Astron. Astrophys. 351, 759-765 (1999)

5. Probability of a nearby GRB

The frequency of nearby supernovae, and hence GRBs assuming they are linked to massive star formation like supernovae, depend on where the solar system was located within the galaxy when it formed. The highest rate of type II supernovae occurs in the two principal spiral arms of the galaxy. The molecular cloud was compressed entering the spiral arm to a condition for star formation and this interaction resulted in a new star cluster that traversed the spiral arm. Massive stars in the cluster evolve rapidly over years terminating in type II supernovae. The width of this supernova zone is about 1 kpc because the stars move at about 100 km s^-1 for years (Clark et al., 1977). It is likely that a nearby supernova caused the collapse of the presolar cloud and also seeded the nebula with the radioactive ²⁶Al needed to explain the ²⁶Mg in CAIs (Cameron et al., 1995). The number of supernovae along the spiral arm, within the 1 kpc zone and over a period of years, has been estimated at 250 supernovae per 100 pc (Clark et al., 1977).

BATSE observes on average about one GRB per day. This corresponds to one burst per million years per galaxy assuming that the rate of GRBs does not change with cosmological time (Fishman & Meegan, 1995). The average rate changes if allowance is made for beaming or a cosmic evolution of the rate of GRBs. The observations that GRB host galaxies are star forming systems (Hogg & Fruchter, 1999; Fruchter et al., 1999; Bloom et al., 1998) indicates that the rate of GRBs may follow the star formation rate (Wijers et al., 1998; Totani, 1999). In this case GRBs are further away and occur at a lower rate and have significantly greater energy output. At present there is no agreement on the nature of the progenitors of the GRB explosion although neutron star mergers are a promising candidate (Eichler et al., 1989; Piran, 1999). The list also includes failed supernovae (Woosley, 1993), white dwarf collapse (Usov, 1992) and hypernovae (Paczynski, 1998). All these models are consistent with the possibility that GRBs are associated with star forming regions. The lifetime of massive stars is quite short and that of a neutron star binary could be sufficiently short to be close to a star forming region.

There is considerable uncertainty in the cosmological rate of GRBs (Cen, 1998; Krumholz et al., 1998; Che et al., 1999) and a rate of one GRB per galaxy per 10⁷ years is adopted which is about 10⁵ times less than the supernova rate (Paczynski, 1998). It is also assumed that GRBs are linked to massive stars and the explosion occurs in the supernova zone of the spiral arm. There is a probability of about 0.001 of a GRB occuring within 100 pc of the solar nebula assuming the length of the spiral arms is about 40 kpc and the thickness of the spiral arm perpendicular to the plane is less than 100 pc. The probability will be smaller by many orders of magnitude if GRB explosions occur at random locations throughout the galaxy. There is evidence such as paired and rimmed chondrules that some of them were melted on more than one occasion (Hewins, 1997; Wasson, 1993). The probability of two GRBs impacting on the solar nebula with sufficient energy to melt chondrules is . The heat source that led to CAI formation is uncertain but it was much more intense and lasted for a longer period than chondrules because most of the refractory dust was evaporated in the process (Wood, 1988). A GRB could have been the heat source but it is very improbable because it must have been within 10 pc to provide the required energy.

If this GRB-chondrule scenario is correct, then only about one planetary system in 1000 should have evolved like the solar system and should preserve evidence for chondrule formation. The solar nebula existed as a detector of intense flashes of radiation for millions of years but recent satellite observations cover less than forty years and have discovered the GRBs and soft -ray repeaters (SGRs). There could be other rare transient sources yet to be discovered that influenced the formation of chondrules. In this context the role of the SGRs might have been important (Kouveliotou et al., 1993). There are four known SGRs that are associated with supernova remnants and which have high velocities relative to the nebula. Two of the SGRs have generated intense transients, ergs and ergs, but these transients are too feeble by about a factor of to influence chondrule formation (Hurley et al., 1999). However the number of SGR sources within the galaxy is very uncertain (Hurley et al., 1994; McBreen & Hurley, 1998; Heyl & Kulkarni, 1998) and SGRs may generate much more powerful outbursts shortly after their formation. The recent detection (Galama et al., 1998) of a weak GRB, about ergs, from a type Ib/c supernova suggests that different mechanisms may give rise to a new class of dim supernova-related GRBs.

A GRB in a nearby galaxy ( 100 Mpc) could be used to reveal protoplanetary disks because of the transient infrared emission from chondrule formation. In K band, the transient source would be at the µJy level and good angular resolution is required to separate the transient emission from the galactic background. The transient sources could occur over a period of hundreds of years after the GRB, assuming isotropic GRB emission.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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