The region around SN 1999bw was first imaged in the mid-IR almost 5 years after dis- covery, via Spitzer SINGS Legacy IRAC and MIPS observations of NGC 3198 (program 00159, PI: Kennicutt). Following its detection in the IRAC images (Figure 4.1) (reported by members of our SEEDS team; Sugerman, Meixner, Fabbri & Barlow 2004), further time was awarded in Spitzer Cycles 2 and 4 (programs 20320, PI: Sugerman; and 40010, PI: Meixner) for continued monitoring of this object. Photometric observations spanned 4 years from May 2004 to May 2008, corresponding to 1843–3331 days after discovery, and included 4 epochs of IRAC and MIPS 24 µm observations, and one IRS-PUI 16 µm observation in Cycle 4. Dates and exposure times of the observations are detailed in Table 4.1. Full details of the SINGS and SEEDS observations can be found in Chapter 3 (Section 3.3). The SEEDS IRAC, PUI and MIPS BCD data from the Spitzer pipeline were further processed with MOPEX (Section 3.4.4) to create the final images with improved pixel scales (compared to the standard pipeline PBCD products) of 0.0075/pixel, 1.002/pixel
and 100.
5/pixel, respectively. The IRAC and MIPS pixel sizes were the same as those of the SINGS enhanced data, processed by the SINGS team.
Figure 4.1: The position of SN 1999bw in Spitzer SINGS Legacy images of NGC 3198 observed in May 2004 (IRAC) and December 2004 (MIPS). Panels (a) and (b) show the whole galaxy in
the IRAC 3.6 µm and 5.8 µm channels respectively. The ∼ 2000
×2000
square is centred on the SN coordinates. Panels (c), (d) and (e) zoom in on the square region of the SN field at 4.5, 8.0 and 24 µm respectively, with the crosshairs indicating the SN position. The SN was detected in all four IRAC filters of the SINGS images which, although taken ∼ 5 years after discovery, were the first mid-IR observations of this object. The bright source to the south-west of the SN, “S2”, dominates the SN position at 24 µm.
Table 4.1 summarises the Spitzer mid-IR flux densities and associated uncertainties of SN 1999bw, as well as the upper limits to the flux densities, from 1843–3331 days after discovery (previously presented in Chapter 3). A source coincident with the SN position was detected in most of the IRAC images at all epochs. Emission was also detected from a point source at the SN position in the IRS-PUI 16-µm image from the most recent Cycle 4 data at day 3331, the only observations at that wavelength. At 24 µm, a bright neighbouring source (∼ 400to the south-west of the SN position, and also seen in the IRAC images), dominated the field at this wavelength (panel e of Figure 4.1) and emission at the SN position was uncertain.
Table 4.1: Spitzermid-IR fluxes of SN 1999bw from PSF-fitted photometry.
UT date Age Exp. time Flux density/ upper limits [µJy]
[days] [s]
IRAC IRS-PUI MIPS1
3.6 µm 4.5 µm 5.8 µm 8.0 µm 16 µm 24 µm 2004-05-01 1843 214.4 11 ± 1 39 ± 2 121 ± 6 213 ± 12 · · · · 2004-12-07 2063 168.8 · · · ≤ 650 2005-11-29 2420 321.6 2.6 ± 0.3 14 ± 1 53 ± 4 105 ± 8 · · · · 2005-12-01 2422 840.0 · · · ≤ 650 2006-05-03 2575 321.6 ≤ 10 6.9 ± 0.6 43 ± 3 87 ± 6 · · · · 2006-05-04 2576 840.0 · · · ≤ 650 2008-05-13 3316 964.8 ≤ 10 4.0 ± 0.5 21 ± 3 46 ± 8 · · · · 2008-05-17 3320 1650 · · · ≤ 650 2008-05-28 3331 534.8 · · · 93 ± 16 · · ·
1 The upper limits for the MIPS 24 µm data were estimated by measuring the flux density of the faint point
source seen ∼ 600
to the east of the SN position. This was found to have an average brightness of 657µJy with a standard deviation of 2.5 % over the four epochs, and this approximate value was used as a robust upper limit for the SN brightness at each epoch. See text for further details.
Figure 4.2: Mid-infrared light curves of SN 1999bw in the IRAC wavebands at 3.6, 4.5, 5.8 and 8.0 µm. Upper limits to the 3.6-µm flux densities are indicated by the downward-pointing arrows.
Figure 4.3:The position of SN 1999bw in Spitzer IRAC 3.6, 5.8 and 8.0 µm images, revealing the mid-IR evolution over the four epochs from days 1843–3316. The point source visible at the SN coordinates (centre of the frame, indicated by the cross-hairs) from day 1843 in all IRAC channels, clearly fades during the 4 years of observations. The 2000
by 2000
region around the SN, corresponds to an area of ∼ 1.3 sq. kpc for the adopted distance of 13.7 Mpc to SN 1999bw.
The fluxes were measured by way of PSF-fitting, as described in Section 3.5.1. Flux upper limits for the non-detections by days 2575 and 3316 at 3.6 µm were derived by adding a fake star of known brightness to the image at the expected position of the SN and increasing the flux until the point source was clearly detected by eye. The flux upper limits for the MIPS 24-µm data were estimated by measuring the flux density of the faintest point source in the region of the SN (∼ 600east of the SN position). The faint point source was found to have an average 24-µm flux density of 657 µJy with a standard deviation of 2.5 % over the four epochs, and this approximate value was used as an upper limit to the SN flux at each epoch. A fake star of this brightness was added to the image at the SN coordinates, confirming the presence of an additional source blended with the bright neighbouring star. Although the ability of PSF-fitting and subtraction to detect fake stars of lower brightness was not investigated, the values reported should provide robust upper limits to the 24-µm flux.
Figure 4.2 depicts the slow decline of the IRAC 3.6–8.0-µm light curves over the four epochs from days 1843–3316, equivalent to ∼ 4–9 years after outburst. The emission at 3.6 and 4.5 µm appears to evolve more quickly than that at the longer wavelengths, fading by factors of ∼ 4 and 3 respectively during the first 1.5 years, compared to the factor of ∼ 2 decline observed at 5.8 and 8.0 µm. The 3.6-µm source had faded completely by the third epoch of observations (day 2575 ≈ 7 years). The 4.5-µm emission continued to decline at a faster rate than the longer wavelength emission, by a factor of almost 10 over the course of the four years, compared to the decline in the 5.8- and 8.0-µm bands by factors of ∼ 6 and 4.5, respectively. The mid-IR evolution is also depicted in Figure 4.3, which shows a 2000× 2000field of view centred on the SN position (indicated by the cross-hairs) in the 3.6, 4.5 and 8.0 µm IRAC images for the four epochs.