Late time Spitzer detection of the 2008 optical transient in NGC 300
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; 01:08 UT
Credential Certification: Jose L. Prieto (jose@obs.carnegiescience.edu)
Subjects: Infra-Red, Optical, Transient
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.
Webpage with text and figures
