The Galactic Center Excess

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Improved Measurements of the MSP Luminosity Function in Globular Clusters

The nature of the Galactic Center excess has been debated for over a decade -- with dark matter and millisecond pulsars as the two leading candidates. We use measurements of globular clustetrs, which have bright GeV gamma-ray fluxes which are thought to be powered by populations of MSPs, in order to calculate the luminosity function of MSPs. We produce a world-leading calculation for the luminosity function of these sources. If a similar luminosity function existed for detectable MSPs in the galactic center, we would expect more observed sources than have presently been found. Unless there is either a large missing cache of unassociated Fermi sources that are MSPs, or the MSP luminosity function is catagorically dimmer than in globular clusters -- this strongly limits the potential for MSPs to contribute to the Galactic center excess.

CTA will Conclusively Probe Pulsar Models of the Galactic Center Excess

The nature of the Galactic Center excess has been debated for over a decade -- with dark matter and millisecond pulsars as the two leading candidates. We utilize recent evidence which shows that MSPs (like young pulsars) accelerate a significant e+e- population up to TeV energies, in order to argue that any pulsar model capable of explaining the excess would produce a bright diffuse TeV flux that would be detected by the upcoming Cherenkov Telescope Array. If such a feature is not detected, it would strongly constrain pular models of the galactic center excess.

Evidence of TeV Halos Around Millisecond Pulsars

Observations over the last few years have detected bright, spatially extended gamma-ray emission (TeV halos) around young and middle-aged pulsars throughout the Milky Way. This brings forth the question of whether the ancient population of millisecond pulsars -- much more numerous but individually less luminous -- also produce TeV halo activity. Unfortunately, if we expect that MSPs are equally efficient at producing TeV halos (compared to their spindown power) as young and middle-aged pulsars, none of these systems is expected to be bright enough to be individually detected by HAWC. However - they are close! In order to enhance the sensitivity of HAWC, we produce a "stacked" joint-likelihood analysis of the 37 MSPs with the highest expected spindown-flux (spindown-power divided by distance squared), and fit this population with a Geminga-like TeV halo model. We find that this method improves the quality of fit to the HAWC data by 4.24 sigma. However, the HAWC backgorunds are not Poissonian. If we re-fit this data by comparing our model with studies of "blank-sky" locations where no TeV halo emission is expected, we find the significance decreases, but is still approximately 2.5 sigma. Stay Tuned!

Hints of TeV Halos Around Millisecond Pulsars

The observation of bright TeV halos surrounding nearby middle-aged pulsars, such as Geminga and Monogem, poses the question of whether such halos also exist around much older, millisecond pulsars. Unfortunately, extrapolations from the "Geminga-like" model predict that individual millisecond pulsars would be too dim to be individually detected with current gamma-ray instruments like HAWC. We produce a stacked population of millisecond pulsars and and utilize the 2HWC catalog to calculate their gamma-ray luminosity. Among the population we find 4 potentially detected sources, which combine to produce nearly 3-sigma evidence of a TeV halo component. While not definitive, this deserves further investigation, as the observation of TeV halos around millisecond pulsars would have important consequences, in particular for models of the Galactic center gamma-ray excess.

Bright Pulsars Do not Power the Galactic Center Excess

Recently, a paper by the Fermi-LAT collaboration found that the majority of the Galactic center excess could be accounted for by a handful of very bright pulsars, which should each be detected close to the individual point-source sensitivity of the Fermi-LAT. We point out that this analysis is at odds with previous literature on the topic, and while attempting to exactly recreate the analysis techniques described in the Fermi-LAT paper, do not find any evidence for such a bright pulsar signal. Upon discussions in the run-up towards the release of this paper, a coding error was found in the original Fermi-LAT analysis, which, when corrected, give results similar to our own - ruling out the possibility that very bright pulsars dominate the Galactic center excess.

TeV Halos Can Explain the Galactic Center "Pevatron"

TeV halos are bright gamma-ray sources that surround young and middle-aged pulsars. Notably, they contain a hard-spectrum gamma-ray component that can extend to nearly 100 TeV in gamma-ray energy. We show that the spectrum, intensity and morphology of this emission closely matches HESS observations of the diffuse 100 TeV gamma-ray flux surrounding the galactic center. This casts doubts on the recent claim by the HESS collaboration that this 100 TeV gamma-ray flux is strong evidence for a hadronic "Pevatron" in the galactic center which accelerates protons to PeV energies in order to power the 100 TeV gmama-ray flux.

Low Mass X-Ray Binary Observations Challenge MSP Models of the Galactic Center Excess

Millisecond Pulsars have long been favored as a potential explanation for the excess gamma-ray emission surrounding the galactic center. However, the individual millisecond pulsars responsible for this emission have remained undetected -- a result that is not particularly unusual given their low individual luminosities. However, millisecond pulsars are expected to evolve through a prior low-mass x-ray binary state, which is often bright enough to be detected. We examing the low-mass x-ray binary population in the galactic center, comparing it to similar populations in globular clusters, finding that there are not enough low-mass x-ray binaries in the galactic center region to explain the extremely bright gamma-ray emission expected from their subsequent millisecond pulsar population.

Full Publication List:

20. Millisecond Pulsars in Globular Clusters and Implications for the Galactic Center Gamma-Ray Excess
Aurelio Amerio, Dan Hooper, Tim Linden
To Be Submitted

19. The Cherenkov Telescope Array Will Test Whether Pulsars Generate the Galactic Center Gamma-Ray Excess
Celeste Keith, Dan Hooper, Tim Linden
Physical Review D 107 10, 103001

18. Evidence of TeV Halos Around Millisecond Pulsars
Dan Hooper, Tim Linden
Physical Review D 105, 103013

17. Millisecond Pulsars, TeV Halos, and Implications for the Galactic Center Gamma-Ray Excess
Dan Hooper, Tim Linden
Physical Review D 98 043005

16. Comment on "Characterizing the Population of Pulsars in the Galactic Bulge with the Fermi Large Area Telescope" [arXiv: 1705.00009v1]
Richard Bartels, Dan Hooper, Tim Linden, Siddharth Mishra-Sharma, Nick Rodd, Ben Safdi, Tracy Slatyer
Physics of the Dark Universe 20 88 2018

15. TeV Gamma Rays from Galactic Center Pulsars
Dan Hooper, Ilias Cholis, Tim Linden
Physics of the Dark Universe 21 40 2018

14. Low Mass X-Ray Binaries in the Inner Galaxy: Implications for MSPs and the GeV Excess
Daryl Haggard, Craig Heinke, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1705 05 056

13. The Gamma-Ray Pulsar Population of Globular Clusters: Implications for the GeV Excess
Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics 1608 08 018

12. The High-Energy Tail of the Galactic Center Gamma-Ray Excess
Tim Linden, Nicholas Rodd, Benjamin Safdi, Tracy Slatyer
Physical Review D 94, 103013

11. Improved Cosmic-Ray Injection Models and the Galactic Center Gamma-Ray Excess
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review D 94 063504

10. Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2
Eric Carlson, Tim Linden, Stefano Profumo
Physical Review Letters 117 111101

9. Known Radio Pulsars Do Not Contribute to the Galactic Center Gamma-Ray Excess
Tim Linden
Physical Review D 93 6 063003

8. The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts
Ilias Cholis, Carmelo Evoli, Francesca Calore, Tim Linden, Christoph Weniger, Dan Hooper
Journal of Cosmology and Astroparticle Physics 1512 12 005

7. The Galactic Center Excess in Gamma-Rays from Annihilation of Self-Interacting Dark Matter
Manoj Kaplinghat, Tim Linden, Haibo Yu
Physical Review Letters, 114 211303

6. Challenges in Explaining the Galactic Center Gamma-Ray Excess with Millisecond Pulsars
Ilias Cholis, Dan Hooper, Tim Linden
Journal of Cosmology and Astroparticle Physics, 06 043 (2015)

5. A New Determination of the Spectrum and Luminosity Function of Millisecond Pulsars
Ilias Cholis, Dan Hooper, Tim Linden
Submitted to PRD

4. The Characterization of the Gamma-Ray Signal from the Central Milky Way
Tansu Daylan, Doug Finkbeiner, Dan Hooper, Tim Linden, Stephen Portillo, Nicholas Rodd, Tracy Slatyer
Physics of the Dark Universe 12 2016 1

3. Pulsars Cannot Account for the Inner Galaxy's GeV Excess
Dan Hooper, Ilias Cholis, Tim Linden, Jennifer Siegal-Gaskins, Tracy Slatyer
Physical Review D, 88 083009

2. Origin of Gamma-Rays from the Galactic Center
Dan Hooper, Tim Linden
Physical Review D, 84 12 123005

1. Gamma-Rays from the Galactic Center and the WMAP Haze
Dan Hooper, Tim Linden
Physics Review D, 83 8 083517 (2011)

Theses

Dark Matter Annihilation at the Galactic Center

Doctoral Thesis
13 June 2013

Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se