GRB and Environment Interaction

We discuss three aspects of the interaction between GRB and their surroundings. The illumination of the progenitor remnant and/or the surroundings by the X-ray afterglow continuum can produce substantial Fe K-alpha line and edge emission, with implications for the progenitor model. The presence of large dust column densities, capable of obscuring the GRB optical afterglow, will lead to characteristic delayed X-ray and far-IR light curve signatures. Pair production induced by the initial gamma-rays in the nearby environment will modify the initial spectrum and the afterglow light curve, and the magnitude of these changes provides a diagnostic for the external density. I FE X-RAY LINES FROM GRB PROGENITORS Important clues for identifying the nature of the progenitors of the long (t> ∼ 2 s) GRBs may be available from the recent report at a 4.7σ level of X-ray Fe line features in the afterglow after 1.5 days of the gamma-ray burst GRB 991216 [10], as well as similar detections at the 3σ level in 5 other bursts with Beppo-SAX and ASCA. X-ray atomic edges and resonance absorption lines are theoretically expected to be detectable from the gas in the immediate environment of the GRB, and in particular from the remnants of a massive progenitor stellar system [15,14,16]. A straightforward interpretation [10] of the GRB 991216 observation would imply a mass > ∼ 0.1− 1M⊙ of Fe at a distance of about 1-2 light-days, possibly due to a remnant of an explosive event or supernova which occurred days or weeks prior to the gamma-ray burst itself (a ’supranova’, [10,12]). The long time delay is necessary both to get the relatively massive, slow moving ejecta out to few light-day distances (to explain the line appearance at a few days with light travel arguments), and in order to get the initial Ni and Co to decay to Fe (∼ 55 days). This requires a two-step process, in which an initial supernova leads to a temporarily stabilized neutron star remnant, which after weeks collapses to a black hole leading to a canonical burst ( [11,12]). It is unclear whether fall-back from the supernova leading to the second collapse to a BH could occur with such a (∼ weeks) long delay (e.g. [18]). Another possibility is that a massive progenitor has previously emitted a copious wind (Ṁ> ∼ 10M⊙/yr), which would need to be unusally Fe-rich and highly inhomogeneous ( [14]; c.f. [10]). An alternative, and perhaps less restrictive scenario for such Fe lines [17] involves an extended, possibly magnetically dominated wind from a GRB impacting the expanding envelope of a massive progenitor star. This could be due either to a spinning-down millisecond super-pulsar or to a highly-magnetised torus around a black hole (e.g. [13]), which could produce a luminosity that was still, one day after the original explosion, as high as Lm ∼ 10 47t day ergs. An outflow with such a dependence can also be powered by accretion of fall-back material onto a central black hole [18]. This jet luminosity may not dominate the continuum afterglow; but its impact on the outer portions of the expanding stellar envelope at distances < ∼ 10 cm, even with just solar abundances, can be efficiently reprocessed into an Fe line luminosity comparable to the observed value, together with a contribution to the X-ray continuum. Under this interpretation, the dominant continuum flux in the afterglow, even in the X-ray band, is still attributable to a standard decelerating blast wave. The relativistic magnetised wind from the compact remnant would develop a stand-off shock before encountering the envelope material, and shocked relativistic plasma would be deflected along the funnel walls. Non-thermal electrons will be accelerated behind the standoff shock in the jet material; the transverse magnetic field strength (which decreases as 1/r in an outflowing wind) would be of order 10 G at 10 cm – strong enough to ensure that the shock-accelerated electrons cool promptly, yielding a powerlaw continuum extending into the X-ray band. Some of these X-rays would escape along the funnel, but at least half (the exact proportion depending on the geometry and flow pattern) would irradiate the material in the stellar envelope. Pressure balance in the shoked envelope wall implies densities of ne = αLm/6πr ckT ∼ 10αL47r −2 13 T −1 8 cm , where α ∼ 1 is a geometric factor, and the recombination time for hydrogenic Fe in the funnel walls photoionized by the non-thermal continuum is trec = 6×10 T 1/2 8 n −1 17 ∼ 6×10 −6αL m47r 2 13T 3/2 8 s. Standard calculations of photoionization of optically-thin slabs (e.g. [7]) show that the equivalent width of the Fe K-alpha line, for solar abundances, is about 0.5 kev, or twice as strong if the Fe has ten times solar abundances. These results are applicable provided that the ionizing photons encounter a Fe ion before being scattered by free electrons i.e. provided that τT = σT dine< ∼ 1. Under these conditions the Fe K-α photon flux is about 0.1 of the X-ray continuum [17], ṄLFe ∼ 10L47β ph/s, where β < 1 is the ratio of ionizing to MHD luminosity. This line luminosity compares well with Fe line luminosity 6× 10 ph/s observed t ∼ 1.5 day after the GRB 991216 burst by [10]. The total amount of Fe needed to explain the observed K-α line flux, arising in a thin layer of the funnel walls of a collapsar model, amounts to a very modest mass of MFe ∼ 10 M⊙, which could be Fe synthesized in the core. The Fe-enriched core material can easily reach a distance comparable to r ∼ 10 cm in 1 day for an expansion velocity below the limit v ∼ 10 cm s inferred by [10] from the line widths. Even without this, a solar abundance (10M⊙ of Fe) in the envelope is sufficient to explain the observations. The initial, energetic portion of the relativistic jet, with a typical burst duration of 1 − 10 s, will rapidly expand beyond the stellar envelope, leading in the usual way to shocks and a decelerating blast wave. A continually decreasing fraction of energy, such as put out by a decaying magnetar, may continue being emitted for periods of a day or longer, and its reprocessing by the stellar envelope can be responsible for the observed Fe line emission in GRB 991216. Since the energy in this tail can decay faster than t, the usual standard shock gamma-ray and afterglow scenario need not be affected, being determined by the first 1-10 s worth of the energy input. II DUSTY GRB DELAYED XR/IR AFTERGLOWS For GRB in large star forming regions, a significant fraction of the prompt X-ray emission will be scattered by dust grains. Since dust grains scatter X-rays by a small angle, time delays of the scattered x-rays will be small (minutes to days, depending on the X-ray energy and the grain size). If the dust column density is substantial, the softer part of the X-ray afterglow on the above timescales will be dominated by the dust scattering, the direct X-ray emission from the blast wave being weaker. This intermediate time, soft(er) X-ray light curve will be steeper than the unscattered X-ray afterglow. As a specific example [6], consider a typical GRB whose unscattered X-ray light curve is parametrized as F0(t) = [1+(t/100s)]/[1+(t/100s) ], with an arbitrary normalization depending on the X-ray energy band. This is represented by the thin line in Fig.1. We assume that the GRB occurs in a large star forming region, of typical radius R about 100pc, where the dust grain populations and optical depths are close to what is observed in our Galactic center region. Thus for numerical estimates we assume that (1) visual extinction is ∼ 10, (2) X-rays are scattered preferentially by those dust grains whose size is in the range a ∼ 0.06μm, (3) the optical depth to dust scattering at the X-ray energy ǫ is τ(ǫ) = 3 ( ǫ 1keV −2 . At X-ray optical depths less than few, dust grains of size a will scatter X-rays of energy ǫ by an angle θ ∼ 0.2λ/a, where λ is the X-ray wavelength, θ(ǫ) ≃ 4 × 10 (