NEOWISE IR photometry

In order to confirm the increase in mid-IR flux of RW Aur observed during the 2014 dimming as reported by Shenavrin et al. (2015) I have looked at publicly available NEOWISE archival data of

3.3. Observations of the 2014 event RW Aur from 2014 and 2015. This is the first large collection of mid-infrared data presented for this system, containing considerably more observations than the ones in Shenavrin et al. (2015).

Figure 3.5 presents the NEOWISE observation of RW Aur from 2014 and 2015. This dataset is made up of four blocks of observations. Each block consists of several photometric data points obtained over the course of 1-2 consecutive nights. The full dataset is included in Table A.1 in Appendix A. In the first block from March 2014 RW Aur is still in its bright state, while the remaining three blocks cover the eclipse. I have used a red symbol to represent the average value of the photometric measurements in its corresponding block. During the eclipse the average magnitude in W1 (3.4µm) decreases and the star brightens by 0.14−0.26 mag, with a photometric error of∼0.03 mag. The W2 (4.6µm) data shows the same trend, with a magnitude change of 0.66−1.28 in eclipse and photometric error of∼0.01 mag. In addition to the overall increase in brightness for the duration of the dimming, the star is also showing smaller time-scale variability on the order of hours to days in all blocks. The brightness trends seen in the NEOWISE data are comparable to the M and L-band observations reported Shenavrin et al. (2015) (see Figure 3.4).

Before moving on, I will briefly discuss the increase of RW Aur’s flux between the WISE and NEOWISE data sets, as well as the scatter in the data seen in the individual observing blocks (1-2 nights) in the context of the instrument’s reliability when performing photometry on saturated sources. First, let us examine the flux increase. NEOWISE measurements are known1to systemat- ically overpredict the flux of sources with magnitudes brighter than W1<8 mag and W2<7 mag in comparison to the cryo phase of the mission. The overestimation is said2to range from little to no difference at the saturation limit to over 1 mag at W1=2mag and over 2 mag at W2=2 mag. With W1<5.5 mag and W2<5.5 mag, RW Aur is subject to these systematics. This raises the question of whether the observed NEOWISE flux increase for RW Aur is dominated by instrument system- atics or intrinsic variation in the system. To address this question, I examined WISE images of RW Aur to look for stars with comparable brightness in the field to use as a reference. I found only one such star (TYC 2389-874-1), which is not known to be variable and has not been a subject of study in literature. The reference star is showing an increase of 0.15−0.30 mag in W1 (comparable to RW Aur), but only 0.17−0.34 mag in W2 (significantly less than RW Aur). We can, therefore, assume that the observed increase in NEOWISE W1 flux of RW Aur is dominated by instrument systematics, but the W2 flux increase is dominated by the system’s variability.

1_{http://wise2.ipac.caltech.edu/docs/release/neowise/expsup/sec2_1c.html} 2_{http://wise2.ipac.caltech.edu/docs/release/neowise/expsup/sec2_2.html}

Chapter 3. The disappearing act: RW Aur

Next, I will address the apparent spread in the data points from individual observing blocks. Since August 2010, the W1 band photometry has been reported to suffer from an adverse interac- tion between an increase in the number of hard-saturated pixels due to the raising temperature of the instrument and the data analysis software3. The main result from this interaction is reported to be an increase in the photometric error or lack thereof, and affects only photometry of bright, saturated objects. The photometric quality flags for all WISE and NEOWISE archive entries for RW Aur used in this work were all marked as high quality. In addition, the scatter in the data points in all observing blocks (1-2 nights) is larger than the photometric error for the individual data points. Still, I compared the spread in the data from individual observing blocks of RW Aur with the spread in the reference star (introduced in the previous paragraph) data, in order to check whether the scatter in the 1-day time scale data for RW Aur is due to variability or an artefact from the instrument systematics. The standard deviation in the W1 1-day subsets from the lightcurve for RW Aur range between 0.08 and 0.15 mag. The standard deviation for the same subsets of the reference star are between 0.07 and 0.17 mag. As the standard deviations for the two stars are comparable and the reference star is not know to be variable, it follows that the spread in the W1 1-day scale data of RW Aur is most likely dominated by instrument systematics. I repeated this measurement for the W2 data as well. The W2 standard deviation for RW Aur is between 0.15 and 0.27 mag, whereas the standard deviation for the reference star is between 0.05 and 0.15 mag. In this case, it appears that the W2 spread in the RW Aur data is dominated by intrinsic variability of the system.

In summary, the W1 data for RW Aur appears to be dominated by instrument systematics, but the W2 data does indeed capture the variability of RW Aur and can be used as evidence in support of the infrared flux increase reported by Shenavrin et al. (2015)