V. G. Kurt

Astron observations of the Rapid Burster MXB 1730-335 and constraints on burster parameters from spectra of trailing bursts

We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5–2 min, rise times of 5–10 s and intervals of 1–1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of ∼28 min with much longer rise times (30–50 s) and longer durations (≃3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated asFp≃(2.0–2.4)×10−9 erg cm−2 s−1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86M⊙,RNS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3–15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints onM,RNS, and distanceD. For instance, for the hydrogen photosphere we obtainedM=(1.0–2.1)M⊙RNS=(7.1–16.4) km ifD=11 kpc. If one postulatesM=1.4M⊙, thenD=(8.5–13) kpc for hydrogen photosphere; if, besides,D=11 kpc, thenRNS=(8.1–13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20–30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.

Spectral evolution of an X-ray burst from MXB 1728-34 and constraints on burster parameters

Abstract Spectral evolution is investigated and interpretation of the spectra are considered for a burst from the X-ray burster MXB 1728-34 detected by ASTRON. Interpretation of the spectra around the burst maximum in terms of radiation from an isothermal neutron star (NS) photosphere with dominating role of the Thomson scattering allows one to obtain reasonable constraints on NS mass M, radius R and distance D to the burster. In particular, M=(1.4−2)M ⊗ , R=(6.5−12) km for D=6 kpc and helium photosphere. This interpretation leads to a conclusion that the photosphere expands up to several tens of kilometers in the burst maximum. The luminosity remains close to the Eddington limit during expansion and contraction, maxima of the observed flux corresponding to minima of the luminosity due to general relativity effects.

Some evidences in favour of the existence of a local galactic population of GRBs

Abstract The observed statistical properties of GRBs including isotropy in spatial distribution and the behaviour of the log N-log S diagram indicate that either GRBs appear in the local vicinity of the Sun (less than a few hundred parsecs) or they appear on distances of over hundreds of kiloparsecs. Accepting the hypothesis of the local (r < 300 pc) origin of GRBs it would give a reason to try to find the influence of local galactic structures like the local (Orion) arm, the Gould belt or separate stellar-dust complexes on the spatial distribution and other properties of GRBs. Using the 3rd BATSE catalogue we found the excesses of numbers of GRBs in two directions coincided with the directions of the local (Orion) arm stretch. In addition, log N–log S towards the arm shows an excess of bright bursts in comparison with that for the longitude range where the arm is hardly felt. To explain these facts we should assume the existence of at least two GRBs populations, one of which (poor but brighter or nearer)...