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Title: The gamma-ray pulsar population of globular clusters: Implications for the GeV excess

Abstract

In this study, it has been suggested that the GeV excess, observed from the region surrounding the Galactic Center, might originate from a population of millisecond pulsars that formed in globular clusters. With this in mind, we employ the publicly available Fermi data to study the gamma-ray emission from 157 globular clusters, identifying a statistically significant signal from 25 of these sources (ten of which are not found in existing gamma-ray catalogs). We combine these observations with the predicted pulsar formation rate based on the stellar encounter rate of each globular cluster to constrain the gamma-ray luminosity function of millisecond pulsars in the Milky Way's globular cluster system. We find that this pulsar population exhibits a luminosity function that is quite similar to those millisecond pulsars observed in the field of the Milky Way (i.e. the thick disk). After pulsars are expelled from a globular cluster, however, they continue to lose rotational kinetic energy and become less luminous, causing their luminosity function to depart from the steady-state distribution. Using this luminosity function and a model for the globular cluster disruption rate, we show that millisecond pulsars born in globular clusters can account for only a few percent or less ofmore » the observed GeV excess. Among other challenges, scenarios in which the entire GeV excess is generated from such pulsars are in conflict with the observed mass of the Milky Way's Central Stellar Cluster.« less

Authors:
 [1];  [2]
  1. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Univ. of Chicago, Chicago, IL (United States)
  2. The Ohio State Univ., Columbus, OH (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1305131
Report Number(s):
FERMILAB-PUB-16-245-A; arXiv:1606.09250
Journal ID: ISSN 1475-7516; 1473014
Grant/Contract Number:
AC02-07CH11359
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Cosmology and Astroparticle Physics
Additional Journal Information:
Journal Volume: 2016; Journal Issue: 08; Journal ID: ISSN 1475-7516
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Hooper, Dan, and Linden, Tim. The gamma-ray pulsar population of globular clusters: Implications for the GeV excess. United States: N. p., 2016. Web. doi:10.1088/1475-7516/2016/08/018.
Hooper, Dan, & Linden, Tim. The gamma-ray pulsar population of globular clusters: Implications for the GeV excess. United States. doi:10.1088/1475-7516/2016/08/018.
Hooper, Dan, and Linden, Tim. 2016. "The gamma-ray pulsar population of globular clusters: Implications for the GeV excess". United States. doi:10.1088/1475-7516/2016/08/018. https://www.osti.gov/servlets/purl/1305131.
@article{osti_1305131,
title = {The gamma-ray pulsar population of globular clusters: Implications for the GeV excess},
author = {Hooper, Dan and Linden, Tim},
abstractNote = {In this study, it has been suggested that the GeV excess, observed from the region surrounding the Galactic Center, might originate from a population of millisecond pulsars that formed in globular clusters. With this in mind, we employ the publicly available Fermi data to study the gamma-ray emission from 157 globular clusters, identifying a statistically significant signal from 25 of these sources (ten of which are not found in existing gamma-ray catalogs). We combine these observations with the predicted pulsar formation rate based on the stellar encounter rate of each globular cluster to constrain the gamma-ray luminosity function of millisecond pulsars in the Milky Way's globular cluster system. We find that this pulsar population exhibits a luminosity function that is quite similar to those millisecond pulsars observed in the field of the Milky Way (i.e. the thick disk). After pulsars are expelled from a globular cluster, however, they continue to lose rotational kinetic energy and become less luminous, causing their luminosity function to depart from the steady-state distribution. Using this luminosity function and a model for the globular cluster disruption rate, we show that millisecond pulsars born in globular clusters can account for only a few percent or less of the observed GeV excess. Among other challenges, scenarios in which the entire GeV excess is generated from such pulsars are in conflict with the observed mass of the Milky Way's Central Stellar Cluster.},
doi = {10.1088/1475-7516/2016/08/018},
journal = {Journal of Cosmology and Astroparticle Physics},
number = 08,
volume = 2016,
place = {United States},
year = 2016,
month = 8
}

Journal Article:
Free Publicly Available Full Text
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Citation Metrics:
Cited by: 6works
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  • The Fermi satellite has recently detected gamma-ray emission from the central regions of our Galaxy. This may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. We show that the observed signal may instead be generated by millisecond pulsars that formed in dense star clusters in the Galactic halo. Most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. The gamma-ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and aremore » in remarkable agreement with the observations. These gamma-rays are from fossil remains of dispersed clusters, telling the history of the Galactic bulge.« less
  • Context. Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. The observation of this emission provides a powerful tool to assess the millisecond pulsar population of a cluster, is essential for understanding the importance of binary systems for the evolution of globular clusters, and provides complementary insights into magnetospheric emission processes. Aims. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. Methods. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Spacemore » Telescope to study the gamma-ray emission towards 13 globular clusters. Results. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices (0.7 < Γ < 1.4) and clear evidence for an exponential cut-off in the range 1.0 - 2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices (1.0 < Γ < 1.7), however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600 - 4700 MSPs in Galactic globular clusters, commensurate with previous estimates. Conclusions. The observation of high-energy gamma-ray emission from globular clusters thus provides a reliable independent method to assess their millisecond pulsar populations.« less
  • Recent surveys of Galactic globular clusters have been very effective at discovering radio pulsars. By a careful analysis of selection effects in the various surveys, and after estimating the relative efficiency of pulsar production in the individual clusters, a census of the cluster population of pulsars is obtained. It is found that there are about 10,000 pulsars in the Galactic globular clusters, subject to uncertainties in pulsar beaming and binarity. Such a large population poses severe problems for the standard model of pulsar production in clusters, unless the uncertain factors due to beaming and binary are pushed to values inconsistentmore » with the present expectations. This suggests the need for a substantial modification of the standard model, either by including accretion-induced collapse of massive white dwarfs or by increasing the retention of primordial neutron stars in the clusters. 55 refs.« less
  • High sensitivity searches of globular clusters (GCs) for radio pulsars by improved pulsar search algorithms and sustained pulsar timing observations have so far yielded some 140 pulsars in more than two dozen GCs. The observed distribution of orbital eccentricity and period of binary radio pulsars in GCs have imprints of the past interaction between single pulsars and binary systems or of binary pulsars and single passing noncompact stars. It is seen that GCs have different groups of pulsars. These may have arisen out of exchange or merger of a component of the binary with the incoming star or a 'fly-by'more » in which the original binary remains intact but undergoes a change of eccentricity and orbital period. We consider the genesis of the distribution of pulsars using analytical and computational tools such as STARLAB, which performs numerical scattering experiments with direct N-body integration. Cluster pulsars with intermediate eccentricities can mostly be accounted for by fly-bys, whereas those with high eccentricities are likely to be the result of exchanges and/or mergers of single stars with the binary companion of the pulsar, although there are a few objects which do not easily fit into this description. The corresponding distribution for galactic field pulsars shows notable differences from the GC pulsar orbital period and eccentricity distribution. The long orbital period pulsars in the galactic field with frozen out low eccentricities are largely missing from the GCs, and we show that the ionization of these systems in GCs cannot alone account for the peculiarities.« less
  • We report the results of pulsar searches in 10 globular clusters (GCs) using the Robert C. Byrd Green Bank Telescope. One new binary millisecond pulsar (MSP) has been discovered in NGC 5986 with P{sub spin} = 2.6 ms, P{sub orb} = 1.3 days, and a minimum companion mass of 0.16 M{sub sun}. The companion is most likely a helium white dwarf. Eight of the GCs we searched have central densities <10{sup 4} L{sub sun} pc{sup -3}, making this a good sample for studying the pulsar content of low-density clusters. We find no evidence for pulsars in clusters with very lowmore » densities (<10{sup 3} L{sub sun} pc{sup -3}), consistent with theoretical predictions. Null results in many of the clusters we searched with moderate densities indicate that these systems do not contain a bright MSP. Two clusters in particular, one with very low metallicity, stand in contrast to theoretical calculations by Ivanova et al. We also find that three-body exchange interaction rates calculated by Phinney seem to overpredict the pulsar content in the clusters we studied.« less