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We report the spectral index of diffuse radio emission between 50 and 100 MHz from data collected with two implementations of the Experiment to Detect the Global EoR Signature (EDGES) low-band system. EDGES employs a wide beam zenith-pointing dipole antenna centred on a declination of $-26.7^circ$. We measure the sky brightness temperature as a function of frequency averaged over the EDGES beam from 244 nights of data acquired between 14 September 2016 to 27 August 2017. We derive the spectral index, $beta$, as a function of local sidereal time (LST) using night-time data and a two-parameter fitting equation. We find $-2.59<beta<-2.54 pm 0.011$ between 0 and 12 h LST, ignoring ionospheric effects. When the Galactic Centre is in the sky, the spectral index flattens, reaching $beta = -2.46 pm 0.011$ at 18.2 h. The measurements are stable throughout the observations with night-to-night reproducibility of $sigma_{beta}<0.004$ except for the LST range of 7 to 12 h. We compare our measurements with predictions from various global sky models and find that the closest match is with the spectral index derived from the Guzm{a}n and Haslam sky maps, similar to the results found with the EDGES high-band instrument for 90-190 MHz. Three-parameter fitting was also evaluated with the result that the spectral index becomes more negative by $sim$0.02 and has a maximum total uncertainty of 0.016. We also find that the third parameter, the spectral index curvature, $gamma$, is constrained to $-0.11<gamma<-0.04$. Correcting for expected levels of night-time ionospheric absorption causes $beta$ to become more negative by $0.008$ - $0.016$ depending on LST.
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The recent detection of the cosmic dawn redshifted 21 cm signal at 78 MHz by the EDGES experiment differs significantly from theoretical predictions. In particular, the absorption trough is roughly a factor of two stronger than the most optimistic theoretical models. The early interpretations of the origin of this discrepancy fall into two categories. The first is that there is increased cooling of the gas due to interactions with dark matter, while the second is that the background radiation field includes a contribution from a component in addition to the cosmic microwave background. In this paper we examine the feasibility of the second idea using new data from the first station of the Long Wavelength Array. The data span 40 to 80 MHz and provide important constraints on the present-day background in a frequency range where there are few surveys with absolute temperature calibration suitable for measuring the strength of the radio monopole. We find support for a strong, diffuse radio background that was suggested by the ARCARDE 2 results in the 3 to 10 GHz range. We find that this background is well modeled by a power law with a spectral index of $-$2.58$pm$0.05 and a temperature at the rest frame 21 cm frequency of 603$^{+102}_{-92}$ mK.
We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35< u<80$ MHz since it is `colder than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).
The radio spectral index is a powerful probe for classifying cosmic radio sources and understanding the origin of the radio emission. Combining data at 147 MHz and 1.4 GHz from the TIFR GMRT Sky Survey (TGSS) and the NRAO VLA Sky Survey (NVSS), we produced a large-area radio spectral index map of ~80 per cent of the sky (Dec > -40 deg), as well as a radio spectral index catalogue containing 1,396,515 sources, of which 503,647 are not upper or lower limits. Almost every TGSS source has a detected counterpart, while this is true only for 36 per cent of NVSS sources. We released both the map and the catalogue to the astronomical community. The catalogue is analysed to discover systematic behaviours in the cosmic radio population. We find a differential spectral behaviour between faint and bright sources as well as between compact and extended sources. These trends are explained in terms of radio galaxy evolution. We also confirm earlier reports of an excess of steep-spectrum sources along the galactic plane. This corresponds to 86 compact and steep-spectrum source in excess compared to expectations. The properties of this excess are consistent with normal non-recycled pulsars, which may have been missed by pulsation searches due to larger than average scattering along the line of sight.
The interpretation of Galactic synchrotron observations is complicated by the degeneracy between the strength of the magnetic field perpendicular to the line of sight (LOS), $B_perp$, and the cosmic-ray electron (CRe) spectrum. Depending on the observing frequency, an energy-independent spectral energy slope $s$ for the CRe spectrum is usually assumed: $s=-2$ at frequencies below $simeq$400 MHz and $s=-3$ at higher frequencies. Motivated by the high angular and spectral resolution of current facilities such as the LOw Frequency ARray (LOFAR) and future telescopes such as the Square Kilometre Array (SKA), we aim to understand the consequences of taking into account the energy-dependent CRe spectral energy slope on the analysis of the spatial variations of the brightness temperature spectral index, $beta$, and on the estimate of the average value of $B_perp$ along the LOS. We illustrate analytically and numerically the impact that different realisations of the CRe spectrum have on the interpretation of the spatial variation of $beta$. We find that the common assumption of an energy-independent $s$ is valid only in special cases. We show that for typical magnetic field strengths of the diffuse interstellar medium ($simeq$2$-$20 $mu$G), at frequencies of 0.1$-$10 GHz, the electrons that are mainly responsible for the synchrotron emission have energies in the range $simeq$100 MeV$-$50 GeV. This is the energy range where the spectral slope, $s$, of CRe has its greatest variation. We also show that the polarisation fraction can be much smaller than the maximum value of $simeq 70%$ because the orientation of ${bf B}_perp$ varies across the telescopes beam and along the LOS. Finally, we present a look-up plot that can be used to estimate the average value of $B_perp$ along the LOS from a set of values of $beta$ measured at centimetre to metre wavelengths, for a given CRe spectrum.
We present an estimate of the polarized spectral index between the Planck 30 and 44 GHz surveys in $3.7^circ$ pixels across the entire sky. We use an objective reference prior that maximises the impact of the data on the posterior and multiply this by a maximum entropy prior that includes information from observations in total intensity by assuming a polarization fraction. Our parametrization of the problem allows the reference prior to be easily determined and also provides a natural method of including prior information. The spectral index map is consistent with those found by others between surveys at similar frequencies. Across the entire sky we find an average temperature spectral index of $-2.99pm0.03(pm1.12)$ where the first error term is the statistical uncertainty on the mean and the second error term (in parentheses) is the extra intrinsic scatter in the data. We use a clustering algorithm to identify pixels with actual detections of the spectral index. The average spectral index in these pixels is $-3.12pm0.03(pm0.64)$ and then when also excluding pixels within $10^circ$ of the Galactic plane we find $-2.92(pm0.03)$. We find a statistically significant difference between the average spectral indices in the North and South Fermi bubbles. Only including pixels identified by the clustering algorithm, the average spectral index in the southern bubble is $-3.00pm0.05(pm0.35)$, which is similar to the average across the whole sky. In the northern bubble we find a much harder average spectral index of $-2.36pm0.09(pm0.63)$. Therefore, if the bubbles are features in microwave polarization they are not symmetric about the Galactic plane.
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