Very rapid turn on of SAX J1808.4-3658
ATel #7379; S. Campana (INAF/OAB), P. D'Avanzo (INAF/OAB), C. Baglio (INAF/OAB), F. Coti Zelati (INAF/OAB), J. Kennea (PSU)
on 13 Apr 2015; 14:08 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)
Subjects: Binary, Neutron Star, Transient
Since Feb. 2015 we started a weekly monitoring program on the accreting millisecond pulsar SAX J1808.4-3658 (SAX J1808) with Swift/XRT. Sanna et al. (2015, Atel #7364) reported a significant rate increase from SAX J1808.4-3658 starting on 2015 April 9 (MJD 57121).
Our closest observation gets back to Apr 3. In this 5.7 ks observation the XRT did not detect SAX J1808 with a 3 sigma upper limit on the 0.3-10 keV count rate of 3.6e-3 c/s.
The following observation was automatically commanded after a type I burst from SAX J1808 on Apr 11 (Malesani et al. 2015, GCN 17686). The count rate (after the burst was gone) was ~32.5+/-0.5 c/s (0.3 ks, after burst cutting, see also Li & Kong 2015, Atel #7376).
UVOT monitored the field too using the filter of the day. Despite the relatively high absorbing column density, the UVOT was able to detect SAXJ1808 with the UVW1 filter at 16.05+/-0.06 (487 s exposure) on Apr 11, whereas only a 3 sigma upper limit of 19.92 can be set on Apr 3 (916 s exposure).
This testifies for a very rapid turn on of the source.
