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Issue A&A
Volume 372, Number 3, June IV 2001
Page(s) 916 - 921
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20010546



A&A 372, 916-921 (2001)
DOI: 10.1051/0004-6361:20010546

The first outburst of SAX J1808.4-3658 revisited

J. J. M. in 't Zand1, 2, R. Cornelisse1, 2, E. Kuulkers1, 2, J. Heise1, L. Kuiper1, A. Bazzano3, M. Cocchi3, J. M. Muller1, 4, L. Natalucci3, M. J. S. Smith1, 5 and P. Ubertini3

1  Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2  Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
3  Istituto di Astrofisica Spaziale (CNR), Area Ricerca Roma Tor Vergata, Via del Fosso del Cavaliere, 00133 Roma, Italy
4  BeppoSAX Science Data Center, Nuova Telespazio, Via Corcolle 19, 00131 Roma, Italy
5  BeppoSAX Science Operation Center, Nuova Telespazio, Via Corcolle 19, 00131 Roma, Italy

(Received 30 August 2000 / Accepted 6 April 2001 )

Abstract
Data of the 1996 outburst of the single-known accreting millisecond pulsar SAX J1808.4-3658, taken with the Wide Field Cameras (WFCs) on BeppoSAX, are revisited with more complete data coverage and more comprehensive analysis techniques than in a previous report. An additional type-I X-ray burst was identified which occurred at a time when the persistent emission is below the detection limit, roughly 30 days after outburst maximum. This burst is three times longer than the first two bursts, and 50% brighter. It is the brightest burst within the ~1700 type-I bursts detected so far with the WFCs. A spectral analysis of the data reveals a distance to SAX J1808.4-3658 of ~2.5 kpc. This is an update from a previously reported value of 4 kpc. We present the evidence that we have for the presence of oscillations at the pulsar frequency during part of the newly found burst. Such an oscillation would lend support to the idea that the frequency of millisecond burst oscillations in other objects is very close to the neutron star rotation frequency.


Key words: binaries: close -- pulsars: individual (SAX J1808.4-3658) -- X-rays: bursts

Offprint request: J. J. M. in 't Zand, jeanz@sron.nl

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