Diffuse Galactic Emission Research Articles

Multimessenger study of the Galactic diffuse emission with LHAASO and IceCube observations

Multimessenger study of the Galactic diffuse emission with LHAASO and IceCube observations

Improved Galactic diffuse emission model strengthens the case for a Millisecond Pulsar explanation of the Fermi GeV excess

After more than a decade since its discovery, the Galactic center gamma-ray excess - discovered with the Fermi Large Area Telescope - remains puzzling. While the spectrum of the signal can be explained by either dark matter or an unresolved population of millisecond pulsars, the spatial morphology of this excess seems to hold the key to separate the two theories. In this contribution, we present the results of a recent study in which we use bleeding edge models for interstellar gas, inverse Compton emission, and stellar mass models to reanalyze the Galactic center excess. We find that the spatial morphology of the excess is highly correlated with stellar matter in the Galactic bulge, providing strong support for the millisecond pulsar hypothesis.

Effects of cosmic rays’ self-modulation on the galactic diffuse gamma-ray emission

We studied how the process of self-modulation of cosmic rays, occurring upon their penetration into dense molecular clouds, affects the total gamma-ray emission of the Galaxy. We estimated how the self-modulation modifies the emission from each individual cloud and integrate the results along the line of sight for a given area in the sky. Our calculations show that the self-modulation reduces the total intensity of gamma-ray emission below 1 GeV by about 10–30%. Even though the magnitude of the effect is not large, it still can substantially affect the background gamma-ray emission at low energies.

Diffuse sources, clustering, and the excess anisotropy of the radio synchrotron background

ABSTRACT We present the largest low frequency (120 MHz) arcminute resolution image of the radio synchrotron background (RSB) to date, and its corresponding angular power spectrum of anisotropies (APS) with angular scales ranging from 3° to 0.3 arcmin. We show that the RSB around the north celestial pole has a significant excess anisotropy power at all scales over a model of unclustered point sources based on source counts of known source classes. This anisotropy excess, which does not seem attributable to the diffuse Galactic emission, could be linked to the surface brightness excess of the RSB. To better understand the information contained within the measured APS, we model the RSB varying the brightness distribution, size, and angular clustering of potential sources. We show that the observed APS could be produced by a population of faint clustered point sources only if the clustering is extreme and the size of the Gaussian clusters is ≲1 arcmin. We also show that the observed APS could be produced by a population of faint diffuse sources with sizes ≲1 arcmin, and this is supported by features present in our image. Both of these cases would also cause an associated surface brightness excess. These classes of sources are in a parameter space not well probed by even the deepest radio surveys to date.

Updated constraints on primordial black hole evaporation

The Hawking evaporation process, leading to the production of detectable particle species, constrains the abundance of light black holes, presumably of primordial origin. Here, we reconsider and correct constraints from soft gamma-ray observations, including of the gamma-ray line, at 511 keV, produced by electron-positron pair-annihilation, where positrons originate from black hole evaporation. First, we point out that the INTEGRAL detection of the Large Magellanic Cloud provides one of the strongest bounds attainable with present observations; and that future MeV gamma-ray telescopes, such as GECCO, will greatly enhance such constraints. Second, we discuss issues with previous limits from the isotropic flux at 511 keV and we provide updated, robust constraints from recent measurements of the diffuse Galactic soft gamma-ray emission and from the isotropic soft gamma-ray background.

Correction to: The C-Band All-Sky Survey (C-BASS): template fitting of diffuse galactic microwave emission in the northern sky

Correction to: The C-Band All-Sky Survey (C-BASS): template fitting of diffuse galactic microwave emission in the northern sky

On the use of temporal filtering for mitigating galactic synchrotron calibration bias in 21 cm reionization observations

ABSTRACT Precision antenna calibration is required for mitigating the impact of foreground contamination in 21 cm cosmological radio surveys. One widely studied source of error is the effect of missing point sources in the calibration sky model; however, poorly understood diffuse galactic emission also creates a calibration bias that can complicate the clean separation of foregrounds from the 21 cm signal. In this work, we present a technique for suppressing this bias with temporal filtering of radio interferometric visibilities observed in a drift-scan mode. We demonstrate this technique on mock simulations of the Hydrogen Epoch of Reionization Array (HERA) experiment. Inspecting the recovered calibration solutions, we find that our technique reduces spurious errors by over an order of magnitude. This improved accuracy approaches the required accuracy needed to make a fiducial detection of the 21 cm signal with HERA, but is dependent on a number of external factors that we discuss. We also explore different types of temporal filtering techniques and discuss their relative performance and trade-offs.

On the morphology of the gamma-ray galactic centre excess

ABSTRACT The characteristics of the galactic centre excess (GCE) emission observed in gamma-ray energies – especially the morphology of the GCE – remain a hotly debated subject. The manner in which the dominant diffuse gamma-ray background is modelled has been claimed to have a determining effect on the preferred morphology. In this work, we compare two distinct approaches to the galactic diffuse gamma-ray emission background: the first approach models this emission through templates calculated from a sequence of well-defined astrophysical assumptions, while the second approach divides surrogates for the background gamma-ray emission into cylindrical galactocentric rings with free independent normalizations. At the latitudes that we focus on, we find that the former approach works better, and that the overall best fit is obtained for an astrophysically motivated fit when the GCE follows the morphology expected of dark matter annihilation. Quantitatively, the improvement compared with the best ring-based fits is roughly 6500 in the χ2 and roughly 4000 in the log of the Bayesian evidence.

QUIJOTE scientific results – V. The microwave intensity and polarization spectra of the Galactic regions W49, W51 and IC443

ABSTRACT We present new intensity and polarization maps obtained with the QUIJOTE experiment towards the Galactic regions W49, W51 and IC443, covering the frequency range from 10 to 20 GHz at $\sim 1\, \text{deg}$ angular resolution, with a sensitivity in the range 35–79 $\mu \text{K}\, \text{beam}^{-1}$ for total intensity and 13–23 $\mu \text{K}\, \text{beam}^{-1}$ for polarization. For each region, we combine QUIJOTE maps with ancillary data at frequencies ranging from 0.4 to 3000 GHz, reconstruct the spectral energy distribution and model it with a combination of known foregrounds. We detect anomalous microwave emission (AME) in total intensity towards W49 at 4.7σ and W51 at 4.0σ with peak frequencies $\nu _{\rm AME}=(20.0\pm 1.4)\, \text{GHz}$ and $\nu _{\rm AME}=(17.7\pm 3.6)\, \text{GHz}$, respectively; this is the first detection of AME towards W51. The contamination from ultracompact H ii regions to the residual AME flux density is estimated at 10 per cent in W49 and 5 per cent in W51, and does not rule out the AME detection. The polarized SEDs reveal a synchrotron contribution with spectral indices αs = −0.67 ± 0.10 in W49 and αs = −0.51 ± 0.07 in W51, ascribed to the diffuse Galactic emission and to the local supernova remnant, respectively. Towards IC443 in total intensity we measure a broken power-law synchrotron spectrum with cut-off frequency $\nu _{\rm 0,s}=(114\pm 73)\, \text{GHz}$, in agreement with previous studies; our analysis, however, rules out any AME contribution which had been previously claimed towards IC443. No evidence of polarized AME emission is detected in this study.

MeV Gamma-Ray Source Contribution to the Inner Galactic Diffuse Emission

The origin of the inner Galactic emission, measured by COMPTEL with a flux of ∼10−2 MeV cm−2 s−1 sr−1 in the 1–30 MeV energy range from the inner Galactic region, has remained unsettled since its discovery. In this paper, we elaborate on a model of individual MeV gamma-ray sources unresolved by COMPTEL. This is conducted for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources between the two catalogs. We find that the source contribution to the COMPTEL emission would be at least ∼20%. Combined with the Galactic diffuse emission, which is not well constrained, the COMPTEL emission can be roughly reproduced in some cases.

The steady-state multi-TeV diffuse γ-ray emission predicted with galprop and prospects for the Cherenkov Telescope Array

ABSTRACT Cosmic rays (CRs) interact with the diffuse gas, radiation, and magnetic fields in the interstellar medium (ISM) to produce electromagnetic emissions that are a significant component of the all-sky flux across a broad wavelength range. The Fermi–Large Area Telescope (LAT) has measured these emissions at GeV γ-ray energies with high statistics. Meanwhile, the high-energy stereoscopic system (H.E.S.S.) telescope array has observed large-scale Galactic diffuse emission in the TeV γ-ray energy range. The emissions observed at GeV and TeV energies are connected by the common origin of the CR particles injected by the sources, but the energy dependence of the mixture from the general ISM (true ‘diffuse’), those emanating from the relatively nearby interstellar space about the sources, and the sources themselves, is not well understood. In this paper, we investigate predictions of the broad-band emissions using the galprop code over a grid of steady-state 3D models that include variations over CR sources, and other ISM target distributions. We compare, in particular, the model predictions in the very-high energy ( VHE; ≳100 GeV) γ-ray range with the H.E.S.S. Galactic plane survey (HGPS) after carefully subtracting emission from catalogued γ-ray sources. Accounting for the unresolved source contribution, and the systematic uncertainty of the HGPS, we find that the galprop model predictions agree with lower estimates for the HGPS source-subtracted diffuse flux. We discuss the implications of the modelling results for interpretation of data from the next generation Cherenkov Telescope Array (CTA).

Structure in the Magnetic Field of the Milky Way Disk and Halo Traced by Faraday Rotation

Magnetic fields in the ionized medium of the disk and halo of the Milky Way impose Faraday rotation on linearly polarized radio emission. We compare two surveys mapping the Galactic Faraday rotation, one showing the rotation measures of extragalactic sources seen through the Galaxy, and one showing Faraday depth of the diffuse Galactic synchrotron emission from the Global Magneto-Ionic Medium Survey. Comparing the two data sets in 5° × 10° bins shows good agreement at intermediate latitudes, 10° < ∣b∣ < 50°, and little correlation between them at lower and higher latitudes. Where they agree, both tracers show clear patterns as a function of Galactic longitude, ℓ; in the northern hemisphere a strong pattern; and in the southern hemisphere a pattern. Pulsars with height above or below the plane ∣z∣ > 300 pc show similar ℓ dependence in their rotation measures. Nearby nonthermal structures show rotation measure shadows as does the Orion–Eridanus superbubble. We describe families of dynamo models that could explain the observed patterns in the two hemispheres. We suggest that a field reversal, known to cross the plane a few hundred parsecs inside the solar circle, could shift to positive z with increasing Galactic radius to explain the pattern in the northern hemisphere. Correlation shows that rotation measures from extragalactic sources are one to two times the corresponding rotation measure of the diffuse emission, implying Faraday complexity along some lines of sight, especially in the southern hemisphere.

Cosmic-ray propagation under consideration of a spatially resolved source distribution

Cosmic rays (CRs) are an integral component of the interstellar medium, producing broadband emission while interacting with other Galactic matter components like the interstellar gas or magnetic fields. In addition to observations, numerical simulations of CR propagation through the Galaxy help to increase the level of understanding of Galactic CR transport and diffuse γ-ray emission as seen by different experiments. Up to now, the standard approach at modelling source distributions used as input for such transport simulations often rely on radial symmetry and analytical functions rather than individual, observation-based sources. We aim at a redefinition of existing CR source distributions by combining sources observed with the H.E.S.S. experiment and simulated random sources, which follow the matter density in the Milky Way. As a result, H.E.S.S.-inspired Galactic CR source distributions are inferred.We use the PICARD code to perform 3D-simulations of nuclei and electrons in CR propagation using our hybrid source distribution models. Furthermore, also gamma-ray maps and spectra, simulated with the redefined source models, are evaluated in different regions in the Galaxy and compared with each other to determine the statistical scatter of the underlying distributions. We find global consistency between our models and in comparison to previous simulations, with only some localised fluctuations, e.g. in the spiral arms.This implementation of a three-dimensional source model based on observations and simulations enables a new quality of propagation modelling. It offers possibilities for more realistic CR transport scenarios beyond radial symmetry and delivers meaningful results in both the arm and interarm regions of the Galaxy. This gives a more realistic picture of the Galactic γ-ray sky by including structures from the source model and not just the gas distributions.

The physics of pulsar halos: Research progress and prospect

Diffusive TeV gamma-ray emissions have been recently discovered extending beyond the pulsar wind nebulae of a few middle-aged pulsars, implying that energetic electron/ positron pairs are escaping from the pulsar wind nebulae and radiating in the ambient interstellar medium. It has been suggested that these extended emissions constitute a distinct class of nonthermal sources, termed “pulsar halos”. In this paper, I will review the research progress on pulsar halos and discuss our current understanding on their physics, including the multiwavelength observations, different theoretical models, as well as implications for the origin of cosmic-ray positron excess and Galactic diffuse gamma-ray emission.

Very massive star winds as sources of the short-lived radioactive isotope 26Al

Context. The 26Al short-lived radioactive nuclide is the source of the observed galactic diffuse γ-ray emission at 1.8 MeV. While different sources of 26Al have been explored, such as asymptotic giant branch stars, massive stellar winds, and supernovae, the contribution of very massive stars has not been studied so far. Aims. We study the contribution of the stellar wind of very massive stars, here, stars with initial masses between 150 and 300 M⊙, to the enrichment in 26Al of the galactic interstellar medium. Methods. We studied the production of 26Al by studying rotating and non-rotating very massive stellar models with initial masses between 150 and 300 M⊙ for metallicities Z = 0.006, 0.014, and 0.020. We compared this result to a simple Milky Way model and took the metallicity and the star formation rate gradients into account. Results. We obtain that very massive stars in the Z = 0.006 − 0.020 metallicity range might be very significant contributors to the 26Al enrichment of the interstellar medium. Typically, the contribution of the winds of massive stars to the total quantity of 26Al in the Galaxy increases by 150% when very massive stars are considered. Conclusions. Despite their rarity, very massive stars might be important contributors to 26Al and might overall be very important actors for nucleosynthesis in the Galaxy.

Gamma-ray light curves and spectra of classical novae

ABSTRACT The nucleosynthesis in classical novae, in particular that of radioactive isotopes, is directly measurable by its γ-ray signature. Despite decades of observations, MeV γ-rays from novae have never been detected – neither individually at the time of the explosion, nor as a result of radioactive decay, nor the diffuse Galactic emission from the nova population. Thanks to recent developments in modelling of instrumental background for MeV telescopes such as INTEGRAL/SPI and Fermi/GBM, the prospects to finally detect these elusive transients are greatly enhanced. This demands for updated and refined models of γ-ray spectra and light curves of classical novae. In this work, we develop numerical models of nova explosions using sub- and near-Chandrasekhar CO white dwarfs as the progenitor. We study the parameter dependence of the explosions, their thermodynamics and energetics, as well as their chemical abundance patterns. We use a Monte Carlo radiative transfer code to compute γ-ray light curves and spectra, with a focus on the early time evolution. We compare our results to previous studies and find that the expected 511-keV-line flash at the time of the explosion is heavily suppressed, showing a maximum flux of only $10^{-9}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$ and thus making it at least one million times fainter than estimated before. This finding would render it impossible for current MeV instruments to detect novae within the first day after the outburst. Nevertheless, our time-resolved spectra can be used for retrospective analyses of archival data, thereby improving the sensitivity of the instruments.

Degree-scale galactic radio emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC

Aims. Contamination from bright diffuse Galactic thermal and non-thermal radio emission poses crucial challenges in experiments aiming to measure the 21-cm signal of neutral hydrogen from the cosmic dawn (CD) and Epoch of Reionisation (EoR). If not included in calibration, this diffuse emission can severely impact the analysis and signal extraction in 21-cm experiments. We examine large-scale diffuse Galactic emission at 122 MHz around the North Celestial Pole, using the Amsterdam-ASTRON Radio Transient Facility and Analysis Centre (AARTFAAC-) High Band Antenna (HBA) system. Methods. In this pilot project, we present the first-ever wide-field image produced with a single sub-band of the data recorded with the AARTFAAC-HBA system. We demonstrate two methods, multi-scale CLEAN and shapelet decomposition, to model the diffuse emission revealed in the image. We used angular power spectrum metrics to quantify different components of the emission and compared the performance of the two diffuse structure modelling approaches. Results. We observed that the point sources dominate the angular power spectrum (ℓ(ℓ + 1)Cℓ/2π≡Δ2(ℓ)) of the emission in the field on scales of ℓ ≳ 60 (≲3 degree). The angular power spectrum after subtraction of compact sources is flat within the 20 ≲ ℓ ≲ 200 range, suggesting that the residual power is dominated by the diffuse emission on scales of ℓ ≲ 200. The residual diffuse emission has a brightness temperature variance of Δℓ=1802 = (145.64 ± 13.61) K2 at 122 MHz on angular scales of 1 degree, and it is consistent with a power law following Cℓ ∝ ℓ−2.0 in the 20 ≲ ℓ ≲ 200 range. We also find that, in the current set-up, multi-scale CLEAN is suitable to model the compact and diffuse structures on a wide range of angular scales, whereas the shapelet decomposition method better models the large scales, which are of the order of a few degrees and wider.

Return of the templates: Revisiting the Galactic Center excess with multimessenger observations

The Galactic center excess (GCE) remains one of the most intriguing discoveries from the Fermi Large Area Telescope (LAT) observations. We revisit the characteristics of the GCE by first producing a new set of high-resolution galactic diffuse gamma-ray emission templates. This diffuse emission, which accounts for the bulk of the observed gamma rays, is ultimately due to cosmic-ray interactions with the interstellar medium. Using recent high-precision cosmic-ray observations, in addition to the continuing Fermi-LAT observations and observations from lower energy photons, we constrain the properties of the galactic diffuse emission. We describe a large set of diffuse gamma-ray emission templates which account for a very wide range of initial assumptions on the physical conditions in the inner galaxy. The broad properties of the GCE that we find in this work are qualitatively unchanged despite the introduction of this new set of templates, though its quantitative features appear mildly different than those obtained in previous analyses. In particular, we find a high-energy tail at higher significance than previously reported. This tail is very prominent in the northern hemisphere, and less so in the southern hemisphere. This strongly affects one prominent interpretation of the excess: known millisecond pulsars are incapable of producing this high-energy emission, even in the relatively softer southern hemisphere, and are therefore disfavored as the sole explanation of the GCE. The annihilation of dark matter particles of mass ${40}_{\ensuremath{-}7}^{+10}\text{ }\text{ }\mathrm{GeV}$ (95% CL) to $b$ quarks with a cross-section of $⟨{\ensuremath{\sigma}}_{A}v⟩=1.{4}_{\ensuremath{-}0.3}^{+0.6}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}26}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ provides a good fit to the excess especially in the relatively cleaner southern sky. Dark matter of the same mass range annihilating to $b$ quarks or heavier dark matter particles annihilating to heavier Standard Model bosons can combine with millisecond pulsars to provide a good fit to the southern hemisphere emission as well, as can a broken power-law spectrum which would be related to recent cosmic-ray burst activity. As part of this paper, we make publicly available all of our templates and the data covariance matrix we have generated to account for systematic uncertainties.

The C-Band All-Sky Survey (C-BASS): template fitting of diffuse galactic microwave emission in the northern sky

ABSTRACTThe C-Band All-Sky Survey (C-BASS) has observed the Galaxy at 4.76 GHz with an angular resolution of 0${_{.}^{\circ}}$73 full-width half-maximum, and detected Galactic synchrotron emission with high signal-to-noise ratio over the entire northern sky (δ &amp;gt; −15○). We present the results of a spatial correlation analysis of Galactic foregrounds at mid-to-high (b &amp;gt; 10○) Galactic latitudes using a preliminary version of the C-BASS intensity map. We jointly fit for synchrotron, dust, and free–free components between 20 and 1000 GHz and look for differences in the Galactic synchrotron spectrum, and the emissivity of anomalous microwave emission (AME) when using either the C-BASS map or the 408-MHz all-sky map to trace synchrotron emission. We find marginal evidence for a steepening (&amp;lt;Δβ&amp;gt; = −0.06 ± 0.02) of the Galactic synchrotron spectrum at high frequencies resulting in a mean spectral index of &amp;lt;β&amp;gt; = −3.10 ± 0.02 over 4.76–22.8 GHz. Further, we find that the synchrotron emission can be well modelled by a single power law up to a few tens of GHz. Due to this, we find that the AME emissivity is not sensitive to changing the synchrotron tracer from the 408-MHz map to the 4.76-GHz map. We interpret this as strong evidence for the origin of AME being spinning dust emission.

Galactic gamma-ray and neutrino emission from interacting cosmic-ray nuclei

We present a study of the expectations for very-high-energy (VHE) to ultra-high-energy (UHE) gamma-ray and neutrino emission from interacting cosmic rays in our Galaxy as well as a comparison to the latest results for the Galactic UHE diffuse emission. We demonstrate the importance of properly accounting for both the mixed cosmic-ray composition and the gamma-ray absorption. We adopt the wounded-nucleon model of nucleus interactions and provide parameterisations of the resulting gamma-ray and neutrino production. Nucleon shielding due to clustering inside nuclei is shown to have a measurable effect on the production of gamma rays and is particularly evident close to breaks and cutoffs in mixed-composition particle spectra. The change in composition around the ‘knee’ in the cosmic ray spectrum has a noticeable impact on the diffuse neutrino and gamma-ray emission spectra. We show that current and near-future detectors can probe these differences in the key energy range from 10 TeV to 1 PeV, testing the paradigm of the universality of the cosmic ray spectrum and composition throughout the Galaxy.