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ATEL # 2406; J. L. Prieto, J. R. Rigby, V. Scowcroft, W. L. Freedman, B. F. Madore, A. Monson, S. E. Persson, M. Seibert (Carnegie Observatories), V. Mager (OU), L. Sturch (BU)
on 27 Jan 2010; 1:08 UT
Password Certification: Jose L. Prieto (firstname.lastname@example.org)
Subjects: Infra-Red, Optical, Transients
We report a late-time detection of the 2008 optical transient in NGC 300 (IAUC # 8946 , ATEL #1544, ATEL #1550) in warm Spitzer/IRAC 3.6 micron images obtained by the Carnegie Hubble Project (CHP) on Dec. 21.8 UT, 2009, 587 days after discovery. The transient is one of the brightest point sources in the IRAC field, with a measured flux density of 2.2 mJy ([3.6] = 12.8 mag; nu * L_nu = 1.9e4 Lsun); this is ~240 times (6 mag) brighter than the progenitor star at 3.6 micron (see Fig. 1). The transient has faded by a factor of ~4 (1.5 mag) at 3.6 micron since a Spitzer/IRS spectrum was obtained on Aug. 14.4 UT, 2008 (92 days after discovery) by Prieto et al. (2009, ApJ, 705, 1425). Assuming a linear decay at late times, this indicates a decay slope at 3.6 micron of ~0.003 mag/day, which is slower than the Co-56 decay slope (see Fig. 2).
Almost simultaneous (Dec. 20.2 UT, 2009) Sloan r-band observations obtained with LDSS-3 on the Magellan/Clay 6.5m telescope show that the transient has faded significantly in the optical (see Bond et al. 2009, ApJ, 695, 154L, for the early light curve), to an r-band magnitude of 23.9 +/- 0.2 (nu * L_nu = 4.3e2 Lsun). We note that the flux in this filter might be significantly affected by H-alpha in emission, which was very strong initially.
The bright mid-IR detection combined with fading in the optical confirms the fast evolution to redder optical and near-IR colors seen by Bond et al. (2009) in late 2008. The source is most likely becoming self-enshrouded in its pre-existing progenitor dust (ATEL #1550), which was left largely intact after the outburst (Prieto et al. 2009). Newly formed dust might also contribute to the strong late-time mid-IR emission.
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