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Title:
On the Measurement of a Cosmological Dipole in the Photon Number Counts of Gamma-Ray Bursts
Authors:
Scharf, C. A.; Jahoda, K.; Boldt, E.
Publication:
Astrophysical Journal v.454, p.573 (ApJ Homepage)
Publication Date:
12/1995
Origin:
APJ; KNUDSEN
Astronomy Keywords:
COSMOLOGY: OBSERVATIONS, GAMMA RAYS: BURSTS, COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE
DOI:
10.1086/176510
Bibliographic Code:
1995ApJ...454..573S

Abstract

If γ-ray bursts are cosmological or in a halo distribution their properties are expected to be isotropic (at least to first order). However, our motion with respect to the burst parent population (whose proper frame is expected to be that of the cosmic microwave background [CMB], or that of a static halo) will cause a dipole effect in the distribution of bursts and in their photon number counts (together termed a Compton-Getting effect).

We argue that the photon number count information is necessary to distinguish a genuine Compton-Getting effect from some other anisotropy and to fully test the proper-frame isotropy of the bursts. Using Monte Carlo simulations we obtain probability distributions for the statistics of dipole alignment, angular power, and dipole aligned component. We demonstrate the agreement expected between number distribution and photon count distribution dipoles in the presence of noise. It is estimated that of the order of 104 bursts would be necessary to constrain a dipole effect of 1 %. However, we can test the consistency of number and photon count distributions for a catalog of any size. Using the 2B catalog (Meegan et al. 1994, available in electronic form via ftp from grossc.gsfc.nasa.gov, user name gronews) of bursts observed with the COMPTON/BATSE instrument (Fishman et al. 1989) (in the energy band 20-50 keV), and the dipole determined from the CMB, we find the surprising result that although the number-weighted distribution is consistent with isotropy, the fluence-weighted dipole has a correlation with the CMB dipole that has a probability of occurring only 10% of the time for an isotropic photon distribution. Furthermore, the photon and number dipoles are inconsistent under the hypothesis of isotropy, at the 2 σ level. Taken together this could be an indication that a nonnegligible fraction of γ-ray bursts originate in the local, anisotropic universe.

These results suggest that future analyses of the angular distribution of γ-ray bursts should include both photon count and number weighting and that larger catalogs should be used to test the robustness of the apparent inconsistency with isotropy found here.

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