Bruce L. Gary (GBL), Hereford Arizona Observatory (G95), 2005.06.26

This web page is devoted to my observations of SN 2005cf. I will list BVRI magnitudes of the SN and maintain a light curve graph. A suggested photometric sequence is given at the bottom of this web page. Sorry, but other observer's measurement are not included on this web page.

Links Internal to This Web Page

    Introduction

    Light Curve

    Finding Chart and Photometric Sequence

Introduction

SN 2005cf, hereafter referred to as SN05cf, is a Type Ia supernova located at RA = 15:21:32.21 -07:24:47.5 (equinox 2000.0). At the present time it transits at about 10:30 pm (local standard time), so this is a good season for observing this SN. Maximum brightness occurred June 10. My interest in this object is prompted by the observing challenges of monitoring an object whose color varies, causing the light curves for each filter band to be slightly different.

In case you think I'm "in love with my data" because I don't show data from other observers, there are several reasons in explanation. First, I had a bad experience playing with someone else's data last year and I've sworn off the practice. Secondly, the observer who took the data knows the strengths and shortcomings of it, and what adjustments are permissible. When I worked professionally with data before my retirement (in the atmospheric sciences) there were rigorous rules about getting permission to do things with other people's data, so I'm adhering to this cautious principle here. All of my data is submitted to the WebObs before it's shown here. Thirdly, as long as I'm on a "soap box," the AAVSO archive does not provide for an unambiguous description of a measurement's precision and accuracy. Technically, a datum is useless until these two error components are available. Ever since I joined the AAVSO I've complained about their archive containing just one number per observation, instead of three numbers. I'm not a revernt person, but I have great reverence for measurements, and I claim that a measurement is not a measurement until it has an associated precision and accuracy. Since there's no unambiguous provision for that in the archive I have been supplementing my AAVSO submissions with web page descriptions of my observations. I'm hopeful that two new data cells will eventually appear on WebObs some day. Until that day I hope others, with an equal reverence for measurements, will also create their web page descriptions. (Incidentally, I might in fact be in love with my data, since I'll spend an hour worrying about the last 0.01 magnitude correction.)

Light Curve

The following table shows BVRI magnitudes based on ensemble photometry using either 5 or 6 of the brightest "Const" reference stars listed in the Photometric Sequence section, below. All reference stars, as well as SN05cf, are treated with proper instrumental corrections for B-V star color (using my Simplified Magnitude Equations procedure, which is more accurate than the AAVSO version of CCD Transformation Equations). The +-SE entries are total SE (orthogonal sum of stochastic SE and estimated systematic SE). In almost every case the stochastic SE is <0.01 magnitude, so the total SE is dominated by my estimated systematic uncertainty.

Light curve using only GBL data. Fitted traces are a 3rd order polynomial.

I hesitate to include other observer's brightness measurements in the same graph for two reasons: 1) The SN is changing color, which requires that a careful star color correction be applied to the SN (using simultaneous 2-color measurements), which I'm not sure every observer is doing, and 2) the reference stars ("comparison" stars) have different colors so SN magnitudes will have different offsets for different observers considering that each observer's systematic errors are likely to be different due to either differences in the treatment of reference star color (i.e., air mass neglect for some observers) or different errors in transformation coefficients.

In other words, there is ample reason to expect that each observer will have a different zero-shift offset for SN magnitudes, and this offset will vary as the SN color varies. I therefore prefer to consider light curves from each observer separately, and on this web page I present mine.

B-V color versus date.

Image showing SN05cf within a star field with FOV = 23.3 x 15.3 'arc, north up, east left. Limiting magnitude is 20.2. Unfiltered, total exposure 6.5 minutes. [Celestron CGE-1400, 14-inch SCT, focal reducer, AO-7 image stabilizer, SBIG ST-8XE CCD; 2005.06.04 UT]

Cropped version of previous image showing reference star numbers whose BVRI magnitudes are listed in the table, below.

Photometric Sequence

The following table currently has tentative magnitudes for some of the 18 reference stars. It is based on two observing nights (5604 and 5606). Considering the V-magnitudes the two nights yield the same average magnitude for all stars. The largest difference is 0.13 mag (Star #17).  The two-night magnitude difference is less than the stochastic SE for 14 of the 18 stars, which is close to the 68% expected that would exist if all stars were constant. The "Notes" column shows which stars were the same within an SE range for the two nights. These stars can probably be counted on as reliable, non-variable reference stars. The brightest 6 stars have been constant during the 5 dates for which I have V-band measurements. The mean difference in all-sky determined V-magnitude for the stars noted as "Const" is 0.025 magnitude, which is consistent with the SE error for each night being 0.018 magnitude (and an "average magnitude" SE of 0.013 magnitude).

Notice: Due to the death of a longtime colleague I am hereby (2005.06.10) suspending my amateur astronomy hobby for a few weeks. This addictive hobby had become the main theme of my life recently, and I'm currently re-establishing a better balance.

This site opened:  June 04, 2005.  Last Update:  June 26, 2005