View a PDF of the paper titled Diprotodon on the sky. The Large Galactic Supernova Remnant (SNR) G278.94+1.35, by Miroslav D. Filipovi\'c and 43 other authors
Abstract:We present a re-discovery of G278.94+1.35 as possibly one of the largest known Galactic supernova remnants (SNR) - that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new EMU and GLEAM radio continuum images at an angular size of 3.33x3.23 deg, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a diameter of 157x152 pc. This size would qualify Diprotodon as the largest known SNR and pushes our estimates of SNR sizes to the upper limits. We investigate the environment in which the SNR is located and examine various scenarios that might explain such a large and relatively bright SNR appearance. We find that Diprotodon is most likely at a much closer distance of $\sim$1 kpc, implying its diameter is 58x56 pc and it is in the radiative evolutionary phase. We also present a new Fermi-LAT data analysis that confirms the angular extent of the SNR in gamma-rays. The origin of the high-energy emission remains somewhat puzzling, and the scenarios we explore reveal new puzzles, given this unexpected and unique observation of a seemingly evolved SNR having a hard GeV spectrum with no breaks. We explore both leptonic and hadronic scenarios, as well as the possibility that the high-energy emission arises from the leftover particle population of a historic pulsar wind nebula.
Comments:
21 pages, 12 figures, 1 table. Accepted for publication in PASA
View a PDF of the paper titled Probing Heavy Axion-like Particles from Massive Stars with X-rays and Gamma Rays, by James H. Buckley and 3 other authors
Abstract:The hot interiors of massive stars in the later stages of their evolution provide an ideal place for the production of heavy axion-like particles (ALPs) with mass up to O(100 keV) range. We show that a fraction of these ALPs could stream out of the stellar photosphere and subsequently decay into two photons that can be potentially detected on or near the Earth. In particular, we estimate the photon flux originating from the spontaneous decay of heavy ALPs produced inside Horizontal Branch and Wolf-Rayet stars, and assess its detectability by current and future $X$-ray and gamma-ray telescopes. Our results indicate that current and future telescopes can probe axion-photon couplings down to $g_{a\gamma} \sim 4\times 10^{-11}$ GeV${}^{-1}$ for $m_a\sim 10-100$ keV, which covers new ground in the ALP parameter space.
Abstract:The detection by the LHAASO Collaboration of the gamma-ray burst GRB 221009A at redshift $z = 0.151$ with energies up to $(13-18) \, \rm TeV$ challenges conventional physics. Photons emitted with energies above $10 \, \rm TeV$ at this redshift can hardly be observed on Earth due to their interaction with the extragalactic background light (EBL). We show that indeed the LHAASO Collaboration should not have observed photons with energies above $10 \, \rm TeV$ if the state-of-the-art EBL model by Saldana-Lopez et al. is taken into account. A problem therefore arises: the Universe should be more transparent than currently believed. We also show that the issue is solved if we introduce the interaction of photons with axion-like particles (ALPs). ALPs are predicted by String Theory, are among the best candidates for dark matter and can produce spectral and polarization effects on astrophysical sources in the presence of external magnetic fields. In particular, for GRB 221009A, photon-ALP oscillations occur within the crossed magnetized media, i.e. the host galaxy, the extragalactic space, the Milky Way, partially reducing the EBL absorption to a level that explains the LHAASO detection of GRB 221009A and its observed spectrum without the need of contrived choices of parameter values, which are instead compulsory within proposed emission models within conventional physics. This fact regarding GRB 221009A represents a strong hint at the ALP existence, which adds to two other indications coming from blazars, a class of active galactic nuclei.
Comments:
4 pages, 1 figure, contribution to the 2024 Very High Energy Phenomena in the Universe session of the 58th Rencontres de Moriond
View a PDF of the paper titled The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays, by Hisashi Hayakawa and 21 other authors
Abstract:We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman-Silverman storm) apparently originated from a complex active region of moderate area (\approx 500 {\mu}sh) that was favorably situated near disk center (S19° E05°). There is circumstantial evidence for an eruption from this region at 9--10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of \approx29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at \approx14:27 UT and allow a minimum Dst estimate of £ -834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19°.9 in magnetic latitude (MLAT) and 24°. 2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below |20|° for the 1872 storm (ranging from |10°. 0|--|19°. 9| MLAT) versus one each for the 1859 storm (|17°. 3| MLAT) and the 1921 storm (|16.°2| MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25°.1 \pm 0°.5 ILAT and -949 \pm 31 nT) and 1921 May (27°.1 ILAT and -907 \pm 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.
View a PDF of the paper titled Fermi-LAT Discovery of a Gamma-ray Outburst from the Peculiar Compact Steep Spectrum Radiogalaxy 3C 216, by Federica Giacchino and 5 other authors
Abstract:3C 216 is an extra-galactic radio source classified as a compact steep spectrum (CSS) object, associated with the source 4FGL J0910.0+4257 detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The source exhibits extended radio structures as well as an inner relativistic jet. In general, jets accelerated by active galactic nuclei (AGNs) are efficient sources of non-thermal radiation, spanning from the radio band to X-ray and gamma-ray energies. Due to relativistic beaming, much of this radiation, particularly in the high-energy domain, is concentrated within a narrow cone aligned with the jet's direction. Consequently, high-energy emission is more easily detected in blazars, where the jet is closely aligned with the line of sight of the observer. Beginning in November 2022, Fermi-LAT observed increased gamma-ray activity from 3C 216, culminating in a strong outburst in May 2023. This event was followed up by observations from the Neil Gehrels Swift Observatory telescope. In this work, we perform a careful analysis of the multifrequency data (gamma-ray, X-ray, UV, optical) collected during this observational campaign. We find that the spectral energy distribution of the flaring source evolves in a coherent way, suggesting a common origin for the multifrequency emission. These results support the interpretation of the gamma-ray emission within a single zone synchrotron self-Compton (SSC) model, with important implications for the mechanisms powering high-energy radiation in AGN jets.
Title:Excess Ultraviolet Emission at High Galactic Latitudes: A New Horizons View
Authors:Jayant Murthy (1), J. Michael Shull (2 and 3), Marc Postman (4), Joel Wm. Parker (5), Seth Redfield (6), Nathaniel Cunningham (7), G. Randall Gladstone (8 and 9), Jon P. Pineau (10), Pontus Brandt (11), Anne J. Verbiscer (12), Kelsi N. Singer (5), Harold A. Weaver (11), Richard C. Henry (13), S. Alan Stern (14) ((1) Indian Institute of Astrophysics, (2) Department of Astrophysical & Planetary Sciences, CASA, University of Colorado, (3) Department of Physics & Astronomy, University of North Carolina, (4) Space Telescope Science Institute, (5) Department of Space Studies, Southwest Research Institute, (6) Astronomy Department and Van Vleck Observatory, Wesleyan University, (7) Nebraska Wesleyan University, (8) Southwest Research Institute, San Antonio, TX (9) University of Texas at San Antonio, (10) Stellar Solutions, Aurora, USA (11) The Johns Hopkins University Applied Physics Laboratory, (12) Department of Astronomy, University of Virginia, (13) Johns Hopkins University, Dept. of Physics and Astronomy, and (14) Southwest Research Institute, Space Sector, Boulder, USA)
View a PDF of the paper titled Excess Ultraviolet Emission at High Galactic Latitudes: A New Horizons View, by Jayant Murthy (1) and 37 other authors
Abstract:We present new observations of the cosmic ultraviolet background (CUVB) at high Galactic latitudes ($|b| > 40^{\circ}$), made using the Alice UV spectrograph on board the New Horizons spacecraft. These observations were taken at about 57 AU from the Sun, outside much of the foreground emission affecting previous missions, and allowed a new determination of the spectrum of the CUVB between 912 -- 1100~Å and 1400 -- 1800~Å. We found a linear correlation between the CUVB and the Planck E(B~-~V) with offsets at zero-reddening of $221 \pm 11$ photon units at 1000~Å and $264 \pm 24$ \photu\ at 1500~Å ($4.4 \pm 0.2$ nW m$^{-2}$ sr$^{-1}$ at 1000~Å and $5.3 \pm 0.5$ nW m$^{-2}$ sr$^{-1}$ at 1500~Å). The former is the first firm detection of the offset in the range 912 -- 1100 Å while the latter result confirms previous results from \galex, showing that there is little emission from the Solar System from 1400 -- 1800 Å. About half of the offset may be explained by known sources (the integrated light of unresolved galaxies, unresolved stars, emission from ionized gas, and two-photon emission from warm hydrogen in the halo) with the source of the remaining emission as yet unidentified. There is no detectable emission below the Lyman limit with an upper limit of $3.2 \pm 3.0$ photon units.
View a PDF of the paper titled Unraveling the kinematic and morphological evolution of the Small Magellanic Cloud, by S. R. Dhanush and 2 other authors
Abstract:We modeled the kinematics of the Small Magellanic Cloud (SMC) by analyzing the proper motion (PM) from Gaia DR3 of nine different stellar populations, which include young main sequence (MS) stars (< 2 Gyr), red giant branch stars, red clump stars, red giants with line-of-sight velocities, and three groups of star clusters. This analysis was carried out using a robust Markov Chain Monte Carlo method to derive up to 7 kinematic parameters. We trace the evolution from a non-rotating flattened elliptical system as mapped by the old population to a rotating highly stretched disk structure as denoted by the young MS stars and clusters (< 400 Myr). We estimated that the inclination, i (~ 58$^\circ$ to 82$^\circ$) decreases and the position angle, $\Theta$ (~ 180$^\circ$ to 240$^\circ$) increases with age. We estimated an asymptotic velocity of ~ 49 - 89 km s$^{-1}$ with scale-radius of ~ 6 - 9 kpc for the young MS populations with velocity dispersion of ~ 11 km s$^{-1}$, suggesting a rotation-supported disk structure. Our models estimate a line-of-sight extension of ~ 30 kpc, in agreement with observations. We identified four regions of the SMC showing anomalies in the residual PM, the East Anomaly (EA), South East Anomaly (SEA), South Anomaly (SA), and West Anomaly (WA). The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that its evolution is considerably shaped by the recent interaction with the Large Magellanic Cloud.
Comments:
Accepted for publication in ApJ. 18 pages, 13 figures, 2 tables
View a PDF of the paper titled Detection of "diffuse" coronal He I 1083 during the April 8 2024 Solar Eclipse: evidence for terrestrial atmospheric scattering origin, by M. E. Molnar and 4 other authors
Abstract:Strong He I 1083 nm atomic line signals have been previously measured during total solar eclipses at coronal heights above the lunar limb. This rather unexpected measurement has kindled a discussion about the hypothesized presence of significant amounts of neutral helium at coronal conditions. We performed spectroscopic observations of the He I 1083 nm spectroscopic region with the newly built CHEESE instrument during the April 8th 2024 total solar eclipse to test the presence of He I 1083 in the solar corona. We detected the He I 1083, the forbidden coronal line Fe XIII 1074.7 nm, as well as the chromospheric H I 1093.8 nm Paschen-{\gamma} line in our eclipse observations. The chromospheric He I 1083 and H I 1093.8 nm Paschen-{\gamma} lines are detected in the corona as well as on the lunar disc. Our findings point toward a non-solar origin of the He I 1083 signal during the April 8th 2024 eclipse that challenge the notion of abundant neutral helium in the solar corona inferred from eclipse observations.
View a PDF of the paper titled Search for continuous gravitational waves from known pulsars in the first part of the fourth LIGO-Virgo-KAGRA observing run, by The LIGO Scientific Collaboration and 1817 other authors
Abstract:Continuous gravitational waves (CWs) emission from neutron stars carries information about their internal structure and equation of state, and it can provide tests of General Relativity. We present a search for CWs from a set of 45 known pulsars in the first part of the fourth LIGO--Virgo--KAGRA observing run, known as O4a. We conducted a targeted search for each pulsar using three independent analysis methods considering the single-harmonic and the dual-harmonic emission models. We find no evidence of a CW signal in O4a data for both models and set upper limits on the signal amplitude and on the ellipticity, which quantifies the asymmetry in the neutron star mass distribution. For the single-harmonic emission model, 29 targets have the upper limit on the amplitude below the theoretical spin-down limit. The lowest upper limit on the amplitude is $6.4\!\times\!10^{-27}$ for the young energetic pulsar J0537-6910, while the lowest constraint on the ellipticity is $8.8\!\times\!10^{-9}$ for the bright nearby millisecond pulsar J0437-4715. Additionally, for a subset of 16 targets we performed a narrowband search that is more robust regarding the emission model, with no evidence of a signal. We also found no evidence of non-standard polarizations as predicted by the Brans-Dicke theory.
Abstract:High-energy collisions at the high-luminosity Large Hadron Collider (HL-LHC) will generate a vast flux of particles along the beam collision axis, a region not accessible by current LHC experiments. The study of multi-particle production in the far-forward region is especially important for astroparticle physics. High-energy cosmic rays create extensive air showers (EAS) in the atmosphere, driven by hadron-ion collisions in the non-perturbative QCD regime. Therefore, understanding high-energy hadronic interactions in the forward region is crucial for interpreting EAS data and estimating backgrounds for searches of astrophysical neutrinos, among other applications. The Forward Physics Facility (FPF) is a proposal to construct a new underground cavern at the HL-LHC, hosting various far-forward experiments designed to detect particles outside the current LHC acceptance. We will outline the current plans for the FPF and highlight its synergies with astroparticle physics. Specifically, we will discuss how FPF measurements will enhance the modeling of high-energy interactions in the atmosphere, helping to reduce the associated uncertainties in multi-messenger astrophysics.
Comments:
Proceedings of the International Symposium on Very High Energy Cosmic Ray Interactions (ISVHECRI 2024); 11 pages, 6 figures. arXiv admin note: text overlap with arXiv:2411.04175, arXiv:2407.03427
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