Highlights
Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy due to millisecond pulsars
R. Crocker , O. Macias, D. Mackey, M. Krumholz, S. Ando, S. Horiuchi, et al., Nature Astrononmy (2022) [arXiv:2204.12054]
Gamma-ray image of the Fermi bubbles (blue) overlaid on a map of RR Lyrae stars (red) observed by the GAIA telescope. The shape and orientation of the Sagittarius (Sgr) dwarf match perfectly well those of the Fermi cocoon – a bright substructure of gamma-ray radiation in the southern part of the Fermi bubbles. This is strong evidence that the Fermi cocoon is due to energetic processes occurring in Sagittarius, which from our perspective, is located behind the Fermi bubbles.
The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s supermassive black hole. Here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the Sagittarius dwarf spheroidal galaxy (dSph). This large Milky Way satellite is viewed through the Fermi bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sagittarius dSph has no ongoing star formation, but we nevertheless demonstrate that the dwarf’s millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron–positron pairs injected by millisecond pulsars belonging to the Sagittarius dSph, combined with these objects’ magnetospheric emission. This finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dSphs.
The Sgr dwarf spheroidal paper explained ! (duration: ~13 min )
The Andromeda Gamma-Ray Excess: Background Systematics of the Millisecond Pulsars and Dark Matter Interpretations
The density plot shows the number density of stars in the direction of the Andromeda Galaxy. The cyan contours display the distribution of interstellar gas belonging to Andromeda. The magenta elliptical region encloses the stellar bulge of our neighbor Galaxy.
Since the discovery of an excess in gamma rays in the direction of Andromeda, its cause has been unclear. Published interpretations focus on a dark matter or stellar related origin. Studies of a similar excess in the Milky Way center motivate a correlation of the spatial morphology of the signal with the distribution of stellar mass in Andromeda. However, a robust determination of the best theory for the observed excess emission is very challenging due to large uncertainties in the astrophysical gamma-ray foreground model. Here we perform a reanalysis of the Andromeda gamma-ray excess using state-of-the-art templates for the distribution of stellar mass in Andromeda and novel astrophysical foreground models for its sky region. We find that our stellar maps, taken as a proxy for the location of a putative population of millisecond pulsars in the bulge of Andromeda, reach a statistical significance of 5.4𝜎. Our detection of the stellar templates is robust to generous variations of the astrophysical foreground model. Once the stellar templates are included in the astrophysical model, we show that the dark matter annihilation interpretation of the signal is unwarranted. In addition, using the results of a binary population synthesis model we demonstrate that a population of about one million unresolved MSPs could naturally explain the observed properties of the Andromeda excess.
Assessing the Impact of Hydrogen Absorption on the Characteristics of the Galactic Center Excess
Maps of column density in units of 10^20 cm^−2 for two concentric rings around the Galactic center. The right panels show models that account for continuum emission where the excitation temperature is allowed to vary with longitude and latitude. The left panels display models from Macias et al. (2018) that do not account for continuum emission.
In this paper we present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation-transport modelling of line and continuum emission and a gas-flow model in the barred Galaxy that provides distance resolution for lines of sight toward the Galactic Center. The main benefits of the new gas model are, (i) the ability to reproduce the negative line signals seen with the HI4PI survey and, (ii) the accounting for gas that primarily manifests itself through absorption.
We apply the new model of Galactic atomic hydrogen to an analysis of the diffuse gamma-ray emission from the inner Galaxy, for which an excess at a few GeV was reported that may be related to dark matter. We find with high significance an improved fit to the diffuse gamma-ray emission observed with the Fermi-LAT, if our new HI model (right column panels) is used to estimate the cosmic-ray induced diffuse gamma-ray emission. The fit still requires a nuclear bulge at high significance. Once this is included there is no evidence for a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favoured by the data. This finding is robust under the variation of various parameters, for example the excitation temperature of atomic hydrogen, and a number of tests for systematic issues.
Measuring the Smearing of the Galactic 511 keV Signal: positron propagation or supernova kicks?
T. Siegert, R. Crocker , O. Macias, F. Panther, F. Calore, D. Song, and S. Horiuchi [MNRAS Lett. 509 (2021) 1, L11-L16]
The vertical-axis shows the Akaike-Information-Criteria (AIC) and the horizontal shows several astrophysical template combinations: Inverse Compton (IC), nuclear bulge (NB), Boxy Bulge (BB), Atomic Hydrogen (HI), molecular gas (CO), Fermi Bubbles (FB), and smooth dark matter (DM0). The arrows show the fit improvement (in units of sigma) after the inclusion of a certain template.
The 511 keV signal from the annihilation of positrons with electrons in the Milky Way has presented a puzzle for half a century. In this article, we used 15 years of data from the INTEGRAL/SPI instrument to revise the long-standing paradigm of positron annihilation in the interstellar medium (ISM) after propagation away from candidate sources. Our spatial analysis reveals that the annihilation signal traces the old stellar population in the Galactic bulge, but with a finite smearing. This smearing indicates that positrons typically propagate for ~150 pc through the bulge ISM before annihilating. An alternate scenario of annihilation in situ at kicked sources (e.g. compact objects from supernovae events) is disfavored.
The figure on the left shows that once the stellar templates (i.e., the nuclear bulge and boxy bulge) are included in the fits, the data no longer requires a dark matter template.
Millisecond pulsars from Accretion Induced Collapse naturally explain the Galactic Center Excess
A. Gautam, R. Crocker , L. Ferrario , A. Ruiter, H. Ploeg, C. Gordon, and O. Macias [Nature Astronomy (2022)]
Schematic diagram showing some of the main evolutionary stages towards, and beyond, accretion induced collapse of a white dwarf.
Gamma-ray data from the Fermi-Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge. The origin of this “Galactic Center Excess” (GCE) has been debated with proposed sources such as self-annihilating dark matter and a hypothetical population of millisecond pulsars. In this work, we used a binary population synthesis forward model to demonstrate that a millisecond pulsar population arising from the accretion induced collapse of Oxygen-Neon white dwarfs (see illustration on the left) in Galactic bulge binaries can naturally explain the GCE.
Importantly, this evolutionary channel of the millisecond pulsars is not expected to produce many Low Mass X-ray Binaries, in accordance with observations.
Spin-down power liberated by the hypothetical millisecond pulsar population responsible for the GCE over cosmological time (band indicates ±1σ error). The luminosity datum at 13.8 Gyr is inferred from the GCE spectrum. This shows that the simulated pulsars have enough power to explain away the GCE.
Excellent popular science video about our work by Anton Petrov. Highly recommended!
CTA sensitivity to the putative millisecond pulsar population responsible for the Galactic center excess
O. Macias, H. van Leijen, D. Song, S. Ando, S. Horiuchi, and R. Crocker [MNRAS 506 (2021) 1741-1760]
What is this paper about?
Past studies of the Fermi Galactic Center Excess (GCE) have found evidence for a “high-energy tail” in the GCE spectrum. If this high-energy tail extends to multi-TeV energies, then it could be detectable by the upcoming Cherenkov Telescope Array (CTA). In this contribution, we present the results of an exhaustive study on simulated data, in which we analyze the CTA sensitivity to the high-energy tail of the GCE.
Why is it relevant/interesting?
Prompt gamma-ray emission from an unresolved population of millisecond pulsars (MSPs) can account for the GCE at GeV-scale energies. This same MSPs popu-
lation could inject multi-TeV electrons/positrons into the interstellar medium, which would produce a high-energy tail through inverse-Compton (IC) radiation. Therefore, detecting this multiwavelength signal with CTA would provide corroborating evidence for the MSPs explanation of the GCE.
What have we done?
We ran detailed simulations of the astrophysical background towards the Galactic Center (GC) using GALPROP. Similarly, we constructed detailed simulations of the expected IC signal from MSPs in the GC. Using the latest CTA instrument response functions, we created mock data and studied the necessary conditions for a reliable CTA discovery of the MSPs population responsible for the GCE.
What is the result?
We found that CTA has the necessary sensitivity to detect the high-energy tail of the GCE for physically plausible electron acceleration efficiencies. Furthermore, in the event that CTA observes an excess of diffuse gamma rays in the GC, it will be able to discriminate between the dark matter and MSPs hypotheses. The paper is summarized in the ~12 min video above.
Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
The sky positions of the Milky Way globular clusters. The red-stars represent the globular clusters found in gamma rays by Fermi LAT, while the green dots show the ones not (yet) detected in gamma rays.
Millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. However, the relative contributions of two separate emission processes: (i) curvature radiation from millisecond pulsar magnetospheres and (ii) inverse Compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars; has long been unclear.
To address this, we have searched for evidence of inverse Compton emission in 8 years of Fermi-LAT data from the directions of 157 Milky Way globular clusters (shown in the left figure).
Measured gamma ray spectrum of the globular cluster NGC 6541. The data is well fitted by curvature radiation (CR) model plus an inverse Compton (IC) component.
We have obtained that the gamma-ray emission of globular clusters can be resolved spectrally into two components: i) an exponentially cut-off power law (red curve in the figure on the left) and ii) a pure power law (blue line). The latter component - which we uncover at a significance of 8.2σ - is most naturally interpreted as inverse Compton emission by cosmic-ray electrons and positrons injected by millisecond pulsars.
In addition, we have found a mildly statistically significant (3.8 σ) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. This is expected of a system emitting gamma rays through inverse Compton radiation processes.
Prospects for detecting heavy WIMP dark matter with the CTA: The Wino and Higgsino
Expected upper limits at 95% confidence level on the Wino annihilation cross section as a function of its mass.
TeV-scale particles that couple to the standard model through the weak force represent a compelling class of dark matter candidates. The search for such Weakly Interacting Massive Particles (WIMPs) has already spanned multiple decades, and whilst it has yet to provide any definitive evidence for their existence, viable parameter space remains. In this paper, we show that the upcoming Cherenkov Telescope Array (CTA) has significant sensitivity to uncharted parameter space at the TeV mass scale.
The figure on the left shows our results for a prototypical dark matter candidate: the Wino. The sensitivity estimate assumes 500 hours of CTA observations towards the GC. The predicted next-to-leading logarithmic (NLL) cross section is shown (solid gray line) and the thermal Wino DM mass is marked (cyan solid line and bands). The only background considered here is the cosmic ray residual background. The full Wino spectrum is included in the expected signal.In this case, we find substantial expected improvements over existing bounds (roughly an order of magnitude on the annihilation cross section) from current imaging atmospheric Cherenkov telescopes. This highlights the amazing discovery potential of CTA.
Probing heavy dark matter decays with multi-messenger astrophysical data
In this paper, we used multi-messenger observations of cosmic rays (protons/antiprotons, electrons/positrons, neutrinos/antineutrinos, and gamma rays) to impose strong conservative constraints on super heavy dark matter candidates. We solved the cosmic ray transport equation self-consistently in the extragalactic, Galactic, and solar regions. Overall, we were able to exclude dark matter lifetimes of 10^28 s or shorter for all the masses investigated in this work.
Limits on the dark matter decay lifetime when the exotic cosmic ray particles are assumed to be injected in our Galaxy. Null observations of dark matter yields from multiple experiments allow to excluded the parameter space shown by shaded regions. The astrophysical data sets correspond to: Pierre Auger Observatory (PAO), IceCube, Telescope Array (TA), and Fermi-LAT.
Same as the left-hand side figure, except that here, the dark matter cosmic rays are assumed to be of extragalactic origin.