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Discovery of a Transient Absorption Edge in the X-ray Spectrum of GRB 990705

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at ~3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of ~0.86 and with an iron abundance of ~75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event

The nature of the progenitors of celestial GRBs is an open issue of key astrophysical importance. Collapse of massive fast rotating stars − the hypernova model (1) − or delayed collapse of a rotationally stabilized neutron star − the supranova model (2) − are among the favored scenarios for the origin of these events. Both models predict that the pre-burst environment is characterized by a high gas density, either as a result of strong winds from the massive progenitor in the case of a hypernova or because of a supernova (SN) event in the case of the supranova model (3). In the latter case the environment is expected to be enriched in heavy elements. The progressive photoionization of the circumburst material (CBM) by the GRB photons should produce in the burst x-ray spectrum transient low-energy cut-off and K-edge absorption features of the elements in the CBM (4). The detection of such features may allow us to estimate the density and composition of the CBM, the GRB redshift, and, ultimately, the nature of the GRB progenitor.
Owing to the coalignment of two detection units of the gamma ray burst monitor (GRBM, 40 to 700 keV) (5,6) with the two wide field cameras (WFCs, 2 to 26 keV) (7), the Italian-Dutch x-ray mission BeppoSAX can provide not only arc minute localizations of GRBs, but also measurements of their spectra in a broad (2 to 700 keV) energy band (8). Among the GRBs detected by the BeppoSAX WFC and GRBM, the event of 5 July 1999 (GRB 990705) is the second brightest in γ-rays (40-700 keV) after GRB 990123 and ranks in the top 15% in x-rays (2-26 keV). This burst trig- A 120,000 s follow-up observation with the BeppoSAX narrow-field instruments (12) was also performed starting ∼11 hours after the GRBM trigger. In the first 7 hours of observing time, an x-ray source of 3.3σ significance, corresponding to 1.9 (±0.6) ×10 −13 erg cm −2 s −1 , was detected from a 2 arc min radius region centered on the near-infrared counterpart position. The source, 1SAX J0509.9-7207, was not visible after this time. On the basis of the fading behavior, 1SAX J0509.9-7207 is the most likely candidate of the GRB 990705 x-ray afterglow.
The event exhibits a highly structured pulse, a duration of 42 s in γ-rays and a longer duration (∼60 s) in x-rays ( Fig. 1). Following the investigation results obtained with a sample of BeppoSAX GRBs (8), we observed the spectral evolution of the GRB prompt emission by accumulating WFC and GRBM spectra in 7 adjacent time intervals (Fig. 1). The 2 to 700 keV spectra of time intervals from C to G can be fit with a simple power law model [I(E) ∝ E −Γ ] with a continuously variable photon index in the range from 1.22 ± 0.02 to 2.24 ± 0.28 (the uncertainties quoted hereafter are 1 standard deviation). This hard-to-soft evolution is typical of GRB prompt emission spectra (8). Instead, the spectra of the first two time intervals (A and B in Fig. 1), of 6 s and 7 s duration, respectively, cannot be described either by this model (see Fig. 2) or by the smoothed broken power-law model proposed by Band et al. (13) for GRB spectra. The description with a power-law photoelectrically absorbed by a gas with cosmic abundance (14) within our galaxy (z = 0) provides a good fit (χ 2 /dof = 5.55/9) for the slice A with equivalent hydrogen column density N H = 8.7(±3.6) × 10 22 cm −2 and power-law photon index Γ = 1.08 ± 0.03. However for time slice B the same model does not provide a good description (χ 2 /dof = 29.9/9); the depression between 4 and 6 keV, which is apparent in this spectrum (Fig. 2 Within this time interval, and for a standard cosmology, this corresponds to a total of 3.6 × 10 57 ionizing photons, where every photon above the threshold has been weighted with the ratio of its cross-section to that at the threshold. Furthermore, the density due to a 10m 1 M ⊙ star dispersed in a volume of radius D is about 10 5 cm −3 , implying long recombination time scales (> 10 5 s), depending on the exact, but rapidly varying, temperature. Thus, we conclude that the burst has about the right number of photons to cause the complete photoionization of iron within the B time interval, with recombination providing no real counter-effect. A similar conclusion, with a more elaborate computation, has been reached, for generic parameters, by Böttcher et al. (4).
The implication of the above discussion is that the iron-rich material is, most likely, located around the burst site and cannot be located by chance along the line of sight.
In fact, from the column density and iron abundance derived from the B spectrum, we have found that the absorbing material has an iron content ∼ 75 times the solar value, and we have deduced that only a SNR can be responsible for this absorption.
Calling R SN R the radius of the SNR, and D its distance from the burst site, if the Our results favor models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova-like explosion.  Distribution of the residuals of the count spectra from the best-fit power-law model in the time intervals A (top), B (middle), and C (bottom). The major deviations of the data from the model are apparent in the lowest energy bin for the A spectrum and in the 3 to 6 keV band for the B spectrum, whereas no statistically significant deviations from a power-law are apparent for the C spectrum. The probability that the observed deviations are due to chance is 2.3×10 −2 for time interval A and 9.0×10 −6 for time slice B. The higher chance probability obtained for the deviations in the time interval A is likely due to the lower statistical quality of these data (see text). The possibility that instrumental effects caused the depression in B spectrum were investigated with negative results. The cross-calibration between the two instruments was verified by measurements of the Crab Nebula spectrum (22). The depression is also found in the ratio between the GRB count rate spectrum of the time interval B and the Crab Nebula spectrum taken with the same instrumentation. The depression is not apparent in the spectra accumulated over the successive time intervals (see bottom panel for the time slice C).