Abstracts of Interest
Selected by:
Gavin Rowell
Abstract: 1804.07730
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Title: HI observations of Sextans A and B with the SKA pathfinder KAT-7
(Submitted on 20 Apr 2018)
Abstract: HI observations of the Local Group dwarf irregular galaxies Sextans A and B, obtained with the Karoo Array Telescope (KAT-7) are presented. The KAT-7 wide field of view and excellent surface brightness sensitivity allows us to verify the true HI extent of the galaxies. We derive HI extends of 30$^{\prime}$ and 20$^{\prime}$ and total HI fluxes of 181 $\pm$ 2.0 Jy.km.s$^{-1}$ and 105 $\pm$ 1.4 Jy.km.s$^{-1}$ for Sextans A and B respectively. This result shows clearly the overestimate of the HI extent and total flux of 54$^{\prime}$ and 264 Jy.km.s$^{-1}$ reported for Sextans A using the Effelsberg observations. Tilted ring models allow us to derive the rotation curves (RCs) of Sextans A and B out to 550$^{\prime \prime}$ ($\sim$ 3.5 kpc) and 650$^{\prime \prime}$ ($\sim$ 4 kpc) respectively. The RCs of the two galaxies are seen to decline in the outer parts. The dark matter distribution in Sextans A is better described by the pseudo-isothermal halo model when a M/L ratio of 0.2 is used. For Sextans B, the mass model fits are not as good but again an isothermal sphere with a M/L of 0.2 represents best the data. Using the MOdified Newtonian Dynamics (MOND), better fits are obtained when the constant a$_{0}$ is allowed to vary. The critical densities for gravitational instabilities are calculated using the Toomre-$Q$ and cloud-growth based on shear criterion. We find that in regions of star formation, the cloud growth criterion based on shear explains better the star formation in both Sextans A and B.
Abstract: 1804.07692
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Title: Simultaneous long-term monitoring of LS I +61°303 by OVRO and Fermi-LAT
(Submitted on 20 Apr 2018)
Abstract: Previous long-term monitorings of the gamma-ray-loud X-ray binary LS I +61{\deg}303 have revealed the presence of a long-term modulation of ~4.5 years. After nine years of simultaneous monitoring of LS I +61{\deg}303 by the Owens Valley Radio Observatory and the Fermi-LAT, two cycles of the long-term period are now available. Here we perform timing-analysis on the radio and the gamma-ray light curves. We confirm the presence of previously detected periodicities at both radio and GeV gamma-ray wavelengths. Moreover, we discover an offset of the long-term modulation between radio and gamma-ray data which could imply different locations of the radio (15 GHz) and GeV emission along the precessing jet.
Abstract: 1804.07542
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Title: Analysis of Fermi-LAT data from Tucana-II: Possible constraints on the Dark Matter models
(Submitted on 20 Apr 2018)
Abstract: Tucana-II (Tuc-II), a recently discovered and confirmed Ultra Faint Dwarf Spheroidal galaxy, has a high mass to light ratio as well as a large line-of-sight stellar velocity dispersion, thus making it an ideal candidate for an indirect dark matter search. In this paper, we have analyzed nine years of $\gamma$-ray data obtained from the Fermi-LAT instrument in the direction of Tuc-II. The fact that no statistically significant $\gamma$-ray excess over the background of Tuc-II have been detected from the location of this galaxy. It called for an estimation of the $95\%$ confidence level upper limit of the possible velocity weighted self-annihilation cross-section of the dark matter particles (WIMPs) within Tuc-II by fitting the observed $\gamma$-ray flux with the DMFit function. The estimated upper limits of the cross-sections are then compared with similar upper limits of the cross-sections determined by us in three other dwarf galaxies that are considered earlier to be the good dark matter candidates in the literature. We have also compared our results with the cross-sections obtained in various popular theoretical models of the WIMPs to find that our results impose reasonable constraints on the parameter spaces of those models. In the concluding section, we compared our results with the similar results obtained from a combined dSph analysis by the Fermi-LAT collaboration as well as the results obtained from the studies of dark matter in the dwarf galaxies by the major ground-based Cherenkov experiments.
Abstract: 1804.07486
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Title: The Contribution of Stellar Winds to Cosmic Ray Production
(Submitted on 20 Apr 2018)
Abstract: Massive stars blow powerful stellar winds throughout their evolutionary stages from the main sequence to Wolf-Rayet phases. The amount of mechanical energy deposited in the interstellar medium by the wind from a massive star can be comparable to the explosion energy of a core-collapse supernova that detonates at the end of its life. In this study, we estimate the kinetic energy deposition by massive stars in our Galaxy by considering the integrated Galactic initial mass function and modeling the stellar wind luminosity. The mass loss rate and terminal velocity of stellar winds during the main sequence, red supergiant, and Wolf-Rayet stages are estimated by adopting theoretical calculations and observational data published in the literature. We find that the total stellar wind luminosity due to all massive stars in the Galaxy is about $L_w\approx 1.1\times 10^{41}$ erg/s, which is about 1/4 of the power of supernova explosions, $L_{SN} \approx 4.8\times 10^{41}$ erg/s. If we assume that $\sim 1-10$ % of the wind luminosity could be converted to Galactic cosmic rays (GCRs) through collisonless shocks such as termination shocks in stellar bubbles and superbubbles, colliding-wind shocks in binaries, and bow-shocks of massive runaway stars, stellar winds might be expected to make a significant contribution to GCR production, though lower than that of supernova remnants.
Abstract: 1804.07460
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Title: A comprehensive study of high-energy gamma-ray and radio emission from Cyg X-3
(Submitted on 20 Apr 2018)
Abstract: We study high-energy $\gamma$-rays observed from Cyg X-3 by the Fermi Large Area Telescope and the 15-GHz emission observed by the Ryle Telescope and the Arcminute Microkelvin Imager. We determine the $\gamma$-ray spectrum averaged over strong flares much more accurately than before, and find it well modelled by Compton scattering of stellar radiation by relativistic electrons with the power law index of $\simeq$3.5 and a low-energy cutoff at the Lorentz factor of $\sim\!10^3$. We find a weaker spectrum in the soft spectral state, but only upper limits in the hard and intermediate states. We measure strong orbital modulation during the flaring state, well modelled by anisotropic Compton scattering by jet relativistic electrons. We measure weaker orbital modulation of the 15 GHz radio emission, which is well modelled by free-free absorption by the stellar wind. We find the peak of the power spectrum of the radio emission is at a period shorter by 20 s than the orbital one, which can be due to a beat of the orbital modulation with a retrograde jet precession at a period of $\simeq$170 d. A definite evidence for the jet precession is provided by a strong dependence of the amplitude and phase of the orbital modulation on the precession phase. We then study cross-correlations between radio, $\gamma$-ray and X-ray emissions. We find no measurable delay of the radio emission with respect to $\gamma$-rays, but find a radio lag of $\sim$50 d with respect to the soft X-rays in the soft state.
Abstract: 1804.07403
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Title: Optical Emission Associated with the Galactic Supernova Remnant G179.0+2.6
(Submitted on 19 Apr 2018)
Abstract: Narrow passband optical images of the large Galactic supernova remnant G179.0+2.6 reveal a faint but nearly complete emission shell dominated by strong [O III] 4959,5007 A line emission. The remnant's optical emission, which consists of both diffuse and filamentary features, is brightest along its southern and northeastern limbs. Deep H-alpha images detect little coincidence emission indicating an unusually high [O III]/H-alpha emission ratio for such a large and apparently old remnant. Low-dispersion optical spectra of several regions confirm large [O III]/H-alpha line ratios with typical values around 10. The dominance of [O III] emission for the majority of the remnant's optical filaments suggests shock velocities above 100 km/s are present throughout most of the remnant, likely reflecting a relatively low density ambient ISM. The remnant's unusually strong [O III] emission adds to the remnant's interesting set of properties which include a thick radio emission shell, radial polarization of its radio emission like that typically seen in young supernova remnants, and an unusually slow-rotating gamma-ray pulsar with a characteristic spin-down age ~50 kyr.
Abstract: 1804.07400
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Title: Eta Carinae's Declining Outflow Seen in the UV, 2002-2015
(Submitted on 19 Apr 2018)
Abstract: Existing HST UV data offer many previously neglected clues to eta Car's behavior since 2000. Here we examine a subset of observations with diverse results. (1) The star's rapid change of state is confirmed by major changes in UV absorption lines, circumstellar extinction, and other features. (2) N III] 1750 is one of the two most luminous emission features in eta Car's observable spectrum, comparable to H-alpha. This and other semi-forbidden lines are useful because they have no P Cyg absorption. (3) N III] multiplet ratios provide the first direct diagnostic of gas densities in eta Car's outflow. They strongly suggest that high-excitation lines originate in condensations within the colliding-wind shocked region. The parameters imply that published models have not adequately represented the essential small size scales. (4) In 2002-2004, a very large amount of N III] emission had anomalous Doppler velocities from +400 to +1200 km/s. This is a mystery; we conjecture that it may have resulted from a burst of mass ejection in the 2003.5 periastron event. Various other effects are also difficult to explain and merit further investigation.
Abstract: 1804.07369
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Title: PeV IceCube signals and Dark Matter relic abundance in modified cosmologies
(Submitted on 19 Apr 2018)
Abstract: The discovery by the IceCube experiment of a high-energy astrophysical neutrino flux with energies of the order of PeV, has opened new scenarios in astroparticles physics. A possibility to explain this phenomenon is to consider the minimal models of Dark Matter (DM) decay, the 4-dimensional operator $\sim y_{\alpha \chi}\overline{{L_{L_{\alpha}}}}\, H\, \chi$, which is also able to generate the correct abundance of DM in the Universe.
Assuming that the cosmological background evolves according to the standard cosmological model, it follows that the rate of DM decay $\Gamma_\chi \sim |y_{\alpha \chi}|^2$ needed to get the correct DM relic abundance ($\Gamma_\chi\sim 10^{-58}$) differs by many orders of magnitude with respect that one needed to explain the IceCube data ($\Gamma_\chi\sim 10^{-25}$), making the four-dimensional operator unsuitable. In this paper we show that assuming that the early Universe evolution is governed by a modified cosmology, the discrepancy between the two the DM decay rates can be reconciled, and both the IceCube neutrino rate and relic density can be explained in a minimal model.
Abstract: 1804.07341
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Title: Kinetic "jets" from fast moving pulsars
(Submitted on 19 Apr 2018)
Abstract: Some fast-moving pulsars, like the Guitar and the Lighthouse, exhibit asymmetric non-thermal emission features that extend well beyond their ram pressure confined pulsar wind nebulae (PWNe). Using 3D relativistic simulations we explain these features as kinetically streaming pulsar wind particles that escaped into the interstellar medium (ISM) due to reconnection between the PWN and ISM magnetic fields. The structure of the reconnecting magnetic fields at the incoming and outgoing regions produce highly asymmetric magnetic bottles, and result in asymmetric extended features. For the features to become visible, the ISM magnetic field should be sufficiently high, $B_{\rm ISM}>10$~$\mu$G. We also discuss archival observations of PWNe displaying evidence of kinetic jets: the Dragonfly PWN (PSR J2021+3651), G327.1--1.1, and MSH 11--62, the latter two of which exhibit "snail eyes" morphologies. We suggest that in those cases the pulsar is moving along the ambient magnetic field in a frisbee-type configuration.
Abstract: 1804.07327
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Title: 3D dynamics and morphology of bow-shock Pulsar Wind Nebulae
(Submitted on 19 Apr 2018)
Abstract: Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute these variations to the intrinsic properties (relative orientation of the pulsar's spin, velocity, and magnetic inclination angle) - as well as the line of sight. We identify three basic types of bow-shock nebulae: (i) a "rifle bullet" (spin and velocity aligned); (ii) a "frisbee" (spin and velocity orthogonal, spin is in the plane of the sky) and (iii) a "cart-wheel" (like frisbee but the spin is perpendicular to the plane of the sky). Using 3D relativistic MHD simulations, as well as analytical calculations, we reproduce both the key morphological features of the bowshock PNEs, as well as variations are seen in different systems. Magnetic stresses within the pulsar wind strongly affect the overall structure, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, non-symmetric shapes etc. On the other hand, the ISM inhomogeneities, as well as the anisotropy of the wind luminosity, produce only mild variations of the PWN shape. In few cases we clearly identify the morphological structure - our results do not favor alignment of the pulsar spin and linear velocity. Our calculations of the underlying radiative process explain low synchrotron $X$-ray efficiency (in terms of the spin-down luminosity) and argue for energetically subdominant contribution of the IC processes.
Abstract: 1804.07216
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Title: The Protons in Primary Cosmic Rays in the Energy Range 10^15-10^17 eV According to Data from the PAMIR Experiment
(Submitted on 19 Apr 2018)
Abstract: Adjusted data on the fraction of protons in the mass composition of primary cosmic rays (PCRs) in the energy range of 10^15-10^17 eV are presented. Adjustments are made according to detailed calculations of the response of the X-ray emulsion chamber in the PAMIR experiment. It is demonstrated that the fraction of protons in a PCR is 16-18% for E0 = 10^15-10^16 eV and does not change within the error for E0 = 10^16-10^17 eV.
Abstract: 1804.07140
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Title: A Light Curve Analysis of Gamma-ray Nova V959 Mon -- Distance and White Dwarf Mass
(Submitted on 19 Apr 2018)
Abstract: V959 Mon is one of the gamma-ray detected novae. It was optically discovered about 50 days after the gamma-ray detection due to proximity to the Sun. The nova speed class is unknown because of lack of the earliest half of optical light curve and short supersoft X-ray phase due to eclipse by the disk rim. Using the universal decline law and time-stretching method, we analyzed the data of V959 Mon and obtained nova parameters. We estimated the distance modulus in the V band to be (m-M)_V=13.15\pm0.3 for the reddening of E(B-V)=0.38\pm0.01 by directly comparing with the similar type of novae, LV Vul, V1668 Cyg, IV Cep, and V1065 Cen. The distance to V959 Mon is 2.5\pm0.5 kpc. If we assume that the early phase light curve of V959 Mon is the same as that of time-stretched light curves of LV Vul, our model light curve fitting suggests that the white dwarf (WD) mass is 0.9-1.15 M_sun, being consistent with a neon nova identification. At the time of gamma-ray detection the photosphere of nova envelope extends to 5-8 R_sun (about two or three times the binary separation) and the wind mass-loss rate is (3-4)\times 10^{-5} M_sun yr^{-1}. The period of hard X-ray emission is consistent with the time of appearance of the companion star from the nova envelope. The short supersoft X-ray turnoff time is consistent with the epoch when the WD photosphere shrank to behind the elevating disk rim, that occurs 500 days before nuclear burning turned off.
Abstract: 1804.06908
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Title: The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics
(Submitted on 18 Apr 2018)
Abstract: Using visible, radio, microwave, and sub-mm data, we study several lines of sight toward stars generally closer than 1 kpc on a component-by-component basis. We derive the component structure seen in absorption at visible wavelengths from Ca II, Ca I, K I, CH, CH$^{+}\!,$ and CN and compare it to emission from H I, CO and its isotopologues, and C$^{+}$ from the GOT C+ survey. The correspondence between components in emission and absorption help create a more unified picture of diffuse atomic and molecular gas in the interstellar medium. We also discuss how these tracers are related to the CO-dark H$_{2}$ gas probed by C$^{+}$ emission and discuss the kinematic connections among the species observed.
Abstract: 1804.06846
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Title: An Unexpected Dip in the Solar Gamma-Ray Spectrum
(Submitted on 18 Apr 2018)
Abstract: The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV up to $\gtrsim200$ GeV. The overall gamma-ray spectrum is much harder ($\sim E_{\gamma}^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\rm CR}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1-10 GeV. Surprisingly, we observe a spectral dip between $\sim$30-50 GeV in an otherwise power-law spectrum. This was not predicted, is not understood, and may provide crucial clues to the gamma-ray emission mechanism. The flux above 100 GeV, which is brightest during the solar minimum, poses exciting opportunities for HAWC, LHAASO, IceCube, and KM3NeT.
Abstract: 1804.06714
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Full Text: [ PostScript, PDF]
Title: Discovery of a jet-like structure with overionized plasma in the SNR IC443
(Submitted on 17 Apr 2018)
Abstract: IC443 is a supernova remnant located in a quite complex environment since it interacts with nearby clouds. Indications for the presence of overionized plasma have been found though the possible physical causes of overionization are still debated. Moreover, because of its peculiar position and proper motion, it is not clear if the pulsar wind nebula (PWN) within the remnant is the relic of the IC443 progenitor star or just a rambling one seen in projection on the remnant. Here we address the study of IC443 plasma in order to clarify the relationship PWN-remnant, the presence of overionization and the origin of the latter. We analyzed two \emph{XMM-Newton} observations producing background-subtracted, vignetting-corrected and mosaicked images in two different energy bands and we performed a spatially resolved spectral analysis of the X-ray emission. We identified an elongated (jet-like) structure with Mg-rich plasma in overionization. The head of the jet is interacting with a molecular cloud and the jet is aligned with the position of the PWN at the instant of the supernova explosion. Interestingly, the direction of the jet of ejecta is somehow consistent with the direction of the PWN jet. Our discovery of a jet of ejecta in IC443 enlarge the sample of core-collapse SNRs with collimated ejecta structures. IC443's jet is the first one which shows overionized plasma, possibly associated with the adiabatic expansion of ejecta. The match between the jet's direction and the original position of the PWN strongly supports the association between the neutron star and IC443.
Abstract: 1804.06603
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Full Text: [ PostScript, PDF]
Title: Ultrahigh energy cosmic rays from nearby starburst galaxies
(Submitted on 18 Apr 2018)
Abstract: Ultrahigh energy cosmic rays are the most energetic of any subatomic particles ever observed in nature. The quest for their mysterious origin is currently a major scientific challenge. Here we explore the possibility that these particles originate from nearby starburst galaxies, a scenario that matches the recent observation by the Telescope Array experiment of a cosmic-ray hotspot above 57~EeV not far from the direction of the starburst galaxy M82. Specifically, we study the stochastic propagation in space of ultrahigh energy cosmic rays through the state-of-the-art simulation framework CRPropa~3, taking into account all relevant particle interactions as well as deflections by the intervening magnetic fields. To ensure a comprehensive understanding of this model, we consider the energy spectrum, the cosmogenic neutrinos and gamma rays, and the distribution of arrival directions. The starburst galaxy scenario reproduces well observations from both the Telescope Array and Pierre Auger Observatories, making it very attractive for explaining the origin of cosmic rays at the highest energies.
Abstract: 1804.06430
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Title: Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge
(Submitted on 17 Apr 2018)
Abstract: We report the discovery of a nearby dwarf galaxy in the constellation of Hydrus, between the Large and the Small Magellanic Clouds. Hydrus 1 is a mildy elliptical ultra-faint system with luminosity $M_V\sim$ -4.7 and size $\sim$ 50 pc, located 28 kpc from the Sun and 24 kpc from the LMC. From spectroscopy of $\sim$ 30 member stars, we measure a velocity dispersion of 2.7 km/s and find tentative evidence for a radial velocity gradient consistent with 3 km/s rotation. Hydrus 1's velocity dispersion indicates that the system is dark matter dominated, but its dynamical mass-to-light ratio M/L $\sim$ 66 is significantly smaller than typical for ultra-faint dwarfs at similar luminosity. The kinematics and spatial position of Hydrus~1 make it a very plausible member of the family of satellites brought into the Milky Way by the Magellanic Clouds. While Hydrus 1's proximity and well-measured kinematics make it a promising target for dark matter annihilation searches, we find no evidence for significant gamma-ray emission from Hydrus 1. The new dwarf is a metal-poor galaxy with a mean metallicity [Fe/H]=-2.5 and [Fe/H] spread of 0.4 dex, similar to other systems of similar luminosity. Alpha-abundances of Hyi 1 members indicate that star-formation was extended, lasting between 0.1 and 1 Gyr, with self-enrichment dominated by SN Ia. The dwarf also hosts a highly carbon-enhanced extremely metal-poor star with [Fe/H] $\sim$ -3.2 and [C/Fe] $\sim$ +3.0.
Abstract: 1804.06093
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Full Text: [ PostScript, PDF]
Title: Very high-energy gamma-ray signature of ultrahigh-energy cosmic-ray acceleration in Centaurus A
(Submitted on 17 Apr 2018)
Abstract: The association of at least a dozen ultrahigh-energy cosmic-ray (UHECR) events with energy $\gtrsim 55$ EeV detected by the Pierre Auger Observatory (PAO) from the direction of Centaurus-A, the nearest radio galaxy, supports the scenario of UHECR acceleration in the jets of radio galaxies. In this work, we model radio to very high energy (VHE,$\gtrsim 100$ \rm GeV) $\gamma$-ray emission from Cen A, including GeV hardness detected by Fermi-LAT and TeV emission detected by HESS. We consider two scenarios: (i) Two zone synchrotron self-Compton (SSC) and external-Compton (EC) models, (ii) Two zone SSC, EC and photo-hadronic emission from cosmic ray interactions. The GeV hardness observed by Fermi-LAT can be explained using these two scenarios, where zone 2 EC emission is very important. Hadronic emission in scenario (ii) can explain VHE data with the same spectral slope as obtained through fitting UHECRs from Cen A. The peak luminosity in cosmic ray proton at 1 TeV, to explain the VHE $\gamma$-ray data is $\approx 2.5 \times 10^{46}$ erg/s. The bolometric luminosity in cosmic ray protons is consistent with the luminosity required to explain the origin of 13 UHECR signal events that are correlated with Cen A.
Abstract: 1804.05877
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Full Text: [ PostScript, PDF]
Title: Constraining cosmic ray acceleration in young star clusters using multi-wavelength observations
(Submitted on 16 Apr 2018)
Abstract: We use $1$D and $3$D two-fluid cosmic ray (CR) hydrodynamic simulations to investigate the role of CRs in the vicinity of a compact young star cluster. We model a self-gravitating cloud (density profile $\rho \propto r^{-1}$), include important thermal and non-thermal processes, and explore two different CR injection scenarios. We show that if internal shocks in the wind-driving region are the main site for CR acceleration, then the resulting $\gamma$-ray luminosity ($L_{\rm \gamma}$) can reach $\approx 5\%$ of the mechanical luminosity ($L_{\rm w}$), independent of the fraction of wind energy ($\sim 1-20\%$) injected into CRs. In contrast, if the forward/reverse shock of a bubble is the injection site then $L_{\rm \gamma}$ increases linearly with the CR injection fraction, as expected analytically. We find that the X-ray luminosity ($L_{\rm x}$) in the forward/reverse shock injection scenario is $\gtrsim 10^{-3} L_{\rm w}$, which is $\sim 10$ times larger than in the central wind-driving injection case. We predict the corresponding range of the synchrotron radio luminosity. We show how multi-wavelength observations can constrain the CR parameters. Comparing the predicted multi-wavelength luminosities with those of 30 Doradus we identify the reverse shock as the most probable CR injection site, and that thermal conduction is important. We do not find significant dynamical impact of CRs in our models.
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