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Strong Variability of Overlapping Iron Broad Absorption Lines in Five Radio-selected Quasars

【标题】Strong Variability of Overlapping Iron Broad Absorption Lines in Five Radio-selected Quasars

【Title】

【DOI】10.1088/0004-637X/803/2/58

【作者】 张少华 ; 周宏岩; Tinggui Wang; Huiyuan Wang; 史习珩; Bo Liu; Wenjuan Liu; Zhenzhen Li; Shufen Wang

【Author】 Zhang Shaohua ; Zhou Hongyan ; Tinggui Wang ; Huiyuan Wang ; Shi Xiheng ; Bo Liu ; Wenjuan Liu ; Zhenzhen Li ; Shufen Wang

【期刊】The Astrophysical Journal

【Journal】The Astrophysical Journal

【期刊年份】2015

【卷】803

【期】2

【关键词】 galaxies: active; quasars: absorption lines; quasars: general

【Keywords】 galaxies: active; quasars: absorption lines; quasars: general

【摘要】We present a study of the variability of broad absorption lines (BALs) in a uniformly radio-selected sample of 28 BAL quasars using the archival data from the FIRST Bright Quasar Survey and the Sloan Digital Sky Survey as well as data we obtained ourselves, covering timescales of ˜ 1-10 yr in the quasar rest frame. To our surprise, five quasars exhibiting strong variations all belong to a special subclass of “overlapping-trough” iron low-ionization BAL (OFeLoBAL) quasars; however, four other non-overlapping FeLoBALs (non-OFeLoBALs) are invariant, except in one case with a weak change in optical depth. Meanwhile, we also identify six typical variations of high-ionization and low-ionization BALs in this BAL quasar sample. Photoionization models suggest that OFeLoBALs are formed in relatively dense (ne > 106 cm-3) outflows at distances from the continuum source that range from the subparsec scale to dozens of parseconds. These formation conditions differ from those of non-OFeLoBALs, which are likely produced by low-density gas located at distances of hundreds to thousands of parseconds. Thus, OFeLoBALs and non-OFeLoBALs, i.e., FeLoBALs with/without strong BAL variations, may represent the bimodality of Fe ii absorption; the former are located in active galactic nucleus environments rather than in the host galaxy. We suggest that a high density and a small distance are the necessary conditions for the creation of OFeLoBALs. As suggested in the literature, strong BAL variability is possibly attributable to variability in the covering factor of the BAL regions caused by clouds transiting across the line of sight rather than to variations in ionization.

【Abstract】We present a study of the variability of broad absorption lines (BALs) in a uniformly radio-selected sample of 28 BAL quasars using the archival data from the FIRST Bright Quasar Survey and the Sloan Digital Sky Survey as well as data we obtained ourselves, covering timescales of ˜ 1-10 yr in the quasar rest frame. To our surprise, five quasars exhibiting strong variations all belong to a special subclass of “overlapping-trough” iron low-ionization BAL (OFeLoBAL) quasars; however, four other non-overlapping FeLoBALs (non-OFeLoBALs) are invariant, except in one case with a weak change in optical depth. Meanwhile, we also identify six typical variations of high-ionization and low-ionization BALs in this BAL quasar sample. Photoionization models suggest that OFeLoBALs are formed in relatively dense (ne > 106 cm-3) outflows at distances from the continuum source that range from the subparsec scale to dozens of parseconds. These formation conditions differ from those of non-OFeLoBALs, which are likely produced by low-density gas located at distances of hundreds to thousands of parseconds. Thus, OFeLoBALs and non-OFeLoBALs, i.e., FeLoBALs with/without strong BAL variations, may represent the bimodality of Fe ii absorption; the former are located in active galactic nucleus environments rather than in the host galaxy. We suggest that a high density and a small distance are the necessary conditions for the creation of OFeLoBALs. As suggested in the literature, strong BAL variability is possibly attributable to variability in the covering factor of the BAL regions caused by clouds transiting across the line of sight rather than to variations in ionization.

【基金/项目】 Chinese Natural Science Foundation (NSFC-11203021) ; National Basic Research Program of China (the “973” Program, 2013CB834905) ; the SOC program (CHINARE2014-02-03). ; Chinese Natural Science Foundation (NSFC-11421303) ; the Strategic Priority Research of Sciences (XDB09000000)

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