GRB 060218 / SN 2006aj
Bruce L. Gary (GBL), Hereford Arizona Observatory (G95)

Links Internal to this Web Page
   Introduction
    Finder Chart & Preliminary Photometric Sequence
    Light Curves for Various Observing Sessions
    Updated V & Rc Mag's
    Confirmation of SDSS Magnitude Bias by CfA
    Related Links

Introduction

The Swift spacecraft discovered GRB 060218 when it outburst at 2006.02.18 at 03:34:31 UT. It's a nearby GRB with z = 0.033. The X-ray light curve is unusual, as is the optical brightness. It has been suggested that we are viewing the polar jet's gamma ray emission from an off-axis orientation, and that light from the underlying supernova emerged after only ~3 days. As the non-thermal gamma ray afterglow fades the SN thermal emission should brighten, which is the explanation for the light curve first fading then brightening. Predicted maximum brightness appears to have just now reached maximum brightness.

This GRB is now an evening object that is 1.5 hours past transit when it gets dark 45 minutes after sunset. It is located at RA/Dec = 03:21:39.7 +16:52:02.

Small FOV image

Figure 1. Small FOV (3.3 x 3.3 'arc) showing GRB 060218/SN2006aj at 48 hours after burst when it was at magnitude 18.3. Seeing was poor, with FWHM = 4.3 "arc. Limiting magnitude is 21.4. [54-minute exposure, unfiltered, 14-inch Celestron, SBIG ST-8XE CCD; 2006.02.20 UT; Hereford, AZ]

Finder Chart & Photometric Sequence

Here's a "finder chart" showing 9 candidate reference stars.

Finder chart with suggested reference stars

Figure 2. Larger FOV (13.4 x 11.3 'arc) showing suggested reference stars. The 5 stars with s-designations are in the SDSS catalog.

                                    Table I - Photometric Sequence

B
 V
 R
 I
 B-V
Comments
1
 17.20 +/- 0.07
 16.26 +/- 0.05 15.81 +/- 0.04
 15.27 +/- 0.05 0.94 +/- 0.10
  Don't use; variable!
2
16.75 +/- 0.08 15.81 +/- 0.04 14.48 +/- 0.05 (~1.7)
  Don't use; very red!
3
 16.22 +/- 0.06
 15.30 +/- 0.04 14.65 +/- 0.04 14.01 +/- 0.05 0.92 +/- 0.08   Acceptable
4
 17.02 +/- 0.07 16.05 +/- 0.05
 15.42 +/- 0.04 14.83 +/- 0.05 0.97 +/- 0.09  Acceptable
5
 11.15 +/- 0.05
  9.57 +/- 0.04
   8.73 +/- 0.04   7.84 +/- 0.05 1.57 +/- 0.07  OK (but red)
6
 13.90 +/- 0.05
 12.55 +/- 0.04
 11.78 +/- 0.04 11.05 +/- 0.05 1.35 +/- 0.06  Acceptable
7
 13.70 +/- 0.05
 13.13 +/- 0.04
 12.74 +/- 0.04 12.31 +/- 0.05 0.57 +/- 0.06  Acceptable
8
 16.33 +/- 0.06
 15.69 +/- 0.05
 15.28 +/- 0.04 14.80 +/- 0.05 0.64 +/- 0.08  Acceptable
9
 13.31 +/- 0.05
 12.72 +/- 0.04
 12.32 +/- 0.04 11.90 +/- 0.05 0.59 +/- 0.06  Acceptable
s1
 16.12 +/- 0.05
 15.27 +/- 0.04
 14.70 +/- 0.04 14.16 +/- 0.05 0.85 +/- 0.06  SDSS star
s2
 15.68 +/- 0.05
 14.95 +/- 0.04
 14.55 +/- 0.04 14.13 +/- 0.05 0.73 +/- 0.06   SDSS star
s3
 16.29 +/- 0.05
 15.24 +/- 0.04
 14.67 +/- 0.04 14.04 +/- 0.05 1.05 +/- 0.06   SDSS star
s4

14.59 +/- 0.04 14.10 +/- 0.05
  SDSS star
s5

14.33 +/- 0.04 13.88 +/- 0.05
  SDSS star
GRB 17.70 +/- 0.11 17.66 +/- 0.10
 17.46 +/- 0.08 17.35 +/- 0.12 0.04 +/- 0.15   2006.02.25, 05 UT

All errors in the table are "SE accuracy" (the orthogonal sum of stochastic and estimated systematic SE).

Note that the GRB is bluer than all reference stars in this table. Care must therefore be taken when transforming differential photometry observations to a standard magnitude (corrected for star-color). It is especially important to NOT USE reference star #2! It is very red, and I have evidence that it is also a variable. The other stars appear to be stable, but when I have more information on this it will appear here.

The Landolt star fields used to derive zero-shifts and star color sensitivitiy exhibited BVRcIc residuals of 0.019, 0.031, 0.026 and 0.036 magnitude (N=29, 35, 18, 14). Recent extinction values are well-established at my site for this season when the skies are photometric. Besides, I observed the Landolt star fields at approximately the same air mass as the GRB 060218 region.

Star #5 is has a Tycho transformed BVRc magnitudes of 11.12, 9.62 and 8.82 (using Henden's linear equations). My BVRc magnitudes differ from the Tycho transformed values by +0.03, -0.05 and -0.09.

There are 5 SDSS stars in my FOV. I have transformed the SDSS g', r' and i' magnitudes to BVRcIc magnitudes using the transformation equations of Smith et al (2002). There are systematic differences for all 5 stars. If my Landolt-based magnitudes are adopted as "true" then the SDSS transformed BVRcIc magnitudes should be adjusted using the following equations:

    B = Bsdss +0.37 +/- 0.03
    V = Vsdss +0.29 +/- 0.03
    R = Rsdss +0.23 +/- 0.01
    I = Isdss +0.12 +/- 0.02
Are the reference stars in the table stable? Only one is variable (in addition to the GRB).

Reference star stability

Figure 3. Reference star stability for a 3-day interval. Each star is compared with it's average observed magnitude and then offset 0.1 magnitude with respect to it's neighbor. The GRB/SN is shown (at bottom) for comparison.

Reference Star #1 appears to be variable (range of 0.06 magnitude). Reference Stars 2 through 9 appear to be stable, exhibiting an RMS deviation from their average of 0.007 magnitude.

I recommend NOT using Reference Stars #1 and #2 (recall that #2 is very red). Thus, there are 6 reference stars that are close to normal in color and appear to be adequately stable: Reference Stars 3 - 9.

Light Curves for Various Observing Sessions

Light curve for 2-hour period

Figure 4. Light curve showing fading at the rate of 0.18 +/- 0.03 magnitude per hour during a 2-hour period starting 47 hours after the burst. (A correction of -0.16 magnitude should be applied to this plot due to the use of 3 red reference stars for monitoring a blue target.)

Note that 4 days later my V-mag is 17.66 +/- 0.10, so this is consistent with a brightening due to the SN rising as the afterglow fades.

I hesitate to present my GRB 060218 / SN 2006aj measurements since there are so many measurements by other observers already in the public domain. However, when trends are to be investigated it is sometimes best to isolate each observer's data from the others on the basis that each observer uses a different set of reference stars, uses different image analysis procedures, has different instrumental sensitivities to star color, etc. Provided each observer uses the same telescope system and employs the same analysis procedures these specific idiosyncracies will be the same, corresponding to one simple zero shift for each observer. Since I want to investigate the SN's light curve shape I will now present my observations of it.

                                            Table II - GRB 060218 LIGHT CURVE MEASUREMENTS (by GBL)

Date
 B
 V
 R
 I
 V-R
 R-I
 Notes
2006.06.20.117


18.00 +/- 0.05


CV
2006.06.20.171

18.25 +/- 0.05


CV
2006.02.24.133
 18.77 +/- 0.29
 17.73 +/- 0.06 17.70 +/- 0.10 16.86 +/- 0.15
 +0.03 +/- 0.12
 +0.84 +/- 0.18
2006.02.25.136
 17.70 +/- 0.11 17.66 +/- 0.10
 17.46 +/- 0.08
 17.36 +/- 0.12
 +0.20 +/- 0.13 +0.10 +/- 0.15
2006.02.27.125
 18.03 +/- 0.17
 17.59 +/- 0.07
 17.27 +/- 0.04
 17.19 +/- 0.17
 +0.32 +/- 0.08 +0.08 +/- 0.18
2006.03.03.085
17.55 +/- 0.08
 17.23 +/- 0.10





















Light curve

Figure 5. Light curve with hand-fitted curves for V and R.

During the first few days the optical emission was dominated by non-thermal GRB afterglow from the jet, which probably was fading at a rapid rate. Figure 4 suggests that on February 20 this jet emission was fading rapidly and the SN had not yet brightened (or its emission was hidden by an expanding shell with an opacity that was still greater than ~1). By Feb 24 it would appear that the SN had brightened (and/or the surrounding shell had become essentially transparent). The apparent reddening of the SN might be partially due to the blue jet emission fading from partial influence on the V-bnad brightness on February 24 to much less influence by February 27. Peak brightness probably occurred March 1 or 2, UT.

The B-band and I-band observations have too large uncertainties for basing a story on them.

Updated V and Rc Magnitudes

On 2006.03.14 I observed this GRB/SN region in alternation with Landolt stars at RA = 04:52 in a manner that provided overlapping air mass. Good solutions were obtained for slight revisions of the V- and Rc-magnitudes for the 14 stars in the above list. For both bands my later magnitudes are 0.02 magnitude brighter than those listed in the above table. I present a weighted average new set of V-a dn Rc-magnitudes in the following table.

  Table III - Updated V and Rc Magnitudes

Star
 V
 Rc
1
 16.25
 15.78
2
 16.74
 15.83
3
 15.29
 14.65
4
 16.06
 15.41
5
 9.55
 8.71
6
 12.52
 11.76
7
 13.12
 12.73
8
 15.68
 15.29
9
 12.70
 12.31
s1
 15.26
 14.69
s2
 14.94
 14.54
s3
 15.23
 14.66
s4
 14.99
 14.57
s5
 14.80
 14.33
G
17.71

Confirmation of SDSS Magnitude Bias, GCN 4898

On 2006.03.20 Malcolm Hicken announced (on behalf of the CfA's Supernova Group) via GCN 4898 that all-sky measurements taken at Mount Hopkins showed that the SDSS magnitudes for stars in this region are too bright by ~0.27 magnitude at V-band, and are too bright by similar amounts at other bands. Additional information on this announcement can be found at GCN 4898 Supporting Info. These results are in good agreement with those presented in Tables I and III, above (created 2006.02.27 and 2006.03.16).

I've plotted the CfA V-band and Rc-band magnitudes versus mine in the following graph.



Figure 6. CfA V and Rc magnitudes versus the average of those I obtained 2006.02.25 (Table I) and 2006.03.14 (Table III). 

It always amazes me when two independent all-sky solutions agree (especially when one was made by an amateur and another was made a few weeks later by Harvard professionals).

Related Links

    Image w/ BVRcIc Notations
    AAVSO campaign
    Web article (based on NASA news release)
    Web article (Penn State news release)
    All-sky Photometry tutorial
    Bruce's AstroPhotos
   
The author of this web page can be contacted at B L G A R Y @ u m i c h . e d u (excuse the inserted spaces to thwart spammers).
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First created: 2006.02.23   Last updated: 2006.03.30