Swift UVOT and XRT observations of the recent outburst of SAX J1808.4-3658

ATel #7376; K. L. Li, A. K.H. Kong (National Tsing Hua University, Taiwan)
on 13 Apr 2015; 10:23 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

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Based on the XRT position of SAX J1808.4-3658 reported in ATel #7371, we found the optical/UV counterpart of SAX J1808.4-3658 in all seven UVOT images (six regular filters plus the white light filter) taken on 2015-04-11 (ObsID: 00637765992). Using a 3 arcsec radius source region and an off-target source-free background region (10 arcsec), we measured the UVOT magnitudes of the outburst in Vega system as following:

v = 16.62 +/- 0.27 (stat) +/- 0.01 (sys)
b = 16.98 +/- 0.16 (stat) +/- 0.02 (sys)
u = 16.01 +/- 0.05 (stat) +/- 0.02 (sys)
uvw1 = 16.24 +/- 0.11 (stat) +/- 0.03 (sys)
uvm2 = 16.08 +/- 0.32 (stat) +/- 0.03 (sys)
uvw2 = 15.98 +/- 0.16 (stat) +/- 0.03 (sys)
uvwh = 16.37 +/- 0.05 (stat) +/- 0.04 (sys)

Comparing with the pre-outburst uvw1 data taken on 2015-04-03 (i.e., uvw1 = 20.44 +/- 0.36 (stat) +/- 0.03 (sys); ObsID: 00030034072), the magnitude change is about 4.2 mag, equivalent to a brightening factor of 48.

We also combined the outburst UVOT data with the simultaneous XRT observation (OBID: 00637765991; 0.7 ks in WT mode) to obtain a broad-band SED. The SED can be reasonably described by an absorbed three-component model, composed of a simple power-law (PL), a neutron star atmosphere (NSA; M(NS) = 1.4 M_sun and R(NS) = 10 km were assumed), and a standard accretion disk (DISKBB). The best-fit parameters are (with 90% c.l.):

nH = 1.05 (-0.28, +0.30) 10^21 cm^-2
Index[PL] = 1.46 (-0.04, +0.04)
log(T_eff)[NSA] = 6.90 (-0.15, +0.16) in Kelvin
T_in[DISKBB] = 0.19 (-0.02, +0.02) keV

The 0.3-10 keV unabsorbed X-ray flux is about 2.5E-9 erg/cm^2/s, which is in a good agreement with the Swift data taken 1.5 hours earlier in the PC mode (ATel #7371). In addition, the distance of SAX J1808.4-3658 inferred from the NSA component is 3.9 kpc, which is roughly consistent with the result of Galloway & Cumming (2006) (i.e., 3.4 - 3.6 kpc).

We acknowledge the use of public data from the Swift data archive.