Annu. Rev. Astron. Astrophys. 1999. 37: 409-443
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11. MICROBLAZARS AND GAMMA-RAY BURSTS

It is interesting that in all three sources where theta (the angle between the line of sight and the axis of ejection) has been determined, a large value is found (that is, the axis of ejection is close to the plane of the sky). These values are theta appeq 79° (SS 433, Margon 1984), theta appeq 66°- 70° (GRS 1915+105, Mirabel & Rodríguez 1994, Fender et al 1999), theta appeq 85° (GRO J1655-40, Hjellming & Rupen 1995), and theta geq 70° for the remaining sources. This result is not inconsistent with the statistical expectation since the probability of finding a source with a given theta is proportional to sintheta. We then expect to find as many objects in the 60° leq theta leq 90° range as in the 0° leq theta leq 60° range. However, this argument suggests that we should eventually detect objects with a small theta. For objects with theta leq 10° we expect the timescales to be shortened by 2gamma and the flux densities to be boosted by 8 gamma3 with respect to the values in the rest frame of the condensation. For instance, for motions with v = 0.98c (gamma = 5), the timescale will shorten by a factor of ~ 10 and the flux densities will be boosted by a factor of ~ 103. Then, for a galactic source with relativistic jets and small theta we expect fast and intense variations in the observed flux. These microblazars may be quite hard to detect in practice, both because of the low probability of small theta values and because of the fast decline in the flux.

Gamma-ray bursts are at cosmological distances and ultra-relativistic bulk motion and beaming appear as essential ingredients to solve the enormous energy requirements (e.g. Kulkarni et al 1999, Castro-Tirado et al 1999). Beaming reduces the energy release by the beaming factor f = Delta Omega / 4pi, where Delta Omega is the solid angle of the beamed emission. Additionally, the photon energies can be boosted to higher values. Extreme flows from collapsars with bulk Lorentz factors > 100 have been proposed as sources of gamma-ray bursts (Mészáros & Rees 1997). High collimation (Dar 1998, Pugliese et al 1999) can be tested observationally (Rhoads 1997), since the statistical properties of the bursts will depend on the viewing angle relative to the jet axis.

Recent studies of gamma-ray afterglows suggest that they are highly collimated jets. The brightness of the optical transient associated to GRB 990123 showed a break (Kulkarni et al 1999), and a steepening from a power law in time t proportional to t-1.2, ultimately approaching a slope t-2.5 (Castro-Tirado et al 1999). The achromatic steepening of the optical light curve and early radio flux decay of GRB 990510 are inconsistent with simple spherical expansion, and well fit by jet evolution (Harrison et al 1999). It is interesting that the power laws that describe the light curves of the ejecta in microquasars show similar breaks and steepening of the radio flux density (Section 7 Rodríguez & Mirabel 1999a). In microquasars, these breaks and steepenings have been interpreted (Hjellming & Johnston 1988) as a transition from slow intrinsic expansion followed by free expansion in two dimensions. Besides, linear polarizations of about 2% were recently measured in the optical afterglow of GRB 990510 (Covino et al 1999), providing strong evidence that the afterglow radiation from gamma-ray bursters is, at least in part, produced by synchrotron processes. Linear polarizations in the range of 2-10% have been measured in microquasars at radio (Rodríguez et al 1995, Hannikainen et al 1999), and optical (Scaltriti et al 1997) wavelengths.

In this context, microquasars in our own Galaxy seem to be less extreme local analogs of the super-relativistic jets associated to the more distant gamma-ray bursters. However, gamma-ray bursters are different to the microquasars found so far in our own Galaxy. The former do not repeat and seem to be related to catastrophic events, and have much larger super-Eddington luminosities. Therefore, the scaling laws in terms of the black hole mass that are valid in the analogy between microquasars and quasars do not seem to apply in the case of gamma-ray bursters.

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