Constraints on the neutron star magnetic field of the two X-ray transients SAX J1808.4–3658 and Aql X–1
Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: email@example.com
2 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy e-mail: firstname.lastname@example.org
Corresponding author: T. Di Salvo, email@example.com
Accepted: 11 October 2002
The recently discovered coherent X-ray pulsations at a frequency of ~400 Hz in SAX J1808.4–3658, together with a measure of the source luminosity in quiescence, allow us to put an upper limit on the neutron star magnetic field, that is Gauss, using simple considerations on the position of the magnetospheric radius during quiescent periods. Combined with the lower limit inferred from the presence of X-ray pulsations, this constrains the SAX J1808.4–3658 neutron star magnetic field in the quite narrow range Gauss. Similar considerations applied to the case of Aql X–1 give a neutron star magnetic field lower than ~109 Gauss.
Key words: accretion disks / stars: individual: SAX J1808.4–3658, Aql X–1 / stars: neutron / X-rays: stars / X-rays: binaries / X-rays: general
© ESO, 2003