Continued Swift and GROND observations of Swift J2058+0516

ATel #3425; Arne Rau, Jochen Greiner, Patricia Schady, Felipe Olivares E (all MPE Garching), Hans Krimm, Stephen Holland (both CRESST/GSFC/USRA)
on 11 Jun 2011; 18:59 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We report further observations of the transient Swift J2058+0516 (Krimm et al. 2011, ATEL #3384; Greiner et al. 2011, ATEL #3385, Rau et al. 2011, ATEL #3390) by Swift and GROND.

Swift J2058.4+0516 has now faded below detectability in the Swift/BAT (15-50 keV). The last significant detection was for the 4-day period 30 May - 2 June 2011 (0.0022 +/- 0.0004 ct/s/cm^2; ~10 mCrab). The 1-sigma upper limit for 10 June 2011 is 0.003 ct/s/cm^2.

We obtained 4 Swift/XRT observations over the last 10 days, sometimes stretching over several orbits. We detect seemingly stochastic variability by a factor of two within hours. There is also a long-term trend of a decreasing X-ray intensity from 1.5 ct/s at maximum to 0.5 ct/s maximum of the short-term variations.

In a stack of the recent Swift observations, Swift J2058+0516 is also detected in UVOT, at the following magnitudes (in Vega system, uncorrected for Galactic foreground extinction):

uvm2 = 21.9 +/- 0.4
uvw2 = 22.2 +/- 0.4
uvw1 = 21.9 +/- 0.4
u = 21.1 +/- 0.3
b > 22.4

The UVOT position is:

RA (J2000) 20:58:19.87 = 314.58279 (deg)
Dec (J2000) +05:13:32.8 = +5.22578 (deg)

with an estimated uncertainty of 0.83 arcsec (radius, 90% confidence, statistical + systematic), fully consistent with the earlier reported GROND position (Rau et al. 2011, ATEL #3390).

We also continued observations of Swift J2058.4+0516 with GROND (Greiner et al. 2008, PASP 120, 405), the 7-channel imager mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile) on June 3, 2011 at 09:32 UT.

In a 24-min exposure we detect the candidate counterpart reported by Rau et al. 2011 (ATEL #3390) with the following magnitudes (all in the AB system):

g = 22.54 +/- 0.05
r = 22.75 +/- 0.09
i = 22.81 +/- 0.14
z > 23.2
J > 21.8
H > 21.3
K > 19.2

These magnitudes were derived by calibrating the images against SDSS and 2MASS field stars and are not corrected for Galactic foreground reddening of E(B-V)~0.1mag (Schlegel et al. 1998). Upper limits are 3-sigma.

Comparison with the previous GROND observation indicates that the optical emission of the source did not change from May 28th to June 3rd. (Note, that the photometry given in Rau et al. 2011, ATEl #3390 suffered from a problem in the reduction and that the corrected magnitudes are consistent with the results reported above.)

The GROND observations have been obtained ~12hr after a 3ks Swift/XRT pointing (00032004003) starting on June 2rd at 09:50 UT and lasting until 21:18 UT. The XRT spectrum is best fit with a powerlaw with slope 1.51 +/- 0.04 and N_H=(7.4 +/- 0.3)E20 cm^-1, the latter largely consistent with the Galactic foreground column density of N_H= 6.5E20 cm^-1 (Kalberla et al. 2005). The count rate was 1.1 ct/s corresponding to an unabsorbed 0.3-10 keV flux of 5.8E11 erg/s/cm^2. Given the short-term X-ray variations and the 12hr time difference between the Swift and GROND observation, the X-ray-to-optical flux ratio remains high at approx. 12,000.