Richard Bartels

Strong Support for the Millisecond Pulsar Origin of the Galactic Center GeV Excess.

Using γ-ray data from the Fermi Large Area Telescope, various groups have identified a clear excess emission in the Inner Galaxy, at energies around a few GeV. This excess resembles remarkably well a signal from dark-matter annihilation. One of the most compelling astrophysical interpretations is that the excess is caused by the combined effect of a previously undetected population of dim γ-ray sources. Because of their spectral similarity, the best candidates are millisecond pulsars. Here, we search for this hypothetical source population, using a novel approach based on wavelet decomposition of the γ-ray sky and the statistics of Gaussian random fields. Using almost seven years of Fermi-LAT data, we detect a clustering of photons as predicted for the hypothetical population of millisecond pulsar, with a statistical significance of 10.0σ. For plausible values of the luminosity function, this population explains 100% of the observed excess emission. We argue that other extragalactic or Galactic sources, a mismodeling of Galactic diffuse emission, or the thick-disk population of pulsars are unlikely to account for this observation.

Boosting the annihilation boost: Tidal effects on dark matter subhalos and consistent luminosity modeling

In the cold dark matter paradigm, structures form hierarchically, implying that large structures contain smaller substructures. These subhalos will enhance signatures of dark matter annihilation such as gamma rays. In the literature, typical estimates of this boost factor assume a concentration-mass relation for field halos, to calculate the luminosity of subhalos. However, since subhalos accreted in the gravitational potential of their host lose mass through tidal stripping and dynamical friction, they have a quite characteristic density profile, different from that of the field halos of the same mass. In this work, we quantify the effect of tidal stripping on the boost factor, by developing a semianalytic model that combines the mass-accretion history of both the host and subhalos as well as subhalo accretion rates. We find that when subhalo luminosities are treated consistently the boost factor increases by a factor 2–5, compared to the typical calculation assuming a field-halo concentration. This holds for host halos ranging from subgalaxy to cluster masses and is independent of the subhalo mass function or specific concentration-mass relation. The results are particularly relevant for indirect dark matter searches in the extragalactic gamma-ray sky.

Inverse-Compton emission from clusters of galaxies: Predictions for ASTRO-H ?

The intra-cluster medium of several galaxy clusters hosts large-scale regions of diffuse synchrotron radio emission, known as radio halos and relics, which demonstrate the presence of magnetic fields and relativistic electrons in clusters. These relativistic electrons should also emit X-rays through inverse-Compton scattering off of cosmic microwave background photons. The detection of such a non-thermal X-ray component, together with the radio measurement, would permit to clearly separate the magnetic field from the relativistic electron distribution as the inverse-Compton emission is independent from the magnetic field in the cluster. However, non-thermal X-rays have not been conclusively detected from any cluster of galaxies so far. In this paper, for the first time, we model the synchrotron and inverse-Compton emission of all clusters hosting radio halos and relics for which the spectral index can be determined. We provide constraints on the volume-average magnetic field by comparing with current X-ray measurements. We then estimate the maximum volume-average magnetic field that will allow the detection of inverse-Compton hard X-rays by the ASTRO-H satellite. We found that several clusters are good targets for ASTRO-H to detect their inverse-Compton emission, in particular that corresponding to radio relics, and propose a list of promising targets for which ASTRO-H can test

Testing the interpretation of the Fermi Galactic center excess in terms of unresolved point sources

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The Fermi-LAT GeV excess as a tracer of stellar mass in the Galactic bulge

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Bayesian model comparison and analysis of the Galactic disc population of gamma-ray millisecond pulsars

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Boosting the boost: the effect of tidal stripping on the subhalo luminosity.

G magnetic fields. We conclude that future hard X-ray observations by the already-operating NuSTAR and the soon-to-be-launched ASTRO-H definitely have the potential to shed light on the long-sought non-thermal hard-X-ray emission in clusters of galaxies.

The Fermi-LAT GeV Excess Traces Stellar Mass in the Galactic Bulge

We introduce a new method to analyse populations of near-threshold sources in gamma-ray data. The method is based on a wavelet transform of the gamma-ray sky, using Mexican hat wavelets. The statistics of peak significances in the wavelet transform provides information about the spatial distribution and luminosity function of near-threshold sources. We apply this method to gammaray data from the inner Galaxy, in order to test the millisecond pulsar interpretation of the Fermi Galactic center excess. We find evidence for the unresolved point sources causing the excess emission at the > 4σ level. For plausible luminosity functions, this new component explains 100% of the observed excess emission.

Comment on “Characterizing the population of pulsars in the Galactic bulge with the Fermi large area telescope” [arXiv:1705.00009v1]

An anomalous emission component at energies of a few gigaelectronvolts and located towards the inner Galaxy is present in the Fermi-LAT data. At present, the two most promising explanations are the annihilation of dark matter particles or the presence of a large population of unresolved point sources, most probably millisecond pulsars, at the Galactic Centre. Here, we report an analysis of the excess characteristics using almost eight years of Pass 8 ULTRACLEAN Fermi-LAT data with SkyFACT—a tool that combines image reconstruction with template-fitting techniques. We find that an emission profile that traces stellar mass in the boxy and nuclear bulge is preferred over conventional dark matter profiles. A model including the bulge is favoured over a model with dark matter at 16σ.An image-template analysis of eight years of Fermi-LAT data shows that the anomalous emission of gigaelectronvolt energies close to the centre of our Galaxy is better fitted with a boxy-shaped bulge generated by stars — possibly millisecond pulsars — than with a dark matter signal.

Prospects for indirect dark matter searches with MeV photons

Pulsed emission from almost one hundred millisecond pulsars (MSPs) has been detected in γ-rays by the Fermi Large-Area Telescope. The global properties of this population remain relatively unconstrained despite many attempts to model their spatial and luminosity distributions. We perform here a self-consistent Bayesian analysis of both the spatial distribution and luminosity function simultaneously. Distance uncertainties, arising from errors in the parallax measurement or Galactic electron-density model, are marginalized over. We provide a public python package (available from http://github.com/tedwards2412/MSPDist) for calculating distance uncertainties to pulsars derived using the dispersion measure by accounting for the uncertainties in Galactic electron-density model YMW16. Finally, we use multiple parametrizations for the MSP population and perform Bayesian model comparison, finding that a broken power-law luminosity function with Lorimer spatial profile are preferred over multiple other parametrizations used in the past. The best-fitting spatial distribution and number of γ-ray MSPs is consistent with results for the radio population of MSPs.