You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
on april 18 , 2001 at 07:55:11.509 ut the gamma ray burst monitor ( grbm , 40 - 700 kev ) onboard bepposax was triggered by a large flare .
the event was also observed by the unit 1 of the bepposax wide field cameras ( wfcs , 2 - 28 kev ) , that was automatically shut - down few seconds after the start of the event by an onboard safeguard algorithm .
the wfc localized this giant flare as originating from the soft gamma - ray repeater sgr 1900 + 14 .    the bepposax grbm unit 1 ,
co - aligned to wfc unit 1 , detected a peak flux of 16,400 counts / s ( 40 - 700 kev , after deadtime correction ) , corresponding to ( 1.1@xmath00.1)@xmath1 erg @xmath2 s@xmath3 .
the event lasted approximately 40  s , and the total fluence was about 240,000 counts .
a 5-s periodic modulation of the light curve is consistent with the 5.17-s period of the persistent x - ray source . in the light curve of the flare
we can identify the repetition of 5 or 6 cycles , and a last pulse that is out of phase with respect to the previous ones . assuming a thermal bremsstrahlung spectral shape , from our 2-channel , 1-s resolved spectrum
, we can derive a plasma temperature kt of @xmath430  kev .
there is no evidence for a hard spike at the start of the event , and the spectral evolution during the event is relatively smooth , with a general hardening trend .
the konus - wind instrument detected this event in the non - trigger mode because of very high background level after strong solar flare ( 30 times more than normal ) .
our estimation for energy more than 15 kev is : fluence - 2@xmath510@xmath6 erg @xmath2 and peak flux - 2.5@xmath510@xmath7 erg @xmath2 s@xmath3 . in spite of rough time resolution in the background mode
the konus - wind data has evidences of the 5-s pulsations .
three weak and short bursts are detected at times 2537 , 755 and 444 seconds _ before _ the giant flare in the wfc unit 1 .
their durations are 100 , 125 and 55 ms , and their peak fluxes exceed 20 crab units in 2 - 28 kev ( feroci et al . 2001b ) .
no gamma - ray counterpart was found for these events in the grbm , nor in _ ulysses _ or konus / wind .
this is the first reported bursting activity from sgr 1900 + 14 since approximately two years .    at the time of the giant flare , as detected by the grbm ,
the wfc1 detected a large increase in the count rate for approximately 3 seconds after the grbm trigger . at this time
the count rate was so high ( larger than 30,000 counts / s ) that it triggered the automatic turn off of the experiment .
the april 18 2001 activity of sgr 1900 + 14 allowed us to activate our target of opportunity observation program with the bepposax narrow field instruments ( nfi , 0.1 - 300 kev ) .
thanks to the remarkable effort by the mission scientist team , the mission planners team and our team , the first follow - up observation was started on april 18 at 15:10 ut , less than 8 hours after the event .
this first observation ( feroci et al .
2001c ) was 28 hours long , for a net exposure time in the mecs instrument of 60 ks .
a second pointing was carried out on april 29 at 20:20 ut , for a similar exposure time .
the persistent x - ray counterpart of sgr 1900 + 14 was detected in both observations .
the source emitted a large number of short x - ray bursts during the first observation , whereas in the second pointing only one burst was detected .
the average spectrum during the first pointing , after removal of the bursts , can be fit with a power law with photon index 2.6@xmath00.1 and n@xmath8=(4.3@xmath00.3)@xmath9 @xmath2 .
the persistent 2 - 10 kev flux showed a rapid decaying behaviour during our first pointing , going approximately from 3.5 to 2@xmath10 erg @xmath2 s@xmath3 ( unabsorbed ) . using the x - ray flux in the same band provided by the chandra observation carried out on april 22 ( kouveliotou et al .
2001 ) , we obtain that the time behaviour of the x - ray flux can be described by a power law , with index @xmath4 - 0.6 .
interestingly , studying the hardness ratio between the 1.6 - 4 kev and the 4 - 10 kev energy ranges , we find a clear correlation flux vs. hardness , indicating that the spectrum softens when the persistent flux decreases .    during our second observation the source appeared to have gone back to its quiescent status .
removing the single burst that we observed , the 0.1 - 10 kev energy spectrum is best fit by the sum of an absorbed power law plus a blackbody with temperature @xmath40.5 kev .
the 2 - 10 kev flux is consistent with the ` standard ' quiescent status , at @xmath41.1@xmath10 erg @xmath2 s@xmath3 ( unabsorbed ) . in figure 1
we show the plot of the x - ray flux vs. time , including also the chandra point , that suggests that the source returned to its quiescent status in less than 3 days after the burst .
we performed a period search in both nfi observations and found the expected 5.17-s periodicity in the 2 - 10 kev flux from the persistent source . in the first observation
the best period was 5.17277 s , with a smooth sinusoidal profile . in the second pointing the period increased to 5.17298 s , without significant differences in the pulse profile .
therefore , the period derivative between the two bepposax observation is about 2.3@xmath11 s s@xmath3 .
the bepposax instrumentation observed the most interesting phases of the 2001 activation of sgr 1900 + 14 with a very complete set of observations .
the grbm and wfc caught the second giant flare in the ( observed ) history of this source and its precursor x - ray activity .
the nfi observed the afterglow of the persistent source after the flare , the bursting activity and its quenching , and the period history soon after the giant flare .
the giant flare is rather unusual with respect to its two predecessors , the march 5 1979 event from sgr 0526 - 66 and the august 27 1998 flare from sgr 1900 + 14 ( e.g. , mazets et al . 1979 , hurley et al . 1999 , feroci et al .
the most important differences in the april 18 are in the following aspects :    * the peak flux and fluence are more than one order of magnitude smaller * the duration is almost one order of magnitude shorter * there is no evidence for the first short and very hard pulse at the beginning of the event , implying that , contrary to the other two events , most likely there was no relativistic particle emission during this event .
the nfi observation is the fastest pointing after a giant flare and provides a _ direct _ observation of a long - duration afterglow emission from an sgr after a giant flare .
our result of an afterglow showing a power law decay , with index -0.6 , is similar to that obtained by woods et al .
( 2001 ) , who measured a decay index of -0.713@xmath00.025 after the august 27 event , based on the assumed constancy of the pulsed fraction .
interestingly , ibrahim et al .
( 2001 ) found a similar decay law in the pulsating soft tail of the august 29 short burst .
the spectral variability of the x - ray afterglow is also interesting . during our first observation , at high flux level ,
the source spectrum showed a softening , and an overall power law shape . in our second observation , at a flux typical of the quiescent state , we clearly detect the emergence of a kt=0.5 kev blackbody component , and an n@xmath8 smaller by about a factor of 2 . based on our observations , together with our analysis of the previous bepposax observations
( see also woods et al . 1999 , 2001 ) of this source in quiescence ( may 1997 and march / april 2000 ) and after the august 27 giant flare ( september 1998 ) , we can state that the blackbody component for this source is peculiar of its quiescent state .
we warmly thank l. piro , g. gandolfi , l. salotti , m. capalbi , l. amati , the bepposax mission planners , the bepposax soc , occ and sdc for their personal efforts that allowed a fast repointing of the source after the flare , as well as a prompt analysis of the grbm , wfc and nfi data .
feroci , m. , hurley , k. , duncan , r.c . , & thompson , c. , 2001a , , 549 , 1021 feroci , m. , mereghetti , s. , gandolfi , g. , piro , l. , & costa , e. 2001b , gcn circular 1055 feroci , m. , in t zand , j.j.m . ,
soffitta , p. , hurley , k. , frontera , f. , & mazets , e. 2001c , gcn circular 1060 guidorzi , c. , et al .
2001 , iau circular 7611 hurley , k. , et al .
1999 , , 397 , 41 ibrahim , a.i . ,
2001 , , 558 , 237 kouveliotou , c. , et al .
2001 , gcn circular 1046 mazets , e. , et al . , 1979 , , 282 , 587 woods , p. , et al . 1999 , , 518 , l103 woods , p. , et al . 2001 , , 552 , 748
after a couple of years of quiescence , the soft gamma repeater sgr 1900 + 14 suddenly reactivated on 18 april 2001 , with the emission of a very intense , long and modulated flare , only second in intensity and duration to the 27 august 1998 giant flare . 
 bepposax caught the large flare with its gamma ray burst monitor and with one of the wide field cameras . 
 the wide field cameras also detected x - ray bursting activity shortly before the giant flare . 
 a target of opportunity observation was started only 8 hours after the large flare with the narrow field instruments , composed of two 60-ks long pointings . 
 these two observations show an x - ray afterglow of the persistent sgr 1900 + 14 source , decaying with time according to a power law of index -0.6 .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in