet -- ejecta interaction reproducing the early blue emission and r-process heating reproducing the late red emission. This supports claims that late low-power engine activity after prompt emission may be common. We encourage very early follow-up observations of future nearby GRBs, and compact binary merger events, to reveal more about the central engine of GRBs and r-process events.
Abstract: 2406.05297
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Title:X-ray Afterglow limits on the viewing angles of short gamma-ray bursts
View PDF HTML (experimental)Abstract:The observed behavior of a short gamma-ray burst (sGRB) afterglow lightcurve can reveal the angular structure of the relativistic jet and constrain the observer's viewing angle $\theta_\textrm{obs}$. Regardless of viewing angle, the afterglow emission is produced by the interaction between the relativistic jet and its surrounding environment. However, the observed deceleration time of the jet, and, therefore, the time of the afterglow peak, depends on the observer's viewing angle. A larger viewing angle leads to a later peak of the afterglow and a lower flux at peak. We use the the earliest X-ray afterglow detections of 58 cosmological sGRBs detected with the Neil Gehrels Swift Observatory X-ray Telescope to set an upper limit on the ratio of the viewing angle $\theta_\textrm{obs}$ to the jet's half-opening angle $\theta_\textrm{c}$. We adopt a power-law angular jet structure in both energy $E(\theta)\propto\theta^{-a}$ and Lorentz factor $\Gamma(\theta)\propto\theta^{-b}$ beyond the core. For this structured jet scenario we find that either sGRBs are viewed within $\theta_\textrm{obs}/\theta_\textrm{c}<1$ or the initial Lorentz factor of material in their jet's core is extremely high ($\Gamma_0>500$). If we consider a tophat jet structure, we constrain 90% of our sample to be viewed within $\theta_\textrm{obs}/\theta_\textrm{c}<1.06$ and 1.15 for our canonical and conservative afterglow scenarios. For a subset of events with measurements of the jet break, under the assumption that they on-axis we can constrain $\Gamma_0\theta_\textrm{c}\gtrsim 30$. This confirmation that cosmological sGRBs are viewed either on-axis or very close to their jet's core has significant implications for the nature of the prompt gamma-ray production mechanism and for the rate of future sGRB detections coincident with gravitational waves (GWs), implying that they are extremely rare.
Abstract: 2401.03750
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Title:Bayesian inference of multimessenger astrophysical data: Joint and coherent inference of gravitational waves and kilonovae
View PDF HTML (experimental)Abstract:We present a Bayesian framework for joint and coherent analyses of multimessenger binary neutron star signals. The method, implemented in our bajes infrastructure, incorporates a joint likelihood for multiple datasets, support for various semi-analytical kilonova models and numerical-relativity (NR) informed relations for the mass ejecta, as well as a technique to include and marginalize over modeling uncertainties. As a first application, we analyze the gravitational-wave GW170817 and the kilonova AT2017gfo data. These results are then combined with the most recent X-ray pulsars analyses of PSR J0030+0451 and PSR J0740+6620 to obtain EOS constraints.Various constraints on the mass-radius diagram and neutron star properties are then obtained by resampling over a set of ten million parametrized EOS built under minimal assumptions. We find that a joint and coherent approach improves the inference of the extrinsic parameters (distance) and, among the instrinc parameters, the mass ratio. The inclusion of NR informed relations strongly improves over the case of using an agnostic prior on the intrinsic parameters. Comparing Bayes factors, we find that the two observations are better explained by the common source hypothesis only by assuming NR-informed relations. These relations break some of the degeneracies in the employed kN models. The EOS inference folding-in PSR J0952-0607 minimum-maximum mass, PSR J0030+0451 and PSR J0740+6620 data constrains, among other quantities, the neutron star radius to $R_{1.4}={12.30}^{+0.81}_{-0.56}$ km ($R_{1.4}={13.20}^{+0.91}_{-0.90}$ km) and the maximum mass to $M_{max}={2.28}^{+0.25}_{-0.17}~{\rm M_\odot}$ ($M_{max}={2.32}^{+0.30}_{-0.19}~{\rm M_\odot}$) where the ST+PDT (PDT-U) analysis of Vinciguerra et a (2023) for PSR J0030+0451 is employed. Hence, the systematics on PSR J0030+0451 data reduction currently dominate the mass-radius diagram constraints.
Abstract: 2405.15657
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Title:Multiple Emission Regions in Jets of Low Luminosity Active Galactic Nucleus in NGC 4278
View PDF HTML (experimental)Abstract:The Large High Altitude Air Shower Observatory (LHAASO) has detected very high energy gamma rays from the LINER galaxy NGC 4278, which has a low luminosity active galactic nucleus, and symmetric mildly relativistic S-shaped twin jets detected by radio observations. Few low-luminosity active galactic nuclei are detected in gamma rays due to their faintness. Earlier, several radio-emitting components were detected in the jets of NGC 4278. We model their radio emission with synchrotron emission of ultra-relativistic electrons to estimate the strength of the magnetic field inside these components within a time-dependent framework after including the ages of the different components. We show that the synchrotron and synchrotron self-Compton emission by these components cannot explain the Swift X-ray data and the LHAASO gamma-ray data from NGC 4278. We suggest that a separate component in one of the jets is responsible for the high energy emission whose age, size, magnetic field and the spectrum of the ultra-relativistic electrons inside it have been estimated after fitting the multi-wavelength data of NGC 4278 with the sum of the spectral energy distributions from the radio components and the high energy component. We note that the radio components of NGC 4278 are larger than the high-energy component which has also been observed in several high-luminosity active galactic nuclei.
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