# Gamma-ray Bursts: 50 Years and Counting!
## 3 The BATSE Era (1991-1996)  
On 5 April 1991, the US spacecraft Compton Gamma Ray Observatory (CGRO) was launched in low Earth orbit. On board, there were four instruments that covered a huge energy range, at the time unprecedented, of six orders of magnitude of the electromagnetic spectrum, ranging from 20 keV X-rays to 30 GeV gamma rays. Among these instruments, the Burst and Transient Source Experiment (BATSE) [14] has led to substantial improvements in our understanding of GRBs. It consisted of eight identical detector modules, each composed of a NaI (sodium iodide, i.e., crystal detector) scintillator Large Area Detector covering the energy range 20 keV-2 MeV to act as a trigger for the observations and a thick NaI Spectroscopy Detector covering the energy range 10 keV-100 MeV. Each module was placed on one of the corners of the satellite in order to obtain complete coverage of the sky, achieving a burst detection sensitivity of \(3\times 10^{-8}\) erg cm\({}^{-2}\) for a 1 s burst. BATSE was thus able to obtain approximate localizations of GRB signals based on the ratios of the count rates between the eight detectors. GRB light curves have a broad range of characteristics and are quite irregular, ranging from single peak light curves to more complex multipeaked extended structures. However, in this period, enormous progress was made in their characterization. In 1996, [PERSON] and colleagues found that the light curves of individual pulses are well fitted with a fast-rise exponential decay (FRED) profile with an average rise-to-decay ratio of 1:3 [15]:  
\[I(t)=\left\{ \begin{array}{ll}A\exp(-(\left|t-t_{\rm max}\right|/\sigma_{d}) ^{\
u})&t\leq t_{\rm max}\\ A\exp(-(\left|t-t_{\rm max}\right|/\sigma_{d})^{\
u})&t>t_{\rm max}\end{array}  \right., \tag{1}\]  
where \(t_{\rm max}\) is the time of the pulse's maximum intensity; \(A\) is the peak value; \(\sigma_{r}\) and \(\sigma_{d}\) are the rise (\(t\leq t_{\rm max}\)) and decay (\(t>t_{\rm max}\)) time constants, respectively; and \(\
u\) is the \"peakedness\", a measure of the pulse's sharpness. In the same paper, a delay between the emission at low energies and that at high energies was highlighted. Low/high energies are defined based on the low vs. high channels of BATSE: 20-50 keV, 50-100 keV, 100-300 keV, and \(>\)300 keV. The BATSE team also reported a width-symmetry-intensity correlation. In particular, high-intensity pulses were (statistically) more symmetric (i.e., with lower decay-to-rise ratios) and had shorter spectral delays. Furthermore, the low-energy light curves of individual pulses are generally wider than the high-energy ones, with a width of \(\sim\)\(E^{-0.4}\)[16]. BATSE observed GRBs with very disparate durations, from \(\sim\)6 ms up to \(\sim\)2000 s. To classify the light curves, a duration measurement standard was introduced, the T90 (T50), in which 90% (50%) of the total counts of the GRB arrive. After subtracting the background counts and normalizing the cumulative counts so that the total is 1, the T90 (T50) is defined as the time interval between the arrival of 5% (25%) and 95% (75%) of the counts. With this definition, [PERSON] and colleagues showed that a GRB can be divided into two distinct groups: long bursts with T90 \(>\) 2 s and short bursts with T90 \(<\) 2 s [17]. The two groups also have different spectral characteristics, with short GRBs tending to be spectrally \"harder\" than long GRBs, where the hardness ratio is the ratio of photons observed in two BATSE channels: channel 3 (100-300 keV) counts divided by channel 2 (50-100 keV) counts. Spectral studies of BATSE GRBs have shown that the emission is non-thermal, with spectra generally characterized by a double power law and a smooth break. [PERSON] and colleagues proposed a phenomenological function, now known as the Band function, to fit GRB spectra [18]:  
\[F_{\text{Band}}(E)=