arXiv:astro-ph/9802154v1 12 Feb 1998 SD3,LsAao ainlLbrtr,LsAao,N 87544 NM Alamos, Los Laboratory, keV National Alamos 400 Los to D436, 2 MS From Fenimore Histories E. E. Time and Spectra Burst Gamma-Ray cnilto one oosretegmary.I a ide was (GRBs). It bursts gamma-rays. ray the observe to counter scintillation rm2t 0 e.Atog the Although keV. 400 to 2 from tutr ob rae tlwreeg ahrta differen a than rather energy “preactivit lower x-ray at broader This be emission. to structure x-ray pure effectively was different. was parameters Ginga [8]. redshift measured for a one way with including the counterparts, opened GRB long-sought has the discovery latter This afterglows x-ray [7]. soft gamma- discovered and has x-rays and [4–6] both rays in bursts several served space- 1991. the October, of reentry in operational the craft until was 1987, GBD March, the from and 1987, of February these the detec- aboard on burst flown Based gamma-ray (GBD) tor the tails). results, some X-ray in intriguing event called GRB (so emis- main X-ray bursts the the outlast although might a 2-to-10 sion [1,2], of the range energy in x-ray the occurs keV of indi- (GRB) percent results burst few Early Gamma-Ray a only range. 100 that energy the cated MeV in 1 emitted usually to is keV power their of most INTRODUCTION t 1. about at peak to coo that tend electrons both relativistic they of but injection rays the involving gamma the than slower was keV) 300 One to band. (50 gamma-ray band zero the gamma-ray than in the greater than to band indexes compared x-ray law keV) power 10 energy to (2 low with bursts of nteXryrgm [3]. regime X-ray spectra the burst in investigate to designed specifically h am-a us eetron detector burst Gamma-Ray The ee esmaiexryrslsfo the from results x-ray summarize we Here, ob- has satellite BeppoSax the recently, More name, aptly are (GRBs) bursts Gamma-ray experiment. Ginga Ginga Ginga detected Ginga Ginga bevdbn nrisi h pcr ont e n a la a had and keV 2 to down spectra the in energies bend observed a anhdin launched was ∼ aelt was satellite 2 Rsad2 fte a ucetsaitc odtries determine to statistics sufficient had them of 22 and GRBs 120 n AS rge rtrawr eysmlr h distribut the similar, very were criteria trigger BATSE and Ginga osse fapootoa one oosretexry an x-rays the observe to counter proportional a of consisted Ginga vn a eido iepeeigtegmary that rays gamma the preceding time of period a had event ysnhornradiation. synchrotron by l lysie osuytexry soitdwt gamma- with associated x-rays the study to suited ally ”mgtb u otepnhn o h R time GRB the for penchant the to due be might y” cle Mmr edOt MO)wudbe would [MRO]) data Out” temporal Read such resolution “Memory Upon high (called scales). special time s trigger, (11 1 increase a or 1/4, significant checked 1/8, a either system for on trigger histories teleme- on-board time the An the of on resolution mode. depended x-rays try temporal data the The time-history between the mea- timing gamma-rays. facilitates relative and This the time. and same started suring the detectors that at both ensured stopped for hardware accumulated samples The time were the detector samples. each en- time the into of within range Photons ergy GRBs. for history-data SC. the effectively and was PC view the both of for field steradian the The for backside window. support reduce PC mechanical to the shielding and of illumination except presence uncollimated the were for detectors The energy indicated channel ranges. their 32 and over 16 respectively, provided SC spectra, and PC The area. .ISRMNA DETAILS INSTRUMENTAL 2. 0 e.Ec eetrhdan 15-to- had detector between Each keV. energies (SC) 400 with counter scintillation photons a recording 2- and the range in keV photons to-25 to sensitive (PC) counter tional hsclpoes h -astn orise to tend x-rays The process. physical t ahdtco rdcdbt pcrladtime and spectral both produced detector Each h B aboard GBD The h vrg ai feeg ntexryband x-ray the in energy of ratio average The . esm ie hsage gis models against argues This time. same he 4.Sm vnshdmr nryi the in energy more had events Some 24%. Ginga osse fapropor- a of consisted ≈ 0cm 60 o fspectral of ion grfraction rger 2 effective and pectra a d fall σ π 1 2 produced. The MRO data has time resolution of conclusions were not affected by the uncertainty 0.03125 s. The PC MRO time history extends in the incidence angle. from 32 s before the trigger to 96 s after the trig- We have adopted the spectral model employed ger. The SC MRO time history extends from 16 by [12] because of its relative simplicity and abil- s before the trigger to 48 s after the trigger. Dur- ity to accurately characterize a wide range of ing the period when both detectors have MRO spectral continua. In addition, this choice facili- time histories, the MRO samples were in phase tates direct comparison of our results with those with each other. In addition to the MRO data, obtained from bursts seen by the Burst and Tran- “Real Time” data were often continuously avail- sient Source Experiment (BATSE). This model able. Three different telemetry modes resulted in has the form PC and SC time histories with either 0.125, 1.0, α − or 4.0 s time resolution. N(E)= AE exp( E/E0) In burst mode, the GBD recorded spectral data if from the PC and SC at 0.5 s intervals for 16 s be- (α − β)E0 ≥ E fore the burst trigger time and for 48 s after the trigger. The PC and SC had 16 and 32 energy and channels, respectively. The MRO data were used α−β β N(E)= A [(α − β)E0] exp(β − α)E for many of the spectral fits described here. In the event that MRO data were not available for a if burst, we utilized the spectral data from the “real (α − β)E0 ≤ E. time” telemetry modes. For these bursts, spectral Here, A is an overall scale factor, α is the low- data were available with either 2, 16, or 64 s accu- energy spectral index, β is the high-energy spec- mulations. For the longer accumulations, spectral tral index, and E0 is the exponential cutoff or studies were not generally feasible. See [3] and [9] bend energy. The model parameters are adjusted for more information concerning the instrument. 2 iteratively until a minimum in χ is obtained. The Ginga GBD was in operation from March 1987 to October 1991. During this time ≈ 120 3. X-RAY CHARACTERISTICS OF γ-ray bursts were identified [10,11]. From this GINGA BURSTS group, a sample of 22 events with statistically significant spectral data were analyzed [9]. These Of particular interest is the behavior of the events occurred within the forward, π steradian Ginga sample at x-ray energies. About 40% of field of view of the detectors (front-side events). the bursts in the sample show a positive spectral Front-side events are easily identified because number index below 20 keV (i.e., α > 0), with they have consistent fluxes in the energy range the suggestion of rolloff toward lower energies in observed by both instruments (15 to 25 keV). Ex- a few of the bursts (α as large as +1.5) [9]. Un- cluded bursts usually show strong rolloffs below fortunately, the lack of data below 1 keV, and the 25 keV and spectral fits with an absorption com- often weak signal below 5-to-10 keV precludes es- ponent due to aluminum (a principal spacecraft tablishing the physical process (photoelectric ab- material) are consistent with this interpretation. sorption, self-absorption) that may be involved Because the incidence angle was known for just in specific bursts. Observations of the low-energy four of the events, we had to assume an incidence asymptote can place serious constraints on several angle for the remaining events. We selected 37 GRB models, most notably the synchrotron shock degrees for the incidence angle when the angle model which predicts that α should be between was unknown. This is a typical angle consider- -3/2 and -1/2 [13]. Crider et al. [14] uses exam- ing that the mechanical support for the window ples from BATSE to argue that some GRB vio- on the PC acts as a collimator limiting the field late these limits during some time-resolved sam- of view to an opening angle of ∼ ±60 degrees. ples. We find violations of these limits in the Extensive simulations [9] demonstrated that our time-integrated events. 3

In our sample of bursts, we find that α can be both less than zero and greater than zero. Nega- tive α’s are often seen in time-integrated BATSE spectra. Positive α’s for which the spectrum rolls over at low energies are usually only seen in time- resolved BATSE spectra [14]. Our distribution of parameters (α, β, E0) is broader than that found by BATSE. For exam- ple, 40% of our events had α greater than 0 com- pared to only 15% of BATSE events, and we had bend energies that occurred as low as 1.7 keV, wheras the lowest BATSE bend energy was 15 keV. The Ginga trigger range (50 to 400 keV) was virtually the same as BATSE’s. Thus, we do not think we are sampling a different popu- lation of bursts, yet we get a different range of Figure 1. The distribution of the ratio of the fit parameters. One possible explanation might energy emitted in x-rays relative to that emitted be that GRBs have two break energies, one often in gamma-rays. The x-ray bandpass is defined to in the 50 to 500 keV range and the other near 5 be from 2 to 10 keV, and the gamma-ray bandpass keV. Both BATSE and Ginga fit with only a sin- is the BATSE range of 50 to 300 keV. Note there gle break energy so BATSE tends to find breaks are some examples of equal energy in the x-rays near the center of its energy range, and we tend and the gamma-rays. The average ratio of the to find breaks in our energy range. Without good energy in the x-rays to the energy in the gamma- high energy observations of bursts with low E0, rays is 24%. From Strohmayer et al., 1998. it is difficult to know whether they also have a high-energy bend.

4. X-RAY EMISSION RELATIVE TO GAMMA-RAY EMISSION depends on the bandpass for which it is evaluated. Early measurements of the x-rays associated Using the best fit α, β, and E0 from [9], one can with gamma-rays were fortuitous observations by find the ratio of emission for a typical x-ray band- collimated x-ray detectors that just happen to pass (2 to 10 keV) compared to the BATSE en- catch a GRB in their field of view. From a few ergy range (50 to 300 keV). The ratio is defined to 10 300 events it appeared that the amount of energy in x- be Rx/g = R2 EN(E)dE/ R50 EN(E)dE. Fig- rays was only a few percent (Laros et al., 1984 [1]) ure 1 presents the distribution for the 22 events confirming that GRBs were, indeed, a gamma-ray analyzed in [9]. Although the ratio is often a phenomena. The Ginga experiment was designed few percent, for some events the ratio is near with a wide field of view to detect a sufficient (or larger than) unity. Some GRBs actually number of events to determine the range of x-ray have more energy in the x-ray bandpass than the characteristics. Early reports from Ginga events gamma-ray bandpass. The simple average of the indicated that sometimes a much larger fraction 22 values is 24%. This large value arises because of the emitted energy was contained in the x-rays. of the few events with nearly equal energy in the For example, by comparing the signal in the pro- x-ray and gamma-ray bandpass. However, even portional counter (roughly 2 to 25 keV) to that the logarithmic average is 7%. of the scintillator (roughly 15 to 400 keV), we re- ported an x-ray to gamma-ray emission ratio up to ∼46% (Yoshida et al., 1989 [15]). Such a ratio 4

5. X-RAY PREACTIVITY OR PULSE the fall are slower at low energy (causing the SPREADING? E−0.45 spreading) but the peak of the emission is not delayed substantially. In the calculations It is common for the x-ray emission to last of Kazanas, Titarchuk, & Hua, the low energy longer than the gamma-ray emission [4–6,15–17]. emission (e.q., 25-to-50 keV) starts to rise after There is one clear example of x-ray activity pre- the high energy emission (e.q., 300-t0-1000 keV) ceding the GRB [16]. Figure 2 presents the time has completely fallen. Because GRBs never seem history of GB900126 in several energy bands. to do this, we think it strongly argues against syn- From top to bottom, one sees an x-ray hard- chrotron cooling as the mechanism that produces ness ratio, the count rate in the PC (1-to-28 the peaks in GRBs. keV, 0.03125 s resolution), and five time histo- ries based on the PC and SC pulse height analy- REFERENCES sis (PHA) data (0.5 s resolution). The horizontal arrow in the bottom panel indicates the period 1. Laros, J. G., et al., 1984, ApJ, 286, 681. that is effectively pure x-rays. During the period 2. Katoh, M., et al., 1984, in High Energy Tran- marked by the arrow, there is no detectable emis- sients in Astrophysics, AIP Conf. 115, ed. S. sion above 7 keV, but there is significant emission E. Woosley (AIP New York), 390. below 7 keV (see Murakami et al. [16]). 3. Murakami, T., et al., 1989, Publ. Astron. Soc. It would be misleading to refer to the x-ray Jap., 41, 405. phase as a “precursor” because it is not a sep- 4. Piro, L., et al., 1997, A&A, in press, astro- arate peak. “Preactivity” gives the connotation ph/9707215. of a separate cause for the emission although not 5. Piro, L., et al., 1998, A&A, in press, astro- necessarily as distinct as “precursor” might imply. ph/9710334. However, the x-ray phase in Figure 2 could be due 6. Frontera, F., et al., 1998, ApJ, in press, astro- to the penchant for peaks to be wider at low en- ph/9711279. ergy. Figure 3 shows the average autocorrelation 7. Costa, E., et al., 1997, Nature, 387, 783, of GRB time histories in four energy bands. At astro-ph/9706065. higher energy, the autocorrelation is wider. This 8. Metgzer, M., et al., 1997, Nature, 387, 878. spreading of the time structure with energy is also 9. Strohmayer, T., et al., 1998, ApJ, in press, seen in the average rise and fall times of individ- astro-ph/9712332. ual pulses [18]. On average, the peak temporal 10. Ogasaka, Y., et al., 1991, ApJ, 383, L61. width scales as E−0.45. The spreading in Figure 11. Fenimore, E. E., et al., 1993, in AIP Confer- 2 is larger but that could reflect that some bursts ence Proceeding 280, Compton Gamma-Ray have larger spreading than the average. Observatory, ed. M. Friedlander, N. Gehrels, Perhaps there is not a separate cause for the x- & D. Macomb, (New York:AIP), 917. ray emission, but rather, the physics responsible 12. Band, D., et al., 1993, ApJ, 413, 281. for the spreading as a function of energy causes 13. Katz, J. L., 1994, ApJ, 432, L107. the x-rays to appear to turn on before the gamma- 14. Crider, A., et al., 1997, 479, L93. rays. 15. Yoshida, A., et al., 1989, PASJ, 41, 509. The physics responsible for the spreading is un- 16. Murakami, T., et al., 1991, Nature, 350, 592. clear. Kazanas, Titarchuk, & Hua (1998) [19] 17. Murakami, T., Inoue, H., van Paradijs, have suggested two processes, synchrotron cool- J., Fenimore, E., & Yoshida, A., 1992, in ing of injected relativistic electrons and Compton Gamma-Ray Bursts, ed. C. Ho, R. I. Epstein downscattering of injected photons. Synchrotron & E. E. Fenimore, (Cambridge: Cambridge cooling of ejected electrons can produce a time University Press), 239. structure that scales as E−1/2. However, such 18. Fenimore, E. E., et al., 1995, ApJ, 448, L101. cooling should cause the emission at lower en- 19. Kazanas, D., Titarchuk, L. G., & Hua, X.-M., ergy to peak later. In most GRBs, the rise and 1998, ApJ, in press, astro-ph/9709180. 5

Figure 2. The temporal evolution of GB900126 demonstrating the x-ray “preactivity.” The top panel is the x-ray hardness and the next panel is the total count rate in the proportional counter. The next 5 panels use the energy-resolved PHA Figure 3. Average autocorrelation of 45 bright data in 5 energy bands from 1 to 370 keV. Note BATSE gamma-ray bursts in four energy chan- that the peak widths become wider with lower nels. At higher energy, GRBs have shorter energy but they are not substantially shifted in timescales. The solid curves are fits of the sum time. Synchrotron cooling of ejected electrons is of two exponentials to the autocorrelation his- expected to produce substantial shifts at lower en- togram. (From Fenimore et al. 1995.) ergy. In the bottom panel, the time period mark with a horizontal arrow is the period of x-ray pre- activity during which the emission occurs only at energies less than 7 keV. (From Murakami et al., 1989.)