We have discovered seven type Ia cluster supernovae ( SNe ) in the course of the Wise Observatory Optical Transients Search in the fields of 0.06 < z < 0.2 galaxy clusters .
Two of these events , SN 1998fc in Abell 403 ( z = 0.10 ) and SN 2001al in Abell 2122/4 ( z = 0.066 ) , have no obvious hosts .
Both events appear projected on the halos of the central cD galaxies , but have velocity offsets of 750-2000 km s ^ { -1 } relative to those galaxies , suggesting they are not bound to them .
We use deep Keck imaging of the locations of the two SNe to put upper limits on the luminosities of possible dwarf hosts , M _ { R } > -14 mag for SN 1998fc and M _ { R } > -11.8 mag for SN 2001al .
The fractions of the cluster luminosities in dwarf galaxies fainter than our limits are < 3 \times 10 ^ { -3 } and < 3 \times 10 ^ { -4 } , respectively .
Thus , 2 / 7 of the SNe would be associated with \leq 3 \times 10 ^ { -3 } of the luminosity attributed to galaxies .
We argue , instead , that the progenitors of both events were probably members of a diffuse population of intergalactic stars , recently detected in local clusters via planetary nebulae and red giants .
Considering the higher detectability of hostless SNe compared to normal SNe , we estimate that 20 ^ { +12 } _ { -15 } percent of the SN Ia parent stellar population in clusters is intergalactic .
This fraction is consistent with other measurements of the intergalactic stellar population , and implies that the process that produces intergalactic stars ( e.g. , tidal disruption of cluster dwarfs ) does not disrupt or enhance significantly the SN Ia formation mechanism .
Hostless SNe are potentially powerful tracers of the formation of the intergalactic stellar population out to high redshift .