GROND reveals optical/NIR brightening of Swift J0513.4-6547

ATel #2013; J. Greiner, P. Afonso, T. Kruehler, A. Rau (MPE Garching)
on 15 Apr 2009; 22:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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GROND (Greiner et al. 2008, PASP 120, 405), the 7-channel imager mounted at the 2.2m ESO/MPI telescope at La Silla Observatory (Chile) started follow-up observations of the transient source Swift J0513.4-6547 (Krimm et al. 2009, ATEL #2011) on 14 Apr 2009, 23:55 UT simultaneously in the g'r'i'z'JHK bands. A total of 8 30-sec exposures in g'r'i'z' and 48 10-sec exposures in JHK were obtained.
Within the 1.9 arcsec Swift XRT error circle (Krimm et al. 2009, ATEL #2011) we detect a single source, at position RA(2000.0) = 05:13:28.29, Decl.(2000.0) = -65:47:18.9 with an error of 0.3 arcsec. This is consistent with the object 2MASS 05132826-6547187, already proposed by Krimm et al. (2009, ATEL #2011) as the optical counterpart of Swift J0513.4-6547.
We measure the following magnitudes (all in AB system):

  
g' (AB) = 14.6 ± 0.1  
r' (AB) = 15.0 ± 0.1  
i' (AB) = 15.1 ± 0.1  
z' (AB) = 15.3 ± 0.1  
J (Vega) = 14.7 -> J (AB) = 15.6 ± 0.1  
H (Vega) = 14.6 -> H (AB) = 16.0 ± 0.1  
K (Vega) = 14.6 -> K (AB) = 16.4 ± 0.1  

Compared to the 2MASS magnitudes (exposures from 10 Dec 1998) of J (Vega) = 15.2 mag, H (Vega) = 15.1 mag, and K (Vega) = 14.8 mag, as well as the DENIS magnitudes (exposures from 22 Dec 1996) of I = 15.09 +/- 0.04 mag and J (Vega) = 15.20 +/- 0.14 mag, this object is now about 0.5 mag brighter.
The majority of high-mass X-ray binaries are Be/X-ray systems, in which the blue part of the optical spectrum is usually dominated by the mass donor, and the red part normally has a significant contribution from the circumstellar disk. Our foreground-extinction corrected color r'-z' = -0.5 mag corresponds to about R-I = +0.55 mag, which is incompatible with the colors of an O or B star. This suggests that already the z'-band is strongly affected by the putative disk. Optical variability in Be/X-ray binaries is not unusual, and amplitudes of up to 0.8 mag over time scale of years have been reported (e.g. Coe et al. 2005, MN 356, 502). While X-ray outbursts are normally associated with the passage of the neutron star close to the circumstellar disk, our observations indicate a substantial brightening of the disk, suggesting that possibly increased mass loss of the donor is responsible for both, the brightening as compared to the 2MASS/DENIS epochs, as well as the X-ray outburst.
We urge for spectroscopic observations to measure the strength and width of the expected emission lines. Given the relatively low degree of crowding (see finding chart at http://www.mpe.mpg.de/~jcg/xrt/swiftj0513p4m6547.html ), a search for long-term variability in archival data might also be interesting.