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Astron. Astrophys. 363, 188-197 (2000)


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Hard X-rays from Type II bursts of the Rapid Burster and its transition toward quiescence

N. Masetti 1, F. Frontera 1,2, L. Stella *  3, M. Orlandini 1, A.N. Parmar 4, S. Del Sordo 5, L. Amati 1, E. Palazzi 1, D. Dal Fiume **  1, G. Cusumano 5, G. Pareschi 6, I. Lapidus 7 and R.A. Remillard 8

1 Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, via Gobetti 101, 40129 Bologna, Italy
2 Dipartimento di Fisica, Università di Ferrara, via Paradiso 11, 44100 Ferrara, Italy
3 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
4 Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
5 Istituto di Fisica Cosmica ed Applicazioni all'Informatica, CNR, via La Malfa 153, 90146 Palermo, Italy
6 Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
7 McKinsey & Co., Inc., No. 1 Jermyn Street, London SW1Y 4UH, UK
8 Center for Space Research and Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Received 13 June 2000 / Accepted 29 August 2000

Abstract

We report on 4 BeppoSAX Target Of Opportunity observations of MXB 1730-335, the Rapid Burster (RB), made during the 1998 February-March outburst. In the first observation, approximately 20 days after the outburst peak, the X-ray light curve showed Type II bursts at a rate of 43 hr-1. Nine days later, during the second BeppoSAX pointing, only 5 Type II bursts were detected at the beginning of the observation. During the third pointing no X-ray bursts were detected and in the fourth and final observation the RB was not detected at all. Persistent emission from the RB was detected up to 10 keV during the first three pointings. The spectra of the persistent and bursting emissions below 10 keV were best fit with a model consisting of two blackbodies. An additional component (a power law) was needed to describe the 1-100 keV bursting spectrum when the persistent emission was subtracted. To our knowledge, this is the first detection of the RB beyond 20 keV. We discuss the evolution of the spectral parameters for the bursting and persistent emission during the outburst decay. The light curve, after the second BeppoSAX pointing, showed a steepening of the previous decay trend, and a sharper decay rate leading to quiescence was observed with BeppoSAX in the two subsequent observations. We interpret this behaviour as caused by the onset of the propeller effect. Finally, we infer a neutron star magnetic field [FORMULA] Gauss.

Key words: stars: individual: MXB 1730-335 – stars: neutron – X-rays: bursts – X-rays: general – X-rays: stars

* Affiliated to International Center for Relativistic Astrophysics
** Deceased on August 5, 2000

Send offprint requests to: N. Masetti (masetti@tesre.bo.cnr.it)

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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