Observing target

The Bicep2observing target is the Southern Hole, a ⇠ 800 degree2 _{patch of sky accessible from} the Southern Hemisphere centered at RA= 0hr, dec= 57.5 degrees. The observing target covers

latitudes distant from the galactic plane, making it exceptionally free from galactic foregrounds. This particular region is the cleanest of its size, with average dust emission averaging 1/100th of the sky median. By sheer coincidence, it is also at a near ideal declination when observed from the South Pole.

Bicep2’s observing band at 150 GHz is chosen for three considerations: i) 150 GHz corresponds to the predicted minimum of the sum of galactic synchrotron and dust emission within the observing region (assuming a synchrotron spectral indexk= 3), ii) as a thermal blackbody at 2.7 K, the CMB 12_{As of 2005. Because of the 10 m/year motion of the polar ice cap at the South Pole, the coordinates of the lab} move with respect to the geographic pole.

6 h 18 h 0 h

12 h

1 3 10 30 100 300 1000 µKCMB

Figure 2.16: TheBicep2observing field overplotted onto the Finkbeiner, Davis, and Schlegel dust emission model (Finkbeiner et al. 1999). Average dust emission in theBicep2target region is over 100 times lower than the sky median. Figure is adapted from Chiang et al. 2010.

peaks at⇠150 GHz, and iii) there is a convenient atmospheric window centered at 150 GHz, between oxygen and water lines at 118 and 183 GHz, respectively. As a single-color instrument, Bicep2is incapable of distinguishing primordialB-modes from galactic foregrounds, and, as a result, can only place upper limits on theB-mode amplitude of the CMB. In the scenario of a detection ofB-mode polarization, further multi-frequency followup will be necessary to distinguish a primordial signature from a foreground.

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Chapter 3

Characterization of the

Bicep2

telescope

As instrument sensitivity has improved over successive generations of CMB polarization experiments, the requirements for polarization systematics have become increasingly stringent. The threat of in- strumental systematics looms large when attempting to measure theB-modes, in part because the analysis relies so heavily on component separation. Small errors in instrument calibration or match- ing between polarized detectors can leak the (comparatively) very bright temperature fluctuations into polarization. Polarization systematics can be identified both in data analysis and through instrument characterization.

The principal goals of the characterization of the optical, thermal, and magnetic performance ofBicep2 are to: i) characterize the optical and polarization response of the telescope for faithful map reconstruction, and ii) assess and isolate potential sources of instrumental polarization. The latter may be accomplished by generating simulated data with instrument parameters captured from calibration data as inputs. The process of taking measured instrument parameters to constraints on falseB-mode polarization will be detailed in Chapter 4.

In this chapter, we summarize our effort to characterize Bicep2. As in the previous chapter, particular attention will be paid to potentially dominant sources of systematics, and to efforts in which I played a substantial role. A non-trivial fraction of my graduate career has been devoted to characterizing the optical response of the telescope. This effort has included three trips to the South Pole, hundreds of hours of data-taking, and thousands of lines of code. Collaborators can view much of the analysis code used to produce the results below in my online working notes, available athttp://bicep.caltech.edu/~rwa/rwa_working_notes/index.html. In addition to the optical calibration efforts, this chapter will also summarize characterization of the polarization, thermal, and magnetic response of the instrument. We will conclude the chapter with characterization of the instrument performance and noise properties.

3.1 Far-field optical characterization of

Bicep2

In this section, we summarize the characterization of the far-field optical response of the instrument in three parts. First, we review the far-field mapping procedure, whereby beam characterization data are acquired, processed, and gathered into maps. This will include a discussion of the beam map construction and beam parameterization. Second, we characterize the beam formed by summing detectors within an orthogonally polarized pair and compare the result with optical models. Third, we assess the pair-difference beam (formed by differencing detectors within a pair).

The primary purpose of characterizing the pair-sum optical response (which we denote asBs(~p))

is to derive an absolute calibration of the Bicep2 temperature maps. As we rely on WMAP for absolute calibration, we re-smooth the WMAP maps with the Bicep2 beam kernel. To leading order the error from uncertainty in Bs(~p) is degenerate with an overall absolute gain factor, but

second-order effects can give rise to a slope in the`-space absolute calibration. These second-order effects can often be ignored, even when Bs(~p)is approximated by a simple Gaussian kernel with a

matched beam width.

The pair-differenced signal that results from detectorsAandBwith idealized polarization angles

A = and B = + 90, elevation nod corrected responsivities ˜gA and g˜B (described in Section

4.1.1), and beam functionsBA(~p)andBB(~p), is:

df(t) = 1 2 Z d~p0[˜gABA(p~A(t) ~p0) g˜BBB(~pA(t) ~p0)]⇥(~p0) +1 2 Z d~p0[˜gABA(p~A(t) ~p0) + ˜gBBB(~pA(t) p~0)](Q(~p0) cos 2 (t) +U(~p0) sin 2 (t))] +nf(t). (3.1) Here,~pi(t)is the detector pointing in spherical coordinates for detectori,Bi(~p)is the “as observed”

frequency-independent beam function as defined in Equation A.3, and nf(t) is the pair-difference

noise. As in previous sections,⇥,Q, andU are the temperature and polarization anisotropy fields. Detector polarization angles have been assumed to be perfectly orthogonal and the cross-polar response to be zero.

In the event that the beams are perfectly matched (BA(ˆn) =BB(ˆn)), the term in Equation 3.1

proportional to ⇥will vanish. If not, there will be some leakage of the temperature field into the pair-difference time series.

The leakage term in Equation 3.1 is a consequence of the choice to pair-difference before con- structing maps. One could constructAand B maps separately, accounting for differences in beam centroids, beam widths, etc., and constructQandU maps as a final step. ForBicep2, this would come at the cost of an intolerable noise penalty. Bicep2relies heavily on pair differencing for atmo- spheric rejection (as well as other common-mode contamination). Alternatively, one could build a

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pipeline that simultaneously estimates the atmosphere, the CMB intensity and polarization, ground pickup, focal plane temperature fluctuations, etc., but this is computationally expensive and much more complicated. We thus choose to pair difference “up front,” before accounting for differences in the detectors’ beams. Characterizing the beam matching betweenAandBis thus a crucial goal for assessing spurious polarization.