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1. Limits on X-Ray Polarization at the Core of Centaurus A as Observed with the Imaging X-Ray Polarimetry Explorer

   https://doi.org/10.3847/1538-4357/ac8056  

   Ehlert, Steven R.; Ferrazzoli, Riccardo; Marinucci, Andrea; Marshall, Herman L.; Middei, Riccardo; Pacciani, Luigi; Perri, Matteo; Petrucci, Pierre-Olivier; Puccetti, Simonetta; Barnouin, Thibault; et al (August 2022, The Astrophysical Journal)

   Abstract We present measurements of the polarization of X-rays in the 2–8 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at 99% confidence (MDP 99 ) of 6.5% using a weighted, spectral model-independent calculation in the 2–8 keV band. The polarization angle ψ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3–400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption ( N H ∼ 10 23 cm −2 ) and a Fe K α emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 keV. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for the production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe K α emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet. 

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2. Constraining particle acceleration in Sgr A ^⋆ with simultaneous GRAVITY, Spitzer , NuSTAR , and Chandra observations

   https://doi.org/10.1051/0004-6361/202140981  

   Abuter, R.; Amorim, A.; Bauböck, M.; Baganoff, F.; Berger, J. P.; Boyce, H.; Bonnet, H.; Brandner, W.; Clénet, Y.; Davies, R.; et al (October 2021, Astronomy & Astrophysics)

   We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A ⋆ . We obtained light curves in the M , K , and H bands in the mid- and near-infrared and in the 2 − 8 keV and 2 − 70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νL ν  ∝  ν 0.5 ± 0.2 ; the spectral slope observed in the X-ray band is νL ν  ∝  ν −0.7 ± 0.5 . Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γ max , which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γ max  ∼ 500, while cooled synchrotron model requires acceleration up to γ max  ∼ 5 × 10 4 . The synchrotron self-Compton scenario requires electron densities of 10 10 cm −3 that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1  R S , the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γ max , implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare. 

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3. 3D MC. II. X-Ray Echoes Reveal a Clumpy Molecular Cloud in the CMZ

   https://doi.org/10.3847/1538-3881/adbaf3  

   Alboslani, Danya; Battersby, Cara; Brunker, Samantha W; Clavel, Maïca; Lipman, Dani; Walker, Daniel L (March 2025, The Astronomical Journal)

   Abstract X-ray observations collected over the past decades have revealed a strongly variable X-ray signal within the Milky Way’s Galactic center, interpreted as X-ray echoes from its supermassive black hole, Sgr A*. These echoes are traced by the strong Fe Kαfluorescent line at 6.4 keV, the intensity of which is proportional to the density of the illuminated molecular gas. Over time, the echo scans through molecular clouds (MCs) in our Galactic center, revealing their 3D structure and highlighting their densest parts. While previous studies have utilized spectral line Doppler shifts along with kinematic models to constrain the geometry of the Central Molecular Zone (CMZ) or to study the structure of individual clouds, these methods have limitations, particularly in the turbulent region of the CMZ. We use archival Chandra X-ray data to construct one of the first 3D representations of one prominent MC, the Stone cloud, located at (ℓ= 0 $\stackrel{\text{°}}{.}$068,b= –0 $\stackrel{\text{°}}{.}$076) at a distance of ∼20 pc from Sgr A* in projection. Using the Chandra X-ray Observatory, we followed the X-ray echo in this cloud from 2008 to 2017. We combine these data with 1.3 mm dust continuum emission observed with the Submillimeter Array (SMA) and the Herschel Space Observatory to reconstruct the 3D structure of the cloud and estimate the column densities for each year’s observed slice. The analysis of the X-ray echoes, along with velocities from SMA molecular line data, indicates that the structure of the Stone cloud can be described as a very diffuse background with multiple dense clumps throughout. 

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4. γ -ray detection toward the Coma cluster with Fermi -LAT: Implications for the cosmic ray content in the hadronic scenario

   https://doi.org/10.1051/0004-6361/202039660  

   Adam, R.; Goksu, H.; Brown, S.; Rudnick, L.; Ferrari, C. (April 2021, Astronomy & Astrophysics)

   null (Ed.)

   The presence of relativistic electrons within the diffuse gas phase of galaxy clusters is now well established, thanks to deep radio observations obtained over the last decade, but their detailed origin remains unclear. Cosmic ray protons are also expected to accumulate during the formation of clusters. They may explain part of the radio signal and would lead to γ -ray emission through hadronic interactions within the thermal gas. Recently, the detection of γ -ray emission has been reported toward the Coma cluster with Fermi -LAT. Assuming that this γ -ray emission arises essentially from pion decay produced in proton-proton collisions within the intracluster medium (ICM), we aim at exploring the implication of this signal on the cosmic ray content of the Coma cluster and comparing it to observations at other wavelengths. We use the MINOT software to build a physical model of the Coma cluster, which includes the thermal target gas, the magnetic field strength, and the cosmic rays, to compute the corresponding expected γ -ray signal. We apply this model to the Fermi -LAT data using a binned likelihood approach, together with constraints from X-ray and Sunyaev-Zel’dovich observations. We also consider contamination from compact sources and the impact of various systematic effects on the results. We confirm that a significant γ -ray signal is observed within the characteristic radius θ 500 of the Coma cluster, with a test statistic TS ≃ 27 for our baseline model. The presence of a possible point source (4FGL J1256.9+2736) may account for most of the observed signal. However, this source could also correspond to the peak of the diffuse emission of the cluster itself as it is strongly degenerate with the expected ICM emission, and extended models match the data better. Given the Fermi -LAT angular resolution and the faintness of the signal, it is not possible to strongly constrain the shape of the cosmic ray proton spatial distribution when assuming an ICM origin of the signal, but preference is found in a relatively flat distribution elongated toward the southwest, which, based on data at other wavelengths, matches the spatial distribution of the other cluster components well. Assuming that the whole γ -ray signal is associated with hadronic interactions in the ICM, we constrain the cosmic ray to thermal energy ratio within R 500 to X CRp = 1.79 −0.30 +1.11 % and the slope of the energy spectrum of cosmic rays to α = 2.80 −0.13 +0.67 ( X CRp = 1.06 −0.22 +0.96 % and α = 2.58 −0.09 +1.12 when including both the cluster and 4FGL J1256.9+2736 in our model). Finally, we compute the synchrotron emission associated with the secondary electrons produced in hadronic interactions assuming steady state. This emission is about four times lower than the overall observed radio signal (six times lower when including 4FGL J1256.9+2736), so that primary cosmic ray electrons or reacceleration of secondary electrons is necessary to explain the total emission. We constrain the amplitude of the primary to secondary electrons, or the required boost from reacceleration with respect to the steady state hadronic case, depending on the scenario, as a function of radius. Our results confirm that γ -ray emission is detected in the direction of the Coma cluster. Assuming that the emission is due to hadronic interactions in the intracluster gas, they provide the first quantitative measurement of the cosmic ray proton content in a galaxy cluster and its implication for the cosmic ray electron populations. 

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5. First Mid-infrared Detection and Modeling of a Flare from Sgr A*

   https://doi.org/10.3847/2041-8213/ada3d2  

   von_Fellenberg, Sebastiano D; Roychowdhury, Tamojeet; Michail, Joseph M; Sumners, Zach; Sanger-Johnson, Grace; Fazio, Giovanni G; Haggard, Daryl; Hora, Joseph L; Philippov, Alexander; Ripperda, Bart; et al (January 2025, The Astrophysical Journal Letters)

   Abstract The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic center, has long been examined in the radio-to-millimeter, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests that synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic fields strengths ~ 40–70 G in the emission zone. Observations at 1.3 mm with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by ≈10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through (a combination of) magnetic reconnection and/or magnetized turbulence. 

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