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Two spacecraft separated by $\sim 1\,\au$ and equipped with gamma-ray burst(GRB) detectors could detect or rule out a cosmological density of MassiveCompact Halo Objects (MACHOs) in the mass range $10^{-15} M_{\odot}\lsim M\lsim 10^{-7} M_{\odot}$ provided that GRBs prove to be cosmological.Previously devised methods for detecting MACHOs have spanned the mass range$10^{-16} M_{\odot}\lsim M \lsim 10^{7} M_{\odot}$, but with a gap of severalorders of magnitude near $10^{-9} M_{\odot}$. For MACHOs and sources both at acosmological distance, the Einstein radius is $\sim 1\,\au\,(M/10^{-7}M_\odot)^{1/2}$. Hence, if a GRB lies within the Einstein ring of a MACHO ofmass $M\lsim 10^{-7}M_\odot$ as seen by one detector, it will not lie in theEinstein ring as seen by a second detector $\sim 1\,\au$ away. This impliesthat if GRBs are measured to have significantly different fluxes by the twodetectors, this would signal the presence of a MACHO $\lsim 10^{-7}M_\odot$. Bythe same token, if the two detectors measured similar fluxes for severalhundred events a cosmological abundance of such low-mass MACHOs would be ruledout. The lower limit of sensitivity, $M\lsim 10^{-15}M_\odot$ is set by thefinite size of the source. If low-mass MACHOs are detected, there are testswhich can discriminate among events generated by MACHOs in the three massranges $M\lsim 10^{-12}\,M_\odot$, $10^{-12}\,M_\odot\lsim M\lsim10^{-7}\,M_\odot$, and $M\gsim 10^{-7}\ M_\odot$. Further experiments wouldthen be required to make more accurate mass measurements.Comment: 8 pages, uuencoded postscript, no figure

astrophysical journal/the astrophysical journal

Probing for MACHOs of Mass [FORMULA][F]10[SUP]-15[/SUP] M[INF]⊙[/INF][/F][/FORMULA] to [FORMULA][F]10[SUP]-7[/SUP] M[INF]⊙[/INF][/F][/FORMULA] with Gamma-Ray Burst Parallax Spacecraft