IRAC High Precision Photometry

Datasets

Examples of known noise pixel spikes

Planetname	AORkey	start_sclk	stop_sclk
HD7924	r46981888	1041429073.301	1041455525.016
HD7924	r44605184	1004594183.339	1004622735.006
XO3	r46482944	1050530251.708	1050573212.475
HD20003	r48408064	1062587169.546	1062628187.337

 

Symptoms

A few times over the course of the mission, noise pixel has been seen to sharply increase in value for a short duration (~30 min) before returning to normal (See Figure 1, 16&30 hours).  These noise pixel spikes are alarming because as noise pixel goes up, the measured aperture flux will decrease which mimics the signal of a transit. During the spikes, positions don’t show out of the ordinary trends.  There is also no apparent correlation with any particular spot on the pixel, so these are not related to a single position.

 

Explanation

Using the IRAC data simulator we have successfully replicated the noise pixel spikes seen in these datasets using a high frequency position oscillation.  At frequencies higher than the observation rate, oscillations of the instrument will have the effect of smearing out the image, and thereby increasing noise pixel, without changing the object centroids (see Figure 2).  A discussion of this was presented at the AAS workshop.  Although we have looked for correlations with telemetry values, we do not yet know what could cause the suggested oscillations onboard Spitzer.