Gamma-ray emission from high Galactic latitude globular clusters

Lloyd, Sheridan J.; Chadwick, P. M.; Brown, A. M.

doi:10.1093/mnras/sty2150

Cited by 10 publications

(8 citation statements)

References 38 publications

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“…Hooper & Linden (2016) studied 25 GCs using 85 months of Fermi data, and found that the γ -ray luminosity function of MSPs in GCs is consistent with that applying to MSPs detected in the field. Lloyd, Chadwick & Brown (2018) studied γ -ray emission from high-latitude GCs and its connection to their X-ray emission. de Menezes, Cafardo & Nemmen (2019) reanalysed 9 yr of Fermi data and found 23 γ -ray emitting GCs; they found that the metallicity only mildly contributes to L γ while a very high encounter rate seemed to reduce the L γ from GCs.…”

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confidence: 99%

Evidence for a high-energy tail in the gamma-ray spectra of globular clusters

Song

Macías

Horiuchi

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. However, the relative contributions of two separate emission processes–curvature radiation from millisecond pulsar magnetospheres versus inverse Compton emission from relativistic pairs launched into the globular cluster environment by millisecond pulsars–have long been unclear. To address this, we search for evidence of inverse Compton emission in 8-year Fermi-LAT data from the directions of 157 Milky Way globular clusters. We find a mildly statistically significant (3.8σ) correlation between the measured globular cluster gamma-ray luminosities and their photon field energy densities. However, this may also be explained by a hidden correlation between the photon field densities and the stellar encounter rates of globular clusters. Analysed in toto, we demonstrate that the gamma-ray emission of globular clusters can be resolved spectrally into two components: i) an exponentially cut-off power law and ii) a pure power law. The latter component–which we uncover at a significance of 8.2σ–has a power index of 2.79 ± 0.25. It is most naturally interpreted as inverse Compton emission by cosmic ray electrons and positrons injected by millisecond pulsars. We find the luminosity of this power-law component is comparable to, or slightly smaller than, the luminosity of the curved component, suggesting the fraction of millisecond pulsar spin-down luminosity into relativistic leptons is similar to the fraction of the spin-down luminosity into prompt magnetospheric radiation.

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confidence: 99%

Evidence for a high-energy tail in the gamma-ray spectra of globular clusters

Song

Macías

Horiuchi

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…The γ-ray emission of GlCs is widely assumed to originate from MSPs that reside in the clusters (Cheng et al 2010;Bednarek & Sobczak 2013), and the detection of pulsed gamma-ray emission from two GlCs has further strengthened the case for this connection (Freire et al 2011;Wu et al 2013). In concert with LMXBs, a strong correlation between γ-ray luminosity L γ and the stellar encounter rate Γ has been established among GlCs (Abdo et al 2010;Hui et al 2011;Hooper & Linden 2016;Tam et al 2016;Zhang et al 2016;Lloyd et al 2018;de Menezes et al 2019), which lend a support to the dynamical origin of MSPs in GlCs.…”

Section: Introductionmentioning

confidence: 94%

A Fermi-LAT Study of Globular Cluster Dynamical Evolution in the Milky Way: Millisecond Pulsars as the Probe

Feng,

Cheng,

Wang

et al. 2024

Res. Astron. Astrophys.

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Using archival {\it Fermi}-LAT data with a time span of $\sim12$ years, we study the population of Millisecond Pulsars (MSPs) in Globular Clusters (GlCs) and investigate their dependence on cluster dynamical evolution in the Milky Way Galaxy. We show that the $\gamma$-ray luminosity ($L_{\gamma}$) and emissivity (i.e., $\epsilon_{\gamma}=L_{\gamma}/M$, with $M$ the cluster mass) are good indicators of the population and abundance of MSPs in GlCs, and they are highly dependent on the dynamical evolution history of the host clusters. Specifically speaking, the dynamically older GlCs with more compact structures are more likely to have larger $L_{\gamma}$ and $\epsilon_{\gamma}$, and these trends can be summarized as strong correlations with cluster stellar encounter rate $\Gamma$ and the specific encounter rate ($\Lambda=\Gamma/M$), with $L_{\gamma}\propto \Gamma^{0.70\pm0.11}$ and $\epsilon_{\gamma}\propto \Lambda^{0.73\pm0.13}$ for dynamically normal GlCs. However, as GlCs evolve into deep core collapse, these trends are found to be reversed, implying that strong encounters may have lead to the disruption of Low-Mass X-ray Binaries (LMXBs) and ejection of MSPs from core-collapsed Systems. Besides, the GlCs are found to exhibit larger $\epsilon_{\gamma}$ with increasing stellar mass function slope ($\epsilon_{\gamma}\propto 10^{(0.57\pm0.1)\alpha}$), decreasing tidal radius ($\epsilon_{\gamma}\propto R_t^{-1.0\pm0.22}$) and distances from the Galactic Center (GC, $\epsilon_{\gamma}\propto R_{gc}^{-1.13\pm0.21}$). These correlations indicate that, as GlCs losing kinetic energy and spiral in towards GC, tidal stripping and mass segregation have a preference in leading to the loss of normal stars from GlCs, while MSPs are more likely to concentrate to cluster center and be deposited into the GC. Moreover, we gauge $\epsilon_{\gamma}$ of GlCs is $\sim10-1000$ times larger than the Galactic bulge, the latter is thought to reside thousands of unresolved MSPs and may responsible for the GC $\gamma$-ray excess, which support that GlCs are generous contributors to the population of MSPs in the GC.

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“…Thanks to the Large Area Telescope (LAT) on board Fermi Gamma-ray Space Telescope (Fermi-LAT; Atwood et al 2009), γ-ray emissions from GCs were first detected in GC 47 Tucanae (Abdo et al 2009). Up to now, approximately 40 GCs' γ-rays have been detected in the fourth Fermi-LAT source catalog for Data Release 3 (4FGL-DR3; Abdollahi et al 2022) and Abdo et al (2010a), Kong et al (2010), Tam et al (2011), Eger & Domainko (2012), Zhou et al (2015), Tam et al (2016), Zhang et al (2016), Lloyd et al (2018) et al (2022). In GC, the first γ-ray pulsation from an individual pulsar, PSR J1823−3021A, was also reported in NGC 6624 (Freire et al 2011) with a 5.44 ms spin period (P) and a larger spin-down rate of P 3.38 10 18 = ´- s s −1 , its spin-down luminosity is E 8.3 10 35 =  erg s −1 .…”

Section: Introductionmentioning

confidence: 99%

A Possible γ-Ray Pulsation from PSR J1740−5340B in the Globular Cluster NGC 6397

Zheng,

Zhang,

Zhang

2024

Res. Astron. Astrophys.

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Recently, a new radio millisecond pulsar (MSP) J1740–5340B, hosted in the globu- lar cluster NGC 6397, was reported with a spin period 5.78 ms in an eclipsing binary system with an orbital period of 1.97 day. Based on a modified radio ephemeris updated by tool tempo2, we analyze the ∼15 year γ-ray data obtained from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope and detect PSR J1740−5340B’s γ-ray pulsation at a confidence level of ∼4σ with a weighted H-test value of ∼26. By performing a phase-resolved analysis, the γ-ray luminosity in on-pulse interval of PSR J1740–5340B is Lγ ∼ 3.8 × 1033 erg/s using the NGC 6397’s distance of 2.48 kpc. And γ-rays from the on-pulse part of PSR J1740−5340B contribute ∼90% of the total observed γ-ray emissions from NGC 6397. No significant γ-ray pulsation of another MSP J1740–5340A in the GC is detected. Considering that the previous four cases of MSPs in GCs, more data in γ-ray, X-ray, and radio are encouraged to finally confirm the γ-ray emissions from MSP J1740−5340B, especially starving for a precise ephemeris.

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Gamma-ray emission from high Galactic latitude globular clusters

Cited by 10 publications

References 38 publications

Evidence for a high-energy tail in the gamma-ray spectra of globular clusters

Evidence for a high-energy tail in the gamma-ray spectra of globular clusters

A Fermi-LAT Study of Globular Cluster Dynamical Evolution in the Milky Way: Millisecond Pulsars as the Probe

A Possible γ-Ray Pulsation from PSR J1740−5340B in the Globular Cluster NGC 6397

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