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<jats:title>Abstract</jats:title> <jats:p>Motivated by the large observed diversity in the properties of extragalactic extinction by dust, we reanalyze the Cepheid calibration used to infer the Hubble constant, <jats:italic>H</jats:italic> <jats:sub>0</jats:sub>, from Type Ia supernovae, using Cepheid data in 19 Type Ia supernova host galaxies from Riess et al. and anchor data from Riess et al. Unlike the SH0ES team, we do not enforce a fixed universal color–luminosity relation to correct the Cepheid magnitudes. Instead, we focus on a data-driven method, where the optical colors and near-infrared magnitudes of the Cepheids are used to derive individual color–luminosity relations for each Type Ia supernova host and anchor galaxy. We present two different analyses, one based on Wesenheit magnitudes, resulting in <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 73.2 ± 1.3 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>, a 4.2<jats:italic>σ</jats:italic> tension with the value inferred from the cosmic microwave background. In the second approach, we calibrate an individual extinction law for each galaxy, with noninformative priors using color excesses, yielding <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 73.9 ± 1.8 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>, in 3.4<jats:italic>σ</jats:italic> tension with the Planck value. Although the two methods yield similar results, in the latter approach, the Hubble constants inferred from the individual Cepheid absolute distance calibrator galaxies range from <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 68.1 ± 3.5 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> to <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> = 76.7 ± 2.0 km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>. Taking the correlated nature of <jats:italic>H</jats:italic> <jats:sub>0</jats:sub> inferred from individual anchors into account, and allowing for individual extinction laws, the Milky Way anchor is in 2.1–3.1 <jats:italic>σ</jats:italic> tension with the NGC 4258 and Large Magellanic Cloud anchors, depending on prior assumptions regarding the color–luminosity relations and the method used for quantifying the tension.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Astrophysical sources of very high energy (VHE; &gt;100 GeV) <jats:italic>γ-</jats:italic>rays are rare, since GeV and TeV photons can be only emitted in extreme circumstances involving interactions of relativistic particles with local radiation and magnetic fields. In the context of the Fermi Large Area Telescope (LAT), only a few sources are known to be VHE emitters, where the largest fraction belongs to the rarest class of active galactic nuclei: the blazars. In this work, we explore Fermi-LAT data for energies &gt;100 GeV and Galactic latitudes <jats:italic>b</jats:italic> &gt; ∣50°∣ in order to probe the origin of the extragalactic isotropic <jats:italic>γ</jats:italic>-ray emission. Since the production of such VHE photons requires very specific astrophysical conditions, we would expect that the majority of the VHE photons from the isotropic <jats:italic>γ</jats:italic>-ray emission originate from blazars or other extreme objects like star-forming galaxies, <jats:italic>γ</jats:italic>-ray bursts, and radio galaxies, and that the detection of a single VHE photon at the adopted Galactic latitudes would be enough to unambiguously trace the presence of such a counterpart. Our results suggest that blazars are, by far, the dominant class of sources above 100 GeV, although they account for only <jats:inline-formula> <jats:tex-math> <?CDATA ${22.8}_{-4.1}^{+4.5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>22.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac771dieqn1.gif" xlink:type="simple" /> </jats:inline-formula>% of the extragalactic VHE photons. The remaining <jats:inline-formula> <jats:tex-math> <?CDATA ${77}_{-4.5}^{+4.1} \% $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>77</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4.1</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac771dieqn2.gif" xlink:type="simple" /> </jats:inline-formula> of the VHE photons still have an unknown origin.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the AGILE observations of GRB 220101A, which took place at the beginning of 2022 January 1 and was recognized as one of the most energetic gamma-ray bursts (GRBs) ever detected since their discovery. The AGILE satellite acquired interesting data concerning the prompt phase of this burst, providing an overall temporal and spectral description of the event in a wide energy range, from tens of kiloelectronvolts to tens of megaelectronvolts. Dividing the prompt emission into three main intervals, we notice an interesting spectral evolution, featuring a notable hardening of the spectrum in the central part of the burst. The average fluxes encountered in the different time intervals are relatively moderate, with respect to those of other remarkable bursts, and the overall fluence exhibits a quite ordinary value among the GRBs detected by MCAL. However, GRB 220101A is the second farthest event detected by AGILE, and the burst with the highest isotropic equivalent energy of the entire MCAL GRB sample, releasing <jats:italic>E</jats:italic> <jats:sub>iso</jats:sub> = 2.54 × 10<jats:sup>54</jats:sup> erg and exhibiting an isotropic luminosity of <jats:italic>L</jats:italic> <jats:sub>iso</jats:sub> = 2.34 × 10<jats:sup>52</jats:sup> erg s<jats:sup>−1</jats:sup> (both in the 400 keV–10 MeV energy range). We also analyzed the first 10<jats:sup>6</jats:sup> s of the afterglow phase, using the publicly available Swift-XRT data, carrying out a theoretical analysis of the afterglow, based on the forward shock model. We notice that GRB 220101A is with high probability surrounded by a wind-like density medium, and that the energy carried by the initial shock shall be a fraction of the total <jats:italic>E</jats:italic> <jats:sub>iso</jats:sub>, presumably near ∼50%.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We discuss the statistical distribution of galaxy shapes and viewing angles under the assumption of triaxiality by deprojecting observed surface brightness profiles of 56 brightest cluster galaxies (BCGs) coming from a recently published large deep-photometry sample. For the first time, we address this issue by directly measuring axis ratio profiles without limiting ourselves to a statistical analysis of average ellipticities. We show that these objects are strongly triaxial, with triaxiality parameters 0.39 ≤ <jats:italic>T</jats:italic> ≤ 0.72, they have average axis ratios 〈<jats:italic>p</jats:italic>(<jats:italic>r</jats:italic>)〉 = 0.84 and 〈<jats:italic>q</jats:italic>(<jats:italic>r</jats:italic>)〉 = 0.68, and they are more spherical in the central regions but flatten out at large radii. Measured shapes in the outskirts agree well with the shapes found for simulated massive galaxies and their dark matter halos from both the IllustrisTNG and the Magneticum simulations, possibly probing the nature of dark matter. In contrast, both simulations fail to reproduce the observed inner regions of BCGs, producing objects that are too flattened.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The study of transiently accreting neutron stars provides a powerful means to elucidate the properties of neutron star crusts. We present extensive numerical simulations of the evolution of the neutron star in the transient low-mass X-ray binary MAXI J0556–332. We model nearly 20 observations obtained during the quiescence phases after four different outbursts of the source in the past decade, considering the heating of the star during accretion by the deep crustal heating mechanism complemented by some shallow heating source. We show that cooling data are consistent with a single source of shallow heating acting during the last three outbursts, while a very different and powerful energy source is required to explain the extremely high effective temperature of the neutron star, ∼350 eV, when it exited the first observed outburst. We propose that a gigantic thermonuclear explosion, a “hyperburst” from unstable burning of neutron-rich isotopes of oxygen or neon, occurred a few weeks before the end of the first outburst, releasing ∼10<jats:sup>44</jats:sup> ergs at densities of the order of 10<jats:sup>11</jats:sup> g cm<jats:sup>−3</jats:sup>. This would be the first observation of a hyperburst, and these would be extremely rare events, as the buildup of the exploding layer requires about a millennium of accretion history. Despite its large energy output, the hyperburst did not produce, due to its depth, any noticeable increase in luminosity during the accretion phase and is only identifiable by its imprint on the later cooling of the neutron star.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of <jats:inline-formula> <jats:tex-math> <?CDATA ${a}_{h}=1\buildrel{\,\prime}\over{.} {88}_{-0.33}^{+0.42}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>h</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.′</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>88</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.33</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.42</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7226ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (<jats:inline-formula> <jats:tex-math> <?CDATA ${118}_{-30}^{+31}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>118</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7226ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> pc) and <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{V}=-{4.17}_{-0.22}^{+0.19};$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4.17</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.19</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>;</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7226ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> for Psc II, we measure <jats:inline-formula> <jats:tex-math> <?CDATA ${a}_{h}=1\buildrel{\,\prime}\over{.} {31}_{-0.09}^{+0.10}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>h</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.′</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>31</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.09</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7226ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> (69 ± 8 pc) and <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{V}=-{4.28}_{-0.16}^{+0.19}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>4.28</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.16</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.19</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac7226ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>. We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only ∼40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in an MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close (∼10–20 kpc) passage about each other just over ∼1 Gyr ago, followed by a collective passage around the LMC (∼30–60 kpc) just under ∼1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ∼30′ from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio–X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnetic reconnection in solar flares can efficiently generate nonthermal electron beams. The energetic electrons can, in turn, cause radio waves through microscopic plasma instabilities as they propagate through the ambient plasma along the magnetic field lines. We aim at investigating the wave emission caused by fast-moving electron beams with characteristic nonthermal electron velocity distribution functions (EVDFs) generated by kinetic magnetic reconnection: two-stream EVDFs along the separatrices and in the diffusion region, and perpendicular crescent-shaped EVDFs closer to the diffusion region. For this purpose, we utilized 2.5D fully kinetic Particle-In-Cell code simulations in this study. We found the following: (1) the two-stream EVDFs plus the background ions are unstable to electron/ion (streaming) instabilities, which cause ion-acoustic waves and Langmuir waves due to the net current. This can lead to multiple-harmonic plasma emission in the diffusion region and the separatrices of reconnection. (2) The perpendicular crescent-shaped EVDFs can cause multiple-harmonic electromagnetic electron cyclotron waves through the electron cyclotron maser instabilities in the diffusion region of reconnection. Our results are applicable to diagnose the plasma parameters, which are associated to magnetic reconnection in solar flares by means of radio wave observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The solar wind in the inner heliosphere has been observed by Parker Solar Probe (PSP) to exhibit abundant wave activities. The cyclotron wave modes responding to ions or electrons are among the most crucial wave components. However, their origin and evolution in the inner heliosphere close to the Sun remains a mystery. Specifically, it remains unknown whether it is an emitted signal from the solar atmosphere or an eigenmode growing locally in the heliosphere due to plasma instability. To address and resolve this controversy, we must investigate the key quantity of the energy change rate of the wave mode. We develop a new technique to measure the energy change rate of plasma waves, and apply this technique to the wave electromagnetic fields measured by PSP. We provide the wave Poynting flux in the solar wind frame, identify the wave nature to be the outward propagating fast-magnetosonic/whistler wave mode instead of the sunward propagating waves. We provide the first evidence for growth of the fast-magnetosonic/whistler wave mode in the inner heliosphere based on the derived spectra of the real and imaginary parts of the wave frequencies. The energy change rate rises and stays at a positive level in the same wavenumber range as the bumps of the electromagnetic field power spectral densities, clearly manifesting that the observed fast-magnetosonic/whistler waves are locally growing to a large amplitude.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the angular power spectrum (APS) of the unresolved gamma-ray background (UGRB) emission and combine it with the measured properties of the resolved gamma-ray sources of the Fermi-LAT 4FGL catalog. Our goals are to dissect the composition of the gamma-ray sky and to establish the relevance of different classes of source populations of active galactic nuclei in determining the observed size of the UGRB anisotropy, especially at low energies. We find that, under physical assumptions for the spectral energy distribution, i.e., by using the 4FGL catalog data as a prior, two populations are required to fit the APS data, namely flat-spectrum radio quasars at low energies and BL Lacs at higher energies. The inferred luminosity functions agree well with the extrapolation of the flat-spectrum radio quasar and BL Lac ones obtained from the 4FLG catalog. We use these luminosity functions to calculate the UGRB intensity from blazars, finding a contribution of 20% at 1 GeV and 30% above 10 GeV. Finally, bounds on an additional gamma-ray emission due to annihilating dark matter are also derived.</jats:p>