Pathfinding With ARCADE

Jack SingalUniversity of Richmond

CMB @ 506/10/2015

ARCADE

• Radiation from sky compared to large (~2ft. across) external microwave blackbody calibrator. Calibrator temperature is adjusted until its emission intensity matches the sky emission. That is then the radiometric temperature of the sky.

• mK-level absolute uncertainties and sub mK-level fractional uncertainties from 3-90 GHz

• Systematic errors are minimized by:- no windows between the sky and the receivers- keeping everything cold (at the top of a 7ft tall open dewar at 120,000 ft)

ARCADE

298 cones of steelcast absorber cast onto an aluminum core, some with thermometers embedded

Frequency(GHz)

Reflected power attenuation (dB)

3.4 42.45.6 55.58.3 68.69.8 62.730 55.690 56.6

Fixsen et al., 2005, Rev. Sci.Inst., 77, 064905

Thermal control maintained via cascading ponds of LHe fed by superfluid pumps from the bath. Components have thermal links to the ponds and heaters.

Primary instrument challenges/innovations are 1) thermal maintenance and control and 2) external calibrator blackness (see e.g. Singal et al., 2010, ApJ, 730, 138)

ARCADE calibrator is one of the blackest microwave objects ever constructed

Toward the Future• Because the low frequency synchrotron excess obscures μ, y , and Yff distortions we need to return to higher frequencies for further limits

→ Need the accuracy and long integration times of space-based observations

• There is still a role for ARCADE in astrophysical measurements e.g. nailing down the spinning dust intensity and spectrum

ARCADE calibrator is clear precursor for e.g. PIXIE

• For blackness make the cones pointy (done!)

• ARCADE calibrator had ~600 mK gradients in flight

- Driven by heat flow from cone tips to ~1.4 K aperture plane through Helium gas atmosphere

600 mK- Heat flow due to conduction ~7 orders of magnitude greater than that due to radiation

- Vacuum of a space mission and control of aperture plane temperature will reduce calibrator thermal gradients by 8+ orders of magnitude

Extra Slides

Calibrator thermal gradients

600 mK

- Driven by heat flow from cones to ~1.4 K aperture plane in Helium atmosphere

𝑃𝑐𝑜𝑛𝑑=𝐾 𝐻𝑒𝐴𝐿 (𝑇 𝐶−𝑇 𝐴𝑝 ) 1𝑊

𝑃𝑟𝑎𝑑 𝜎 𝐴(T C4−T Ap4 )1𝜖𝐶

+1𝜖𝐴𝑝

−1≈ 4×10−8𝑊

A=1.3 m2

KHe≈.14 W/mK

ϵC=1

ϵAp~0.01

~.15 m (distance varies greatly)

The absolute spectrum below 30 GHz

• There are good reasons to believe that this monopole synchrotron-like excess is extragalactic(e.g. Singal et al., 2015, ApJL, 799, L10)

• Combining ARCADE 2 measurements at 3-90 GHz and absolutely calibrated single dish all-sky maps at low frequencies shows that there is a large power-law excess at these frequencies (Fixsen et al., 2011, ApJ, 734, 5; Kogut et al., 2011, ApJ, 734, 4)

• CRB → interesting in its own right

Residuals after TCMB and power law

Yff at 2σ FIRAS upper limit (Yff ≤ 1.9x10-5)

μ at 50 x FIRAS upper limit (μ ≤ 9x10-5)

CMB spectral distortions with ARCADE

From Seiffert et al., 2011, ApJ, 734, 6

Future prospects for Yff (2σ)

Measurements at these freqsat these precisions

Combined with 2 mK precision ARCADE measurement at 3-90 GHz

• Because the low frequency synchrotron excess obscures Yff and μ distortions we need to return to higher frequencies for further limits

→ Need the accuracy and long integration times of space-based observations

• There is still a role for ARCADE in astrophysical measurements e.g. nailing down the spinning dust intensity and spectrum

Spinning Dust

Fig 4 from WMAP 7 Galaxy paper (Gold et al. 2011, ApJS, 192, 15G)

Red = Hard Synch+ FF + Thermal Dust

Blue = Synch+ FF + Thermal Dust + SD

Dashed = Synch

Dotted = FF

Dash-dot = Dust

Stars=WMAPTriangles=ARCADE

CMB spectral distortionsFree-free: Bremsstrahlung radiation – photons emitted from free electrons scattering off of each other in ionized gas

Emission is proportional to the free electron number density squared – a measurement of the integrated amount of ionization (given by parameter Yff)

Adds intensity to low frequency part of spectrum

Path Length

Ionized IGM

Chemical Potential: Processes in early universe inject energy into plasma too soon before last scattering for photons to fully equilibrate with baryons

Energy would be from component previously decoupled from plasma (e.g. dark sector)

Photons would then have a Bose-Einstein distribution with a chemical potential μ

∆ 𝑇 𝑓𝑓 (𝜈 )=𝑘𝐵

2 𝑇03

h2𝜈2 𝑌 𝑓𝑓

𝜀 (𝜈 ) 𝑑𝜈=8𝜋 h𝑐3

𝜈3

𝑒h𝜈𝑘𝐵𝑇

+𝜇

−1

FIRAS: μ ≤ 9x10-5

FIRAS: Yff ≤ 1.9x10-5 (2σ)

ARCADE power law + ff: Yff ≤ 5.7x10-5 (1σ)

ARCADE power law + μ : μ ≤ 7x10-4 (1σ)

Cosmic Radio Background• The radio sky has a bright isotropic (on < 1 degree scales) synchrotron component which is either Galactic or extragalactic

• It must originate from regions of relatively high magnetic field strength ~1 μG (Singal et al., 2010, MNRAS, 409, 1172)

• If from z<5 it should be from in or near galaxies, and with those galaxies characterized by an evolving radio far-infrared correlation (e.g. Vernstrom et al., 2011, MNRAS, 415, 3641)

• However, originating from z<5 galaxies may lead to a background that is clumpier than limits (Holder, 2014, ApJ, 780, 112; Cline Vincent, 2013, JCAP, 02, 011C), and there may not be enough low flux galaxies (e.g. Condon et al., 2012, ApJ, 758, 23)

• Synchrotron from Pop III supernovae is an intriguing possibility (Beirmann et al., 2014, MNRAS, 441, 1147)

• A number of authors have suggested annihilating dark matter in halos and filaments (e.g. Hooper et al., 2012, PRD, 863, 003H)

• If Galactic, it must be either from the halo in regions with a strangely high magnetic field, or more locally with inexplicably low polarization (e.g. Fornengo et al. 2014, JCAP, 04, 008; Singal et al., 2010, MNRAS, 409, 1172)

• The preponderance of the evidence favors the extragalactic scenario

The Case Against Galactic

3) What about the Local Bubble?

• It would be highly polarized since the Galactic magnetic field is locally pretty constant. In that case, we would expect to see some quadrupole polarization structure in WMAP 22 GHz aligned with the local Galactic magnetic field, but we don’t.• We don’t see similar localized radio halos in other places

(A region that the sun and nearby stars are passing through that has hot x-ray emitting gas and a low density compared to the average ISM)

1) Large radio halos which could produce the observed intensity are not seen in similar spirals (Singal et al., 2015, ApJL, 799, L10)

2) The synchrotron-emitting electrons in a halo would overproduce the X-ray background through inverse-Compton (Singal et al., 2010, MNRAS, 409, 1172) and the radio-C+ correlation would overproduce the C+ 0-level

Is it the instrument?• Could it be thermometry in the calibrator or emission? It would

have to be a conspiracy, because we see a CMB temperature at 10-90 GHz consistent with FIRAS

• Could it be emission from peripheral instrument components? Not very plausible. Again, it would have to show up only in the 3 & 8 GHz channels. Also we tested instrument emission by warming components, and have a conservative estimate on the uncertainty in the instrument contribution

• Could it be RFI? Not likely to be isotropic across the sky, and coincidentally in four frequency bands.

• It isn’t just ARCADE. Any two of the three measurements in combination (ARCADE 2, FIRAS, and low frequency single dish surveys) reproduce the same results

Is it something in the solar system?

• Synchrotron from charged particles interacting with the Earth’s magnetic field? That would appear highly anisotropic.

• Charged particles interacting with the magnetopause? That would also appear highly anisotropic and probably highly polarized, so WMAP should see it. We should also see similar radio halos around other stars

H. Sharpe et al., 2009, arXiv:0902:0181 "Heliosheath Synchrotron: A Possible Source for the ARCADE 2 CMB Distortions"