Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
ABSTRACT 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 launc...
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Veröffentlicht in: | Monthly notices of the Royal Astronomical Society 2021-11, Vol.507 (4), p.5161-5176 |
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creator | Song, Deheng Macias, Oscar Horiuchi, Shunsaku Crocker, Roland M Nataf, David M |
description | ABSTRACT
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-yr 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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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-yr 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.</description><identifier>ISSN: 0035-8711</identifier><identifier>EISSN: 1365-2966</identifier><identifier>DOI: 10.1093/mnras/stab2406</identifier><language>eng</language><publisher>United States: Oxford University Press</publisher><subject>ASTRONOMY AND ASTROPHYSICS ; gamma-rays: general ; globular clusters: general ; pulsars: general</subject><ispartof>Monthly notices of the Royal Astronomical Society, 2021-11, Vol.507 (4), p.5161-5176</ispartof><rights>2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society 2021</rights><lds50>peer_reviewed</lds50><oa>free_for_read</oa><woscitedreferencessubscribed>false</woscitedreferencessubscribed><citedby>FETCH-LOGICAL-c366t-a3bacb38d98b7bc43bcc7e404b41f7f5b2210992a0672998de421bd52b0553da3</citedby><cites>FETCH-LOGICAL-c366t-a3bacb38d98b7bc43bcc7e404b41f7f5b2210992a0672998de421bd52b0553da3</cites><orcidid>0000-0003-3441-4212 ; 0000-0002-2036-2426 ; 0000000220362426 ; 0000000334414212</orcidid></display><links><openurl>$$Topenurl_article</openurl><openurlfulltext>$$Topenurlfull_article</openurlfulltext><thumbnail>$$Tsyndetics_thumb_exl</thumbnail><link.rule.ids>230,314,780,784,885,1604,27924,27925</link.rule.ids><linktorsrc>$$Uhttps://dx.doi.org/10.1093/mnras/stab2406$$EView_record_in_Oxford_University_Press$$FView_record_in_$$GOxford_University_Press</linktorsrc><backlink>$$Uhttps://www.osti.gov/servlets/purl/1837494$$D View this record in Osti.gov$$Hfree_for_read</backlink></links><search><creatorcontrib>Song, Deheng</creatorcontrib><creatorcontrib>Macias, Oscar</creatorcontrib><creatorcontrib>Horiuchi, Shunsaku</creatorcontrib><creatorcontrib>Crocker, Roland M</creatorcontrib><creatorcontrib>Nataf, David M</creatorcontrib><creatorcontrib>Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)</creatorcontrib><title>Evidence for a high-energy tail in the gamma-ray spectra of globular clusters</title><title>Monthly notices of the Royal Astronomical Society</title><description>ABSTRACT
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-yr 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.</description><subject>ASTRONOMY AND ASTROPHYSICS</subject><subject>gamma-rays: general</subject><subject>globular clusters: general</subject><subject>pulsars: general</subject><issn>0035-8711</issn><issn>1365-2966</issn><fulltext>true</fulltext><rsrctype>article</rsrctype><creationdate>2021</creationdate><recordtype>article</recordtype><recordid>eNqFkDtPwzAUhS0EEqWwMltsDG79ipOMqCoPqYgFZuvacRKjPCrbQeq_J9AyM93lu0fnfAjdMrpitBTrfggQ1zGB4ZKqM7RgQmWEl0qdowWlIiNFztgluorxk1IqBVcL9Lr98pUbrMP1GDDg1jctcYMLzQEn8B32A06tww30PZAABxz3zqYAeKxx041m6iBg200xuRCv0UUNXXQ3p7tEH4_b980z2b09vWwedsQKpRIBYcAaUVRlYXJjpTDW5k5SaSSr8zoznM-LSg5U5bwsi8pJzkyVcUOzTFQglujumDvG5HW0Pjnb2nEY5mqaFSKXpZyh1RGyYYwxuFrvg-8hHDSj-seY_jWm_4zND_en1Gn_H_sNUgZvDQ</recordid><startdate>20211101</startdate><enddate>20211101</enddate><creator>Song, Deheng</creator><creator>Macias, Oscar</creator><creator>Horiuchi, Shunsaku</creator><creator>Crocker, Roland M</creator><creator>Nataf, David M</creator><general>Oxford University Press</general><general>Royal Astronomical Society</general><scope>AAYXX</scope><scope>CITATION</scope><scope>OIOZB</scope><scope>OTOTI</scope><orcidid>https://orcid.org/0000-0003-3441-4212</orcidid><orcidid>https://orcid.org/0000-0002-2036-2426</orcidid><orcidid>https://orcid.org/0000000220362426</orcidid><orcidid>https://orcid.org/0000000334414212</orcidid></search><sort><creationdate>20211101</creationdate><title>Evidence for a high-energy tail in the gamma-ray spectra of globular clusters</title><author>Song, Deheng ; Macias, Oscar ; Horiuchi, Shunsaku ; Crocker, Roland M ; Nataf, David M</author></sort><facets><frbrtype>5</frbrtype><frbrgroupid>cdi_FETCH-LOGICAL-c366t-a3bacb38d98b7bc43bcc7e404b41f7f5b2210992a0672998de421bd52b0553da3</frbrgroupid><rsrctype>articles</rsrctype><prefilter>articles</prefilter><language>eng</language><creationdate>2021</creationdate><topic>ASTRONOMY AND ASTROPHYSICS</topic><topic>gamma-rays: general</topic><topic>globular clusters: general</topic><topic>pulsars: general</topic><toplevel>peer_reviewed</toplevel><toplevel>online_resources</toplevel><creatorcontrib>Song, Deheng</creatorcontrib><creatorcontrib>Macias, Oscar</creatorcontrib><creatorcontrib>Horiuchi, Shunsaku</creatorcontrib><creatorcontrib>Crocker, Roland M</creatorcontrib><creatorcontrib>Nataf, David M</creatorcontrib><creatorcontrib>Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)</creatorcontrib><collection>CrossRef</collection><collection>OSTI.GOV - Hybrid</collection><collection>OSTI.GOV</collection><jtitle>Monthly notices of the Royal Astronomical Society</jtitle></facets><delivery><delcategory>Remote Search Resource</delcategory><fulltext>fulltext_linktorsrc</fulltext></delivery><addata><au>Song, Deheng</au><au>Macias, Oscar</au><au>Horiuchi, Shunsaku</au><au>Crocker, Roland M</au><au>Nataf, David M</au><aucorp>Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)</aucorp><format>journal</format><genre>article</genre><ristype>JOUR</ristype><atitle>Evidence for a high-energy tail in the gamma-ray spectra of globular clusters</atitle><jtitle>Monthly notices of the Royal Astronomical Society</jtitle><date>2021-11-01</date><risdate>2021</risdate><volume>507</volume><issue>4</issue><spage>5161</spage><epage>5176</epage><pages>5161-5176</pages><issn>0035-8711</issn><eissn>1365-2966</eissn><abstract>ABSTRACT
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-yr 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.</abstract><cop>United States</cop><pub>Oxford University Press</pub><doi>10.1093/mnras/stab2406</doi><tpages>16</tpages><orcidid>https://orcid.org/0000-0003-3441-4212</orcidid><orcidid>https://orcid.org/0000-0002-2036-2426</orcidid><orcidid>https://orcid.org/0000000220362426</orcidid><orcidid>https://orcid.org/0000000334414212</orcidid><oa>free_for_read</oa></addata></record> |
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subjects | ASTRONOMY AND ASTROPHYSICS gamma-rays: general globular clusters: general pulsars: general |
title | Evidence for a high-energy tail in the gamma-ray spectra of globular clusters |
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