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D. J. Fixsen

Publications and source records attributed to D. J. Fixsen.

51 records · Page 3Linked to original sources
arXiv1998

Calibrator Design for the COBE Far Infrared Absolute Spectrophotometer (FIRAS)

The photometric errors of the external calibrator for the FIRAS instrument on the COBE are smaller than the measurement errors on the cosmic microwave background (CMBR) spectrum (typically 0.02 MJy/sr, 1 sigma), and smaller than 0.01% of the peak brightness of the CMBR. The calibrator is a re-entrant cone, shaped like a trumpet mute, made of Eccosorb iron-loaded epoxy. It fills the entire beam of the instrument and is the source of its accuracy. Its known errors are caused by reflections, temperature gradients, and leakage through the material and around the edge. Estimates and limits are given for all known error sources. Improvements in understanding the temperature measurements of the calibrator allow an improved CMBR temperature determination of 2.725 +/- 0.002 K.

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arXiv1998

The Spectrum of the Extragalactic Far Infrared Background from the COBE Firas Observations

The COBE FIRAS data contain foreground emission from interplanetary, Galactic interstellar dust and extragalactic background emission. We use three different methods to separate the various emission components, and derive the spectrum of the extragalactic Far InfraRed Background (FIRB). Each method relies on a different set of assumptions, which affect the FIRB spectrum in different ways. Despite this, the FIRB spectra derived by these different methods are remarkably similar. The average spectrum that we derive in the ν= 5 - 80 cm^{-1} (2000-125 \um) frequency interval is: I(ν) = (1.3 \pm 0.4)\times10^{-5} (ν/ν_0)^{0.64 \pm 0.12} P_ν(18.5 \pm 1.2 K), where ν_0=100 cm^{-1} (λ_0=100 \um) and P is the Planck function. The derived FIRB spectrum is consistent with the DIRBE 140 and 240 \um detections. The total intensity received in the 5 - 80 cm^{-1} frequency interval is 14 nW m^{-2} sr^{-1}, and comprises about 20% of the total intensity expected from the energy release from nucleosynthesis throughout the history of the universe.

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arXiv1997

Detection of Cosmic Microwave Background Anisotropy by the Third Flight of MSAM

The third flight of the Medium Scale Anisotropy Measurement (MSAM1), in June 1995, observed a new strip of sky, doubling the sky coverage of the original MSAM1 dataset. MSAM1 observes with a 0.5 deg beam size in four bands from 5-20 icm. From these four bands we derive measurements of cosmic microwave background radiation (CMBR) anisotropy and interstellar dust emission. Our measurement of dust emission correlates well with the 100 um IRAS Sky Survey Atlas; from this comparison we determine an effective emissivity spectral index between 100 um and 444 um of 1.46 +/- 0.28. Analysis of our measurement of CMBR anisotropy shows that for Gaussian-shaped correlation functions with theta_c = 0.3 deg, we place a limit on total rms anisotropy of 2.2 x 10^{-5} < Delta T/T < 3.9 x 10^{-5} (90% confidence interval, including calibration error). The band-power limits are = ^{1/2} = 50^{+16}_{-11} uK at l = 160, and = 65^{+18}_{-13} uK at l = 270 (1 sigma limits, including calibration error). The corresponding limits with statistical errors only are = 50^{+13}_{-9} uK and = 65^{+14}_{-10} uK respectively. These measurements are consistent with a standard adiabatic cold dark matter model; we discuss constraints on h, n, and the redshift of reionization.

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arXiv1997

Comparison of the COBE FIRAS and DIRBE Calibrations

We compare the independent FIRAS and DIRBE observations from the COBE in the wavelength range 100-300 microns. This cross calibration provides checks of both data sets. The results show that the data sets are consistent within the estimated gain and offset uncertainties of the two instruments. They show the possibility of improving the gain and offset determination of DIRBE at 140 and 240 microns.

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arXiv1997

The Spectrum of the CMB Anisotropy from the Combined COBE FIRAS and DMR Observations

We analyze the Cosmic Microwave Background (CMB) anisotropy data from the independent COBE FIRAS and DMR observations. We extract the frequency spectrum of the FIRAS signal that has the spatial distribution seen by DMR and show that it is consistent with CMB temperature fluctuations in the radiation well into the Wien region of the spectrum. Conversely, we form a map of the Planckian component of the sky temperature from FIRAS and show that it correlates with the DMR anisotropy map. The rms fluctuations at angular scales of 7 degrees are 48 \pm 14 uK for the FIRAS data compared with 35 \pm 2 uK for the DMR data and 31 \pm 6 uK for the correlated combination (1 sigma uncertainties). The consistency of these data, from very different instruments with very different observing strategies, provide compelling support for the interpretation that the signal seen by DMR is, in fact, temperature anisotropy of cosmological origin. The data also limit rms fluctuations in the Compton y parameter, observable via the Sunyaev- Zel'dovich effect, to Delta_y < 3 x 10^{-6} (95% CL) on 7 degree angular scales.

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arXiv1996

A Millimeter/Submillimeter Search for the Sunyaev-Zel'dovich Effect in the Coma Cluster

Observations from the first flight of the Medium Scale Anisotropy Measurement (MSAM1-92) are analyzed to search for the Sunyaev-Zel'dovich (SZ) effect towards the Coma cluster. This balloon-borne instrument uses a $28\arcmin$ FWHM beam and a three position chopping pattern with a throw of $\pm40\arcmin$. With spectral channels at 5.7, 9.3, 16.5, and 22.6~\icm, the observations simultaneously sample the frequency range where the SZ spectral distortion in the intensity transitions from a decrement to an increment and where the fractional intensity change is substantially larger than in the Rayleigh-Jeans region. We set limits on the Comptonization parameter integrated over our antenna pattern, $Δy \leq 8.0 \times 10^{-5}$($2 σ$). For a spherically symmetric isothermal model, this implies a central Comptonization parameter, $y_o \leq 2.0 \times 10^{-4}$, or a central electron density, $n_o \leq 5.8 \times10^{-3}$cm$^{-3}h_{50}$, a result consistent with central densities implied by X-ray brightness measurements and central Comptonization estimates from lower frequency observations of the SZ effect.

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arXiv1996

The Cosmic Microwave Background Spectrum from the Full COBE/FIRAS Data Set

We have refined the analysis of the data from the FIRAS (Far InfraRed Absolute Spectrophotometer) on board the COBE (COsmic Background Explorer). The FIRAS measures the difference between the cosmic microwave background and a precise blackbody spectrum. We find new tighter upper limits on general deviations from a blackbody spectrum. The RMS deviations are less than 50 parts per million of the peak of the CMBR. For the Comptonization and chemical potential we find $|y| < 15\times10^{-6}$ and $|μ| < 9\times10^{-5}$ (95\% CL). There are also refinements in the absolute temperature, 2.728 $\pm$ 0.004 K (95\% CL), and dipole direction, $(\ell,b)=(264.14^\circ\pm0.30, 48.26^\circ\pm0.30)$ (95\% CL), and amplitude, $3.372 \pm 0.007$ mK (95\% CL). All of these results agree with our previous publications.

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arXiv1996

A Detection of Bright Features in the Microwave Background

We report the characterization of bright, compact features in the cosmic microwave background radiation (CMBR) detected during the June 1992 and June 1994 balloon flights of the Medium Scale Anisotropy Measurement (MSAM1-92 and MSAM1-94, respectively). Spectral flux densities are determined for each feature at 5.7, 9.3, and 16.5 cm$^{-1}$. No viable counterparts for these features were found in source catalogs at 5 GHz or at 100 $μ$m. The measured spectrum of each feature is consistent with a temperature fluctuation in the CMBR. The existence of these features is consistent with adiabatic fluctuation models of anisotropy in the CMBR.

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arXiv1996

A CMBR Measurement Reproduced: A Statistical Comparison of MSAM1-94 to MSAM1-92

The goal of the second flight of the Medium Scale Anisotropy Measurement (MSAM1-94) was to confirm the measurement of cosmic microwave background radiation (CMBR) anisotropy made in the first flight (MSAM1-92). The CMBR anisotropy and interstellar dust emission signals from the two flights are compared by forming the sum and difference of those portions of the data with the same pointings on the sky. The difference data are consistent with a null detection, while the summed data show significant signal. We conclude that MSAM1-92 and MSAM1-94 measured the same celestial signal.

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arXiv1995

A Balloon-Borne Millimeter-Wave Telescope for Cosmic Microwave Background Anisotropy Measurements

We report on the characteristics and design details of the Medium Scale Anisotropy Measurement (MSAM), a millimeter-wave, balloon-borne telescope that has been used to observe anisotropy in the Cosmic Microwave Background Radiation (CMBR) on 0\fdg5 angular scales. The gondola is capable of determining and maintaining absolute orientation to a few arcminutes during a one-night flight. Emphasis is placed on the optical and pointing performance as well as the weight and power budgets. We also discuss the total balloon/gondola mechanical system. The pendulation from this system is a ubiquitous perturbation on the pointing system. A detailed understanding in these areas is needed for developing the next generation of balloon-borne instruments.

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arXiv1995

MSAM1-94: Repeated Measurement of Medium-Scale Anisotropy in the Cosmic Microwave Background Radiation

The second flight of the Medium Scale Anisotropy Measurement (MSAM1-94) observed the same field as the first flight (MSAM1-92) to confirm our earlier measurement of cosmic microwave background radiation (CMBR) anisotropy. This instrument chops a 30\arcmin\ beam in a 3 position pattern with a throw of $\pm40\arcmin$, and simultaneously measures single and double differenced sky signals. We observe in four spectral channels centered at 5.6, 9.0, 16.5, and 22.5~\icm, providing sensitivity to the peak of the CMBR and to thermal emission from interstellar dust. The dust component correlates well with the \IRAS\ 100~\micron\ map. The CMBR observations in our double difference channel correlate well with the earlier observations, but the single difference channel shows some discrepancies. We obtain a detection of fluctuations in the MSAM1-94 dataset that match CMBR in our spectral bands of $ΔT/T = 1.9^{+1.3}_{-0.7}\times 10^{-5}$ (90\% confidence interval, including calibration uncertainty) for total rms Gaussian fluctuations with correlation angle 0\fdg3, using the double difference demodulation.

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arXiv1995

FAR-INFRARED SPECTRAL OBSERVATIONS OF THE GALAXY BY COBE

We derive Galactic continuum spectra from 5-96/cm from COBE/FIRAS observations. The spectra are dominated by warm dust emission, which may be fit with a single temperature in the range 16-21 K (for nu^2 emissivity) along each line of sight. Dust heated by the attenuated radiation field in molecular clouds gives rise to intermediate temperature (10-14 K) emission in the inner Galaxy only. A widespread, very cold component (4-7 K) with optical depth that is spatially correlated with the warm component is also detected. The cold component is unlikely to be due to very cold dust shielded from starlight, because it is present at high latitude. We consider hypotheses that the cold component is due to enhanced submillimeter emissivity of the dust that gives rise to the warm component, or that it may be due to very small, large, or fractal particles. Lack of substantial power above the emission from warm dust places strong constraints on the amount of cold gas in the Galaxy. The microwave sky brightness due to interstellar dust is dominated by the cold component, and its angular variation could limit our ability to discern primordial fluctuations in the cosmic microwave background radiation.

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arXiv1993

A Measurement of the Medium-Scale Anisotropy in the Cosmic Microwave Background Radiation

Observations from the first flight of the Medium Scale Anisotropy Measurement (MSAM) are analyzed to place limits on Gaussian fluctuations in the Cosmic Microwave Background Radiation (CMBR). This instrument chops a 30\arcmin\ beam in a 3 position pattern with a throw of $\pm40\arcmin$; the resulting data is analyzed in statistically independent single and double difference datasets. We observe in four spectral channels at 5.6, 9.0, 16.5, and 22.5~\icm, allowing the separation of interstellar dust emission from CMBR fluctuations. The dust component is correlated with the \IRAS\ 100~\micron\ map. The CMBR component has two regions where the signature of an unresolved source is seen. Rejecting these two source regions, we obtain a detection of fluctuations which match CMBR in our spectral bands of $0.6 \times 10^{-5} < ΔT/T < 2.2 \times 10^{-5}$ (90\% CL interval) for total rms Gaussian fluctuations with correlation angle 0\fdg5, using the single difference demodulation. For the double difference demodulation, the result is $1.1 \times 10^{-5} < ΔT/T < 3.1 \times 10^{-5}$ (90\% CL interval) at a correlation angle of 0\fdg3.

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arXiv1993

Morphology of the Interstellar Cooling Lines Detected by COBE

The FIRAS instrument on the COBE satellite has conducted an unbiased survey of the far-infrared emission from our Galaxy. The first results of this survey were reported by Wright et al. (1991). We report the results of new analyses of this spectral survey, which includes emission lines from 158 um C+, 122 um and 205 um N+, 370 um and 609 um C, and CO J=2-1 through 5-4. We report the morphological distribution along the galactic plane (b=0) of the spectral line emission, and the high galactic latitude intensities of the C+ and 205 um N+ emission. The high galactic latitude intensity cosecant of the 158 um fine structure transition from C+ is presented, and C+ is seen to decrease more rapidly than the far infrared intensity with increasing galactic latitude. C+ and H I emission are closely correlated with a C+ cooling rate of (2.65 +/- 0.15)x10^{-26} erg/s/H-atom. We conclude that this emission arises almost entirely from the Cold Neutral Medium. The high galactic latitude intensity of the 205 um fine structure transition from N+ arises entirely from the Warm Ionized Medium, and its cosecant is presented. We estimate the total ionizing photon rate in the Galaxy to be 3.5x10^{53} ionizing photons per second, based on the 205 um N+ transition.

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