Galactic Emission Research Articles

One-point fluctuation analysis of IceCube neutrino events outlines a significant unassociated isotropic component and constrains the Galactic contribution

The origins of the extraterrestrial neutrinos observed in IceCube have yet to be determined. In this study we perform a one-point fluctuation analysis of the six-year high-energy starting event (HESE) shower data, with fixed non-Poissonian contributions from atmospheric, Galactic and some extragalactic components, as well as an isotropic (and weakly non-Poissonian) template. In addition to the star-forming galaxies and blazars, our analysis suggests the presence of an additional isotropic component, not associated with any known class of sources, with best-fit intensity of $(2.8\pm0.2)\times 10^{-18}\,(E/100~{\rm TeV})^{-2.7\pm 0.5}$ ${\mathrm{cm}^{-2}\, \mathrm{s}^{-1}\,\mathrm{sr}^{-1}\,\mathrm{GeV}^{-1}}$. For the first time, we also consider high-energy extrapolations of several phenomenological models for the diffuse Galactic emission (tuned to both local cosmic-ray data and diffuse gamma-ray emission in the GeV-TeV domain). We demonstrate the potential of our framework in discriminating between different scenarios, with possible implications on the physics of cosmic ray transport in the TeV-PeV range.

Statistical 21-cm Signal Separation via Gaussian Process Regression Analysis

Detecting and characterizing the Epoch of Reionization and Cosmic Dawn via the redshifted 21-cm hyperfine line of neutral hydrogen will revolutionize the study of the formation of the first stars, galaxies, black holes and intergalactic gas in the infant Universe. The wealth of information encoded in this signal is, however, buried under foregrounds that are many orders of magnitude brighter. These must be removed accurately and precisely in order to reveal the feeble 21-cm signal. This requires not only the modeling of the Galactic and extra-galactic emission, but also of the often stochastic residuals due to imperfect calibration of the data caused by ionospheric and instrumental distortions. To stochastically model these effects, we introduce a new method based on `Gaussian Process Regression' (GPR) which is able to statistically separate the 21-cm signal from most of the foregrounds and other contaminants. Using simulated LOFAR-EoR data that include strong instrumental mode-mixing, we show that this method is capable of recovering the 21-cm signal power spectrum across the entire range $k = 0.07 - 0.3 \ \rm{h\, cMpc^{-1}}$. The GPR method is most optimal, having minimal and controllable impact on the 21-cm signal, when the foregrounds are correlated on frequency scales $\gtrsim 3$\,MHz and the rms of the signal has $\sigma_{\mathrm{21cm}} \gtrsim 0.1\,\sigma_{\mathrm{noise}}$. This signal separation improves the 21-cm power-spectrum sensitivity by a factor $\gtrsim 3$ compared to foreground avoidance strategies and enables the sensitivity of current and future 21-cm instruments such as the {\sl Square Kilometre Array} to be fully exploited.

Exploring cosmic origins with CORE: Gravitational lensing of the CMB

Lensing of the cosmic microwave background (CMB) is now a well-developed probe of the clustering of the large-scale mass distribution over a broad range of redshifts. By exploiting the non-Gaussian imprints of lensing in the polarization of the CMB, the CORE mission will allow production of a clean map of the lensing deflections over nearly the full-sky. The number of high-S/N modes in this map will exceed current CMB lensing maps by a factor of 40, and the measurement will be sample-variance limited on all scales where linear theory is valid. Here, we summarise this mission product and discuss the science that will follow from its power spectrum and the cross-correlation with other clustering data. For example, the summed mass of neutrinos will be determined to an accuracy of 17 meV combining CORE lensing and CMB two-point information with contemporaneous measurements of the baryon acoustic oscillation feature in the clustering of galaxies, three times smaller than the minimum total mass allowed by neutrino oscillation measurements. Lensing has applications across many other science goals of CORE, including the search for B-mode polarization from primordial gravitational waves. Here, lens-induced B-modes will dominate over instrument noise, limiting constraints on the power spectrum amplitude of primordial gravitational waves. With lensing reconstructed by CORE, one can “delens” the observed polarization internally, reducing the lensing B-mode power by 60 %. This can be improved to 70 % by combining lensing and measurements of the cosmic infrared background from CORE, leading to an improvement of a factor of 2.5 in the error on the amplitude of primordial gravitational waves compared to no delensing (in the null hypothesis of no primordial B-modes). Lensing measurements from CORE will allow calibration of the halo masses of the tens of thousands of galaxy clusters that it will find, with constraints dominated by the clean polarization-based estimators. The 19 frequency channels proposed for CORE will allow accurate removal of Galactic emission from CMB maps. We present initial findings that show that residual Galactic foreground contamination will not be a significant source of bias for lensing power spectrum measurements with CORE.

Galactic bulge preferred over dark matter for the Galactic centre gamma-ray excess

An anomalous gamma-ray excess emission has been found in Fermi Large Area Telescope data covering the centre of the Galaxy. Several theories have been proposed for this `Galactic Centre Excess'. They include self-annihilation of dark matter particles, an unresolved population of millisecond pulsars, an unresolved population of young pulsars, or a series of burst events. Here we report on a new analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic Centre Excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic Centre Excess is not a dark matter phenomenon but rather associated with the stellar population of the Galactic bulge and nuclear bulge.

Hitomi observation of radio galaxy NGC 1275: The first X-ray microcalorimeter spectroscopy of Fe-Kα line emission from an active galactic nucleus

Abstract The origin of the narrow Fe-Kα fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In 2016 February–March, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) on board the Hitomi satellite of the Fanaroff–Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high-energy resolution of ∼5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-Kα line with ∼5.4 σ significance. The velocity width is constrained to be 500–1600 km s−1 (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus up to ∼20 keV, giving an equivalent width of ∼20 eV for the 6.4 keV line. Because the velocity width is narrower than that of the broad Hα line of ∼2750 km s−1, we can exclude a large contribution to the line flux from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-Kα line comes from a region within ∼1.6 kpc of the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-Kα line from NGC 1275 is likely a low-covering-fraction molecular torus or a rotating molecular disk which probably extends from a parsec to hundreds of parsecs scale in the active galactic nucleus system.

Where is OH and Does It Trace the Dark Molecular Gas (DMG)?

Abstract Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H2 under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1–0 transitions of 12CO, 13CO, and C18O, and the J = 2–1 transition of 12CO. The excitation temperatures of OH were found to follow a modified lognormal distribution the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (A V ∼ 0.05–2 mag), the detection rate of OH with a detection limit N(OH) ≃ 1012 cm−2 is approximately independent of A V . We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of “dark molecular gas (DMG).” The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.

Polarization leakage in epoch of reionization windows – III. Wide-field effects of narrow-field arrays

Leakage of polarized Galactic diffuse emission into total intensity can potentially mimic the 21-cm signal coming from the epoch of reionization (EoR), as both of them might have fluctuating spectral structure. Although we are sensitive to the EoR signal only in small fields of view, chromatic sidelobes from further away can contaminate the inner region. Here, we explore the effects of leakage into the 'EoR window' of the cylindrically averaged power spectra (PS) within wide fields of view using both observation and simulation of the 3C196 and NCP fields, two observing fields of the LOFAR-EoR project. We present the polarization PS of two one-night observations of the two fields and find that the NCP field has higher fluctuations along frequency, and consequently exhibits more power at high-$k_\parallel$ that could potentially leak to Stokes $I$. Subsequently, we simulate LOFAR observations of Galactic diffuse polarized emission based on a model to assess what fraction of polarized power leaks into Stokes $I$ because of the primary beam. We find that the rms fractional leakage over the instrumental $k$-space is $0.35\%$ in the 3C196 field and $0.27\%$ in the NCP field, and it does not change significantly within the diameters of $15^\circ$, $9^\circ$ and $4^\circ$. Based on the observed PS and simulated fractional leakage, we show that a similar level of leakage into Stokes $I$ is expected in the 3C196 and NCP fields, and the leakage can be considered to be a bias in the PS.

An Empirical Correlation of Tmax–MWD of Dwarf Novae and the Average White Dwarf Mass in Cataclysmic Variables in the Galactic Bulge

Abstract The mean white dwarf (WD) mass in the Galactic bulge cataclysmic variables (CVs) was measured by applying the shock temperature-WD mass correlation of magnetic cataclysmic variables (mCVs) to the Galactic bulge X-ray emission (GBXE) spectra. However, the resulting mean WD mass is lower than that of the local CVs. This discrepancy could be explained by the dominating sources in the GBXE, which are non-mCVs instead of mCVs. In this work, we conduct a thorough investigation of the X-ray spectra of local DNe from the Suzaku archives and derive semi-empirical correlations between the shock temperature T max, the flux ratio of Fe xxvi–Lyα to Fe xxv–Heα lines, and WD mass for quiescent, nonmagnetic CVs. By applying these correlations to the GBXE, we derive the average WD mass of CVs in the Galactic bulge to be 0.81 ± 0.07M ⊙. This value is consistent with previous optical measurements of WD mass in local CVs.

A 3D model of polarized dust emission in the Milky Way

We present a three-dimensional model of polarised galactic dust emission that takes into account the variation of the dust density, spectral index and temperature along the line of sight, and contains randomly generated small scale polarisation fluctuations. The model is constrained to match observed dust emission on large scales, and match on smaller scales extrapolations of observed intensity and polarisation power spectra. This model can be used to investigate the impact of plausible complexity of the polarised dust foreground emission on the analysis and interpretation of future CMB polarisation observations.

Galactic Hα emission line studies using DEFPOS at the TÜBİTAK National Observatory

H$\alpha $ spectral line (6563 Å) profiles of eight northern H II regions in our galaxy (Sh2-25, Sh2-131, Sh2-237, IC 434, IC 1318, NGC 1982, NGC 6543, and NGC 6611) have been obtained using the Dual Etalon Fabry-Pérot Optical spectrometer (DEFPOS) with a 4' field of view, located at the coudé focus of the 150 cm RTT150 telescope at the TÜBİTAK National Observatory (TUG, Antalya, Turkey). Observations were carried out during the nights of 25-27 December 2015 and 6-7 June 2017 with different long exposure times ranging from 900 s to 3600 s. These spectra provide information about the local standard of rest (LSR) velocities, the full width at half maximum (FWHM), and densities of the sources. The LSR velocities and the FWHM of the H$\alpha $ emission lines were found to be in the range of -50.31 km/s to 27.57 km/s and 34.88 km/s to 44.54 km/s (mean FWHM: 37.19 km/s), respectively. The NGC 6543 (Cat's Eye Nebula) is the faintest one (75.06 R), while the NGC 1982 H II region is brightest source (38,477.3 R). The LSR velocity and the FWHM results of the DEFPOS/RTT150 system were compared with the data by several authors given in the literature and the results of DEFPOS data were found to be in good agreement with data given in the literature. Since there is not enough information on LSR velocities and intensities of such galactic sources with low angular size in the literature, we believe that this spectrometer will provide a powerful tool for the study of diffuse ionized gas and these new results may have significant contributions to the literature. In the future, we are planning to prepare H$\alpha $ maps with the 4' field of view of some diffuse ionized emission sources selected from VTSS and WHAM H$\alpha $ maps with large fields of view.

Tensor Minkowski Functionals for random fields on the sphere

We generalize the translation invariant tensor-valued Minkowski Functionals which are defined on two-dimensional flat space to the unit sphere. We apply them to level sets of random fields. The contours enclosing boundaries of level sets of random fields give a spatial distribution of random smooth closed curves. We outline a method to compute the tensor-valued Minkowski Functionals numerically for any random field on the sphere. Then we obtain analytic expressions for the ensemble expectation values of the matrix elements for isotropic Gaussian and Rayleigh fields. The results hold on flat as well as any curved space with affine connection. We elucidate the way in which the matrix elements encode information about the Gaussian nature and statistical isotropy (or departure from isotropy) of the field. Finally, we apply the method to maps of the Galactic foreground emissions from the 2015 PLANCK data and demonstrate their high level of statistical anisotropy and departure from Gaussianity.

Bandpass mismatch error for satellite CMB experiments I: estimating the spurious signal

Future Cosmic Microwave Background (CMB) satellite missions aim to use the B mode polarization to measure the tensor-to-scalar ratio r with a sensitivity σr ≲ 10−3. Achieving this goal will not only require sufficient detector array sensitivity but also unprecedented control of all systematic errors inherent in CMB polarization measurements. Since polarization measurements derive from differences between observations at different times and from different sensors, detector response mismatches introduce leakages from intensity to polarization and thus lead to a spurious B mode signal. Because the expected primordial B mode polarization signal is dwarfed by the known unpolarized intensity signal, such leakages could contribute substantially to the final error budget for measuring r. Using simulations we estimate the magnitude and angular spectrum of the spurious B mode signal resulting from bandpass mismatch between different detectors. It is assumed here that the detectors are calibrated, for example using the CMB dipole, so that their sensitivity to the primordial CMB signal has been perfectly matched. Consequently the mismatch in the frequency bandpass shape between detectors introduces differences in the relative calibration of galactic emission components. We simulate this effect using a range of scanning patterns being considered for future satellite missions. We find that the spurious contribution to r from the reionization bump on large angular scales (ℓ < 10) is ≈ 10−3 assuming large detector arrays and 20 percent of the sky masked. We show how the amplitude of the leakage depends on the nonuniformity of the angular coverage in each pixel that results from the scan pattern.

Interstellar Silicon Depletion and the Ultraviolet Extinction

Abstract Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far-ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Å interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [ Si / H ] dust and the 2175 Å bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance [ Si / H ] gas is known. We derive [ Si / H ] dust either from [ Si / H ] ISM − [ Si / H ] gas or from the Kramers–Kronig relation, which relates the wavelength-integrated extinction to the total dust volume, where [ Si / H ] ISM is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive [ Si / H ] dust from fitting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We find that in all three cases [ Si / H ] dust shows no correlation with the 2175 Å bump, while the carbon depletion [ C / H ] dust tends to correlate with the 2175 Å bump. This supports carbon grains instead of silicates as the possible carriers of the 2175 Å bump. We also find that neither [ Si / H ] dust nor [ C / H ] dust alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.

Separation of galactic and extragalactic radio emission at decameter wavelengths

We present the results of separation of observed radio emission at decameter wavelengths into extragalactic background and halo components of the Galaxy. We have used a multifrequency UTR-2 (decametric wave radio telescope) survey to perform multifrequency temperature-versus-temperature (T-T) plots. The low-frequency radio maps of the area near the minimum of Galactic radiation $(41^{\circ } < \delta_{2000} < 46^{\circ }, 7^{\mathrm{h}} 45^{\mathrm{m}} < \alpha_{2000} < 8^{\mathrm{h}} 35^{\mathrm{m}}) $ has been investigated in detail. The differential spectral index of the Galaxy halo at the frequencies 14.7, 16.7, 20, and 25 MHz was determined in this direction using pairwise of all the above frequencies. Its value was found to be $\beta_{G} = 2.81 \pm 0.03 $ . In addition, using the multifrequency T-T plots, the integral extragalactic brightness temperatures of isotropic background were determined to range 12 kK at 25 MHz to 42 kK at 14.7 MHz, and the extragalactic spectral index $\beta_{E} = 2.41 \pm 0.1 $ .

Precision Measurements of High-Energy Cosmic Gamma-Ray Emission with the GAMMA-400 Gamma-Ray Telescope

The GAMMA-400 γ-ray telescope installed at the Russian space observatory is intended for precision measurements in the energy range of 20 MeV–1000 GeV of γ-ray emission (with the angular and energy resolutions several times better than that of current γ-ray telescopes) from discrete sources; measurement of the energy spectra of Galactic and extragalactic diffuse γ-ray emission; studies of γ-ray emission from the active Sun; and measurements of fluxes of γ-ray emission and electron–positron cosmicray component, which are probably associated with the annihilation or decay of dark-matter particles.

Constraints on Galactic Neutrino Emission with Seven Years of IceCube Data

Abstract The origins of high-energy astrophysical neutrinos remain a mystery despite extensive searches for their sources. We present constraints from seven years of IceCube Neutrino Observatory muon data on the neutrino flux coming from the Galactic plane. This flux is expected from cosmic-ray interactions with the interstellar medium or near localized sources. Two methods were developed to test for a spatially extended flux from the entire plane, both of which are maximum likelihood fits but with different signal and background modeling techniques. We consider three templates for Galactic neutrino emission based primarily on gamma-ray observations and models that cover a wide range of possibilities. Based on these templates and in the benchmark case of an unbroken E − 2.5 power-law energy spectrum, we set 90% confidence level upper limits, constraining the possible Galactic contribution to the diffuse neutrino flux to be relatively small, less than 14% of the flux reported in Aartsen et al. above 1 TeV. A stacking method is also used to test catalogs of known high-energy Galactic gamma-ray sources.

Diffuse X-ray sky in the Galactic center

Abstract The Galactic diffuse X-ray emission (GDXE) in the Milky Way Galaxy is spatially and spectrally decomposed into the Galactic center X-ray emission (GCXE), the Galactic ridge X-ray emission (GRXE), and the Galactic bulge X-ray emission (GBXE). The X-ray spectra of the GDXE are characterized by the strong K-shell lines of the highly ionized atoms, and the brightest lines are the K-shell transition (principal quantum number transition of n = 2 → 1) of neutral iron (Fe i-Kα), He-like iron (Fe xxv-Heα), and He-like sulfur (S xv-Heα). Accordingly, the GDXE is composed of a high-temperature plasma of ∼7 keV (HTP) and a low-temperature plasma of ∼1 keV, which emit the Fe xxv-Heα and S xv-Heα lines, respectively. The Fe i-Kα line is emitted from nearly neutral irons, and hence the third component of the GDXE is a cool gas (CG). The Fe i-Kα distribution in the GCXE region is clumpy (Fe i-Kα clump), associated with giant molecular cloud (MC) complexes (Sagittarius A, B, C, D, and E) in the central molecular zone. The origin of the Fe i-Kα clumps is the fluorescence and Thomson scattering from the MCs irradiated by past big flares of the supermassive black hole Sagittarius A*. The scale heights and equivalent widths of the Fe i-Kα, Fe xxv-Heα, and Fe xxvi-Lyα (n = 2 → 1 transition of H-like iron) lines are different among the GCXE, GBXE, and GRXE. Therefore, their structures and origins are separately examined. This paper gives an overview of the research history and the present understandings of the GDXE, while in particular focusing on the origin of the HTP and CG in the GCXE.

A Dense Plasma Globule in the Solar Neighborhood

Abstract The radio source J1819+3845 underwent a period of extreme interstellar scintillation between circa 1999 and 2007. The plasma structure responsible for this scintillation was determined to be just 1–3 pc from the solar system and to posses a density of cm−3, which is three orders of magnitude higher than the ambient interstellar density. Here we present radio-polarimetric images of the field toward J1819+3845 at wavelengths of 0.2, 0.92, and 2 m. We detect an elliptical plasma globule of approximate size (major-axis position angle of ), via its Faraday-rotation imprint ( ) on the diffuse Galactic synchrotron emission. The extreme scintillation of J1819+3845 was most likely caused at the turbulent boundary of the globule (J1819+3845 is currently occulted by the globule). The origin and precise nature of the globule remain unknown. Our observations represent the first time that plasma structures which likely cause extreme scintillation have been directly imaged.

Probing the Intergalactic Medium with Lyα and 21 cm Fluctuations

Abstract We study 21 cm and Lyα fluctuations, as well as Hα, while distinguishing between Lyα emission of galactic, diffuse, and scattered intergalactic medium (IGM) origin. Cross-correlation information about the state of the IGM is obtained, testing neutral versus ionized medium cases with different tracers in a seminumerical simulation setup. In order to pave the way toward constraints on reionization history and modeling beyond power spectrum information, we explore parameter dependencies of the cross-power signal between 21 cm and Lyα, which displays a characteristic morphology and a turnover from negative to positive correlation at scales of a couple Mpc−1. In a proof of concept for the extraction of further information on the state of the IGM using different tracers, we demonstrate the use of the 21 cm and Hα cross-correlation signal to determine the relative strength of galactic and IGM emission in Lyα. We conclude by showing the detectability of the 21 cm and Lyα cross-correlation signal over more than one decade in scale at high signal-to-noise ratio for upcoming probes like SKA and the proposed all-sky intensity mapping satellites SPHEREx and CDIM, while also including the Lyα damping tail and 21 cm foreground avoidance in the modeling.

The angular power spectrum measurement of the Galactic synchrotron emission using the TGSS survey

AbstractCharacterizing the diffuse Galactic synchrotron emission (DGSE) at arcminute angular scales is needed to remove this foregrounds in cosmological 21-cm measurements. Here, we present the angular power spectrum (Cℓ) measurement of the diffuse Galactic synchrotron emission using two fields observed by the TIFR GMRT Sky Survey (TGSS). We apply 2D Tapered Gridded Estimator (TGE) to estimate the Cℓ from the visibilities. We find that the residual data after subtracting the point sources is likely dominated by the diffuse Galactic synchrotron radiation across the angular multipole range 240 ≤ ℓ ≲ 500. We fit a power law to the measured Cℓ over this ℓ range. We find that the slopes in both fields are consistent with earlier measurements. For the second field, however, we interpret the measured Cℓ as an upper limit for the DGSE as there is an indication of a significant residual point source contribution.