A1 Refereed original research article in a scientific journal
Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst
Authors: Li Z, De Falco V, Falanga M, Bozzo E, Kuiper L, Poutanen J, Cumming A, Galloway DK, Zhang S
Publisher: EDP SCIENCES S A
Publication year: 2018
Journal: Astronomy and Astrophysics
Journal name in source: ASTRONOMY & ASTROPHYSICS
Journal acronym: ASTRON ASTROPHYS
Article number: ARTN A114
Volume: 620
Number of pages: 6
ISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/201833857
Self-archived copy’s web address: https://arxiv.org/abs/1810.05490
Abstract
SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It is also known as an X-ray burster source discovered by Beppo-SAX. We analyzed the persistent emission and type-I X-ray burst properties during its 2015 outburst. The source changed from hard to soft state within half day. We modeled the broadband spectra of the persistent emission in the (1-250) keV energy band for both spectral states using the quasi-simultaneous INTEGRAL and Swift data. The broadband spectra are well fitted by an absorbed thermal Componization model, COMPPS, in a slab geometry. The best-fits for the two states indicate significantly different plasma temperature of 18 and 5 keV and the Thomson optical depths of three and four, respectively. In total, 56 type-I X-ray bursts were observed during the 2015 outburst, of which 26 detected by INTEGRAL in the hard state, 25 by XMM-Newton in the soft state, and five by Swift in both states. As the object transited from the hard to the soft state, the recurrence time for X-ray bursts decreased from approximate to 2 to approximate to 1 h. The relation between the recurrence time, Delta t(rec )and the local mass accretion rate per unit area onto the compact object, in, is fitted by a power-law model, and yielded as best fit at Delta t(rec )similar to <(m) over dot >(-1)(.0)(2 +/-)(0.)(03) using all X-ray bursts. In both cases, the observed recurrence times are consistent with the mixed hydrogen and helium bursts. We also discuss the effects of type-I X-ray bursts prior to the hard to soft transition.
SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It is also known as an X-ray burster source discovered by Beppo-SAX. We analyzed the persistent emission and type-I X-ray burst properties during its 2015 outburst. The source changed from hard to soft state within half day. We modeled the broadband spectra of the persistent emission in the (1-250) keV energy band for both spectral states using the quasi-simultaneous INTEGRAL and Swift data. The broadband spectra are well fitted by an absorbed thermal Componization model, COMPPS, in a slab geometry. The best-fits for the two states indicate significantly different plasma temperature of 18 and 5 keV and the Thomson optical depths of three and four, respectively. In total, 56 type-I X-ray bursts were observed during the 2015 outburst, of which 26 detected by INTEGRAL in the hard state, 25 by XMM-Newton in the soft state, and five by Swift in both states. As the object transited from the hard to the soft state, the recurrence time for X-ray bursts decreased from approximate to 2 to approximate to 1 h. The relation between the recurrence time, Delta t(rec )and the local mass accretion rate per unit area onto the compact object, in, is fitted by a power-law model, and yielded as best fit at Delta t(rec )similar to <(m) over dot >(-1)(.0)(2 +/-)(0.)(03) using all X-ray bursts. In both cases, the observed recurrence times are consistent with the mixed hydrogen and helium bursts. We also discuss the effects of type-I X-ray bursts prior to the hard to soft transition.
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