Mixed H/He bursts in SAX J1748.9–2021 during the spectral change of its 2015 outburst

¹ Department of Physics, Xiangtan University, Xiangtan, 411105, PR China
e-mail: lizhaosheng@xtu.edu.cn
² International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
³ Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics/Laboratory for High-Energy Physics, University of Bern, Switzerland
⁴ Departement Physik, Universität Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
⁵ International Space Science Institute Beijing, No.1 Nanertiao, Zhongguancun, Haidian District, 100190 Beijing, PR China
⁶ Department of Astronomy, University of Geneva, Chemin d’ecogia 16, 1290 Versoix, Switzerland
⁷ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁸ Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku, Finland
⁹ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
¹⁰ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
¹¹ Department of Physics and McGill Space Institute, McGill University, 3600 Rue University, Montreal, QC, H3A 2T8, Canada
¹² School of Physics and Astronomy, Monash University, Australia, Australia VIC, 3800, Australia
¹³ Monash Centre for Astrophysics, Monash University, Australia, Australia VIC, 3800, Australia
¹⁴ Laboratory for Particle Astrophysics, Institute of High Energy Physics, Beijing, 100049, PR China

Received: 15 July 2018
Accepted: 12 October 2018

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 ≈2 to ≈1 h. The relation between the recurrence time, Δt_rec, and the local mass accretion rate per unit area onto the compact object, ṁ, is fitted by a power-law model, and yielded as best fit at Δt_rec ∼ ⟨ṁ⟩^{−1.02±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.