Aug 26, 2011 UT: Photometry of the young SN 2011fe in M101

Michael Richmond
Aug 26, 2011

On the night of Aug 25/26, 2011, I observed the newly discovered supernova known as PTF 11kly = SN 2011fe in the galaxy M101. This object was discovered just two nights ago, and appears to be a young type Ia event. At a distance of only about 6 or 7 Mpc, this might become bright enough to see visually in small telescopes.

The setup was:

Notes from the night

This is a chart of the field, based on a stack of five R-band images. The field of view is about 10 by 8 arcminutes.

The marked objects appear in a list of comparison stars with magnitudes in BVRI, created by the AAVSO. You can find the original at the AAVSO web site; go to the Variable Star Plotter (VSP) area. I've made a simplified ASCII version of the catalog below, or you can look at the AAVSO's original version in a local copy.


#AUID   RA.   Dec.   Label   U   B   V   Rc   Ic   J   H   K   Comments
# Label            RA         Ra(deg)       Dec        Dec(deg)  ID       U    B                 V                  R                 I
000-BKD-527   14:03:49.86 [210.95775d]   54:09:05.8 [54.15161d]   117 x   -   12.268 (0.057)29  11.735 (0.040)29  11.359 (0.048)29  11.006 (0.054)29  -   -   -    
000-BKD-528   14:04:21.68 [211.09033d]   54:19:22.4 [54.32289d]   119 x   -   12.572 (0.068)29  11.948 (0.047)29  11.528 (0.056)29  11.134 (0.063)29  -   -   -    
000-BKD-529   14:02:00.79 [210.50330d]   54:23:15.8 [54.38772d]   128 x   -   13.553 (0.062)29  12.751 (0.031)29  12.253 (0.045)29  11.788 (0.056)29  -   -   -    
000-BKD-530   14:01:55.32 [210.48050d]   54:16:21   [54.27250d]   131 x   -   13.540 (0.064)29  13.094 (0.044)29  12.716 (0.051)29  12.361 (0.056)29  -   -   -    
000-BKD-531   14:02:02.00 [210.50833d]   54:20:43.9 [54.34553d]   133 x   -   14.188 (0.094)29  13.291 (0.057)29  12.772 (0.074)29  12.287 (0.088)29  -   -   -    
000-BKD-532   14:01:49.79 [210.45746d]   54:23:02.7 [54.38408d]   134 x   -   14.187 (0.047)29  13.436 (0.033)29  12.970 (0.049)29  12.534 (0.061)29  -   -   -    
000-BKD-533   14:03:13.67 [210.80696d]   54:15:43.4 [54.26205d]   138 A  -   14.699 (0.091)29  13.801 (0.046)29  13.230 (0.062)29  12.698 (0.074)29  -   -   -    
000-BKD-534   14:02:54.17 [210.72571d]   54:16:29.5 [54.27486d]   140 B   -   14.557 (0.107)29  13.991 (0.074)29  13.581 (0.078)29  13.197 (0.083)29  -   -   -    
000-BKD-535   14:02:29.54 [210.62308d]   54:16:55.2 [54.28200d]   141 C   -   14.959 (0.102)29  14.100 (0.045)29  13.598 (0.063)29  13.130 (0.077)29  -   -   -    
000-BKD-536   14:02:31.15 [210.62979d]   54:14:03.9 [54.23442d]   146 G   -   15.158 (0.078)29  14.605 (0.029)29  14.204 (0.061)29  13.828 (0.082)29  -   -   -    
000-BKD-540   14:01:50.86 [210.46191d]   54:16:08.1 [54.26892d]   149 x   -   15.373 (0.010)29  14.925 (0.000)29  14.635 (0.006)29  14.361 (0.008)29  -   -   -    
000-BKD-538   14:03:23.74 [210.84892d]   54:14:32.6 [54.24239d]   155 H   -   16.015 (0.042)29  15.475 (0.000)29  15.133 (0.046)29  14.811 (0.065)29  -   -   -    
000-BKD-539   14:03:05.64 [210.77350d]   54:17:26.2 [54.29061d]   162 x   -   16.776 (0.091)29  16.177 (0.000)29  15.640 (0.079)29  15.140 (0.112)29  -   -   -    
#Report this sequence as: 5249bgj in the chart field of your observation report. This table is for a 0.25000° fov around PTF11KLY, RA: 14:03:05.81 (210.77421) & Decl.: 54:16:25.40 (54.27372). 

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 3 pixels = 4.2 arcseconds in VRI and a radius of 5 pixels = 7 arcsec in B, and sky defined by an annulus around each star. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution for the R-band.

The floor of this diagram is about 0.020 mag, which indicates pretty poor data. The floors for each passband were

passband     magsig floor
--------------------------
  B            0.04
  V            0.03
  R            0.02
  I            0.02
--------------------------

I used the output of this ensemble photometry to estimate the uncertainty in the measurement of the SN's magnitude in each passband, using the scatter in measurements of stars at the same apparent brightness as the SN. This uncertainty was smaller than the uncertainty in the catalog magnitude for star "B", which I used to convert the ensemble magnitudes to the standard Johnson-Cousins scale (because star "B" was closest to the SN in color, though still quite a bit less blue). I added the catalog uncertainty for star B and the ensemble uncertainty for the SN in quadrature to estimate the overall uncertainty in the SN's final magnitude.

I did not include color terms in the final magnitudes, because I don't know them well enough. I'll plan to measure them the next clear night in at least a rough manner.

After doing all the work, I end up with these values for the magnitude of SN 2011fe.

Passband       JD-2,455,000        Mag     +/-
--------------------------------------------------
   B             799.596          14.05    0.12
   V             799.560          13.75    0.08
   R             799.554          13.66    0.08
   I             799.580          13.63    0.11
--------------------------------------------------

I reported these measurements to the AAVSO via their WebObs tool this morning.


Last modified 8/26/2011 by MWR.