R. K. Manchanda

Discovery of the short gamma-ray burst GRB 050709.

Gamma-ray bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale and the progenitors of the short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst; this in turn led to the identification of the host galaxy of the burst as a late-type galaxy at z = 0.16 (ref. 10). These results show that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be caused by the merging of compact binaries.

Global Characteristics of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by HETE-2

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first 3 years of its mission, focusing on the properties of X-ray flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable, and that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy E of the burst spectrum in νFν, the peak energy flux Fpeak, and the energy fluence SE of XRFs are much smaller (and those of XRRs are smaller) than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the [SE(2-30 keV), SE(30-400) keV] plane, the [SE(2-400 keV), Epeak] plane, and the [Fpeak(50-300 keV), Epeak] plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon.

Scientific highlights of the HETE-2 mission

Abstract The High Energy Explorer Satellite 2 (HETE-2) mission has been highly productive. It has observed more than 250 γ-ray bursts (GRBs) so far. It is currently localizing 25–30 GRBs per year, and has localized 43 GRBs to date. Twenty-one of these localizations have led to the detection of X-ray, optical, or radio afterglows, and as of now, 11 of the bursts with afterglows have redshift determinations. HETE-2 has also observed more than 45 bursts from soft γ-ray repeaters, and more than 700 X-ray bursts. HETE-2 has confirmed the connection between GRBs and Type Ic supernovae, a singular achievement and certainly one of the scientific highlights of the mission so far. It has provided evidence that the isotropic-equivalent energies and luminosities of GRBs may be correlated with redshift; such a correlation would imply that GRBs and their progenitors evolve strongly with redshift. Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBs as a probe of cosmology and the early universe. HETE-2 has placed severe constraints on any X-ray or optical afterglow of a short GRB. It has made it possible to explore the previously unknown behavior of optical afterglows at very early times, and has opened up the era of high-resolution spectroscopy of GRB optical afterglows. It is also solving the mystery of “optically dark” GRBs, and revealing the nature of X-ray flashes.

High-energy observations of XRF 030723: Evidence for an off-axis gamma-ray burst?

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E of the νFν burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E = 8.4 keV. The ratio of X-ray to γ-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy Eiso and Epk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late-time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.

The flattening of the X-ray spectrum of Sco X-1 at energies beyond 40 keV

The measurements of the hard X-ray spectrum of Sco X-1 in the energy interval 20–150 keV in three balloon flights from Hyderabad, India are reported. These results show conclusively that the spectrum of Sco X-1 is very flat in the energy interval 40–150 keV and the measured fluxes beyond 60 keV are several orders of magnitude higher than those expected on the basis of an extrapolation of bremsstrahlung spectrum from a thin hot plasma at a temperature corresponding tokT=5 keV, which is applicable for Sco X-1 for energies <40 keV. The results are compared with those of other investigators of hard X-rays from Sco X-1, and the implication of the results is briefly discussed.

Energy spectrum and time variations of hard x-rays from Cyg X-1

Experimental results on the intensity, energy spectrum and time variations in hard X-ray emission from Cyg X-1 based on a balloon observation made on 1971, April 6 from Hyderabad (India) are described. The average energy spectrum of Cyg X-1 in the 22–154 keV interval on 1971 April 6 is best represented by a power law dN/dE=(5.41±1.53)E−(1.92±0.10) photons cm−2s−1 keV−1 which is in very good agreement with the spectrum of Cyg X-1 derived from an earlier observation made by us on 1969 April 16 in the 25–151 keV band and given by dN/dE=(3.54±2.44)E−(1.89±0.22) photons cm−2s−1 keV−1. A thermal bremsstrahlung spectrum fails to give a good fit over the entire energy range for both the observations. Comparison with the observations of other investigators shows that almost all balloon experiments consistently give a spectrum of ∼E−2, while below 20 keV the spectrum varies fromE−1.7 toE−5. There is some indication of a break in the Cyg X-1 spectrum around 20 keV. Spectral analysis of data in different time intervals for the 1971 April 6 flight demonstrates that while the source intensity varies over time scales of a few minutes, there is no appreciable variation in the spectral slope. Analysis of various hard X-ray observations for long term variations shows that over a period of about a week the intensity of Cyg X-1 varies upto a factor of four. The binary model proposed by Dolan is examined and the difficulties in explaining the observed features of Cyg X-1 by this model are pointed out.

Gas amplification in high pressure proportional counters

Abstract We have made absolute gas gain measurements for proportional counters filled with argon/xenon at various pressure up to ∼ 30 atm (3000 kPa). Our data show that of the various empirical formulae which give gas gain as a function of electric field and gas pressure, the relation proposed by Charles [J. Phys. E5 (1972) 95] gives the best fit over a wide range of pressures for both argon and xenon. The measurements further reveal that gas gain data at low pressures cannot be extrapolated to higher pressures to explain the behaviour of proportional counters filled to such high pressures, because of the changing mobility of positive ions. The details of our measurements and the results obtained are described in this paper.

HETE-2 observation of two gamma-ray bursts at z > 3

GRB 020124 and GRB 030323 constitute half the sample of gamma-ray bursts with a measured redshift greater than 3. This paper presents the temporal and spectral properties of these two gamma-ray bursts detected and localized with HETE-2. While they have nearly identical redshifts (z=3.20 for GRB 020124, and z=3.37 for GRB 030323), these two GRBs span about an order of magnitude in fluence, thus sampling distinct regions of the GRB luminosity function. The properties of these two bursts are compared with those of the bulk of the GRB population detected by HETE-2. We also discuss the energetics of GRB 020124 and GRB 030323 and show that they are compatible with the Epeak - Eiso relation discovered by Amati et al. (2002). Finally, we compute the maximum redshifts at which these bursts could have been detected by HETE-2 and we address various issues connected with the detection and localization of high-z GRBs.

Fabrication and flight performance of a large area balloon borne hard X-ray telescope

Abstract We describe the fabrication and flight performance of a balloon-borne large area hard X-ray (20–100 keV) telescope for spectral studies of discrete cosmic X-ray sources. The telescope consists of two multi-wire Xenon filled proportional counters of effective area 1200 cm 2 each, mounted on an orientable platform. It can be pre-programmed to track any celestial source with a pointing accuracy of 0.5 degrees. For one hour of observation the telescope has a 5 σ detection sensitivity of 10 −5 ph cm −2 s −1 . The laboratory test results and the performance in a series of balloon flights conducted in 1984–1986 period is discussed and the preliminary results obtained for some X-ray sources are presented.

Sudden Changes in the Intensity of High Energy X-Rays from Sco X-1

WE report the observation of sudden changes in the intensity of Sco X-1 by a factor of about three in the energy interval 29.9–52.3 keV on December 22, 1968, between 0427 h and 0553 h UT. The observation was made with an X-ray telescope in a balloon flight from Hyderabad (latitude 17.6° N, longitude 78.5° E), India. The telescope consisted of an NaI(Tl) crystal with an area of 97.3 cm2 and thickness 4 mm surrounded by both active and passive collimators. The passive collimator was a cylindrical graded shield of lead, tin and copper and the active collimator was a plastic scintillator surrounding the shield. The field of view of the telescope at f.w.h.m. was 18.6°. The axis of the telescope was inclined at an angle of 32° to the zenith. The telescope was mounted on an oriented platform programmed to look in four specified directions successively, spending about 4 min in each direction during a cycle of period about 16 min. The four specified directions were N(ϕ = 0), SW(ϕ=110°, with the convention ϕ = 90° being due west), S(ϕ = 180°, due south) and NE(ϕ = 310°). In this flight the orienter performed satisfactorily and the telescope picked up Sco X-1 in the south direction, Cyg X-1 in the direction NE(ϕ = 310°) and a new source in the direction SW(ϕ = 110°). In the north direction (ϕ = 0) there was no source during the period of observation and therefore the information on background X-rays was obtained from this direction. The observations of the short term changes of intensity of Sco X-1 are discussed here—the results from the other data will be published separately. A pair of crossed flux gate magnetometers provided information every 8.2 s on the azimuth of the telescope. The pulse heights from the X-ray detector were sorted into nine contiguous channels extending from 10 to 120 keV. Counting rates of several channels were combined for analysis to improve the statistics. A 241Am source came into the field of view of the telescope once in 15 min for about 30 s to provide in-flight calibration of the detector and this indicated that there was stable operation and no drift in the channel positions during the flight. The balloon was launched at 0200 h UT and reached the ceiling altitude at 0435 h. Between 0435 and 0530 h, the ceiling altitude was 7.6 ± 0.3 g cm−2. Just before the balloon reached the ceiling Sco X-1 was in the field of view of the telescope for 3 min 41 s when the balloon was ascending from 9.2 to 8.3 g cm−2. After the balloon reached ceiling, Sco X-1 was in the field of view on five occasions between 0443 and 0553 h. During the last observation, 0551–0553 h, however, the floating altitude of the balloon was a little lower, 8.5 g cm−2. The meridian transit of Sco X-1 was at 0454 h and the flight was terminated at 0615 h.