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:
galactic globular clusters are surprisingly rich in luminous x - ray sources @xcite .
scenarios involving tidal capture by two stars @xcite and/or exchange reactions involving three stars @xcite are widely believed to be the sources of these strongly interacting binaries in clusters . the remarkable correlation between stellar encounter rate and number of x - ray sources in globular cluster cores
@xcite supports this viewpoint .
accreting white dwarf - main sequence star binaries  the cataclysmic variable ( cv ) stars  are almost certainly produced by the same mechanisms in clusters , and should also be plentiful in globulars @xcite . perhaps the best cluster to search for this predicted population is ngc 6397 . at a mere 3.3 kpc @xcite
it is the closest globular cluster to the sun .
thus even cvs resembling the least luminous known such objects ( with @xmath0 ) in this cluster should be detected with deep hst imagery .
the initial discoveries of multiple x - ray sources @xcite and h@xmath1-bright stars @xcite in ngc 6397 were rapidly followed by spectrographic identification of essentially certain cvs with hydrogen and helium emission lines @xcite . remarkably ,
all four cvs in this cluster with hst spectra show significant , and in three of four cases , prominent heii 4686 .
this line is seen almost exclusively in magnetic cvs and nova - like variables @xcite , prompting @xcite and @xcite to suggest that magnetic cvs might well be the dominant cv population in globular clusters .    about half of all known field cvs @xcite are dwarf novae ( dn ) .
most field dn have been discovered because they undergo 2 - 5 magnitude outbursts every few weeks to months @xcite .
in contrast , strongly magnetic cvs are _ not _ observed to undergo dn outbursts .
this is because dn eruptions are likely caused by a disk instability @xcite , and the accretion disks of magnetic cvs have been truncated or are absent .
since large archival hst datasets of globular cluster images are available , we have been systematically looking for erupting dwarf novae in the cores of all such clusters @xcite . here
we report the results of a search of ngc 6397 to determine if any of the magnetic cv candidates undergo eruptions .
remarkably we do find dwarf nova eruptions of two of the putative ngc 6397 magnetic cvs .
the hubble space telescope has imaged ngc 6397 during 5 separate epochs from 1996 to 2003 .
our own 2003 datasets focused on stis uv imagery , heretofore unavailable and particularly useful in detecting cvs .
the dates of observation , pi and number of hst program , filters used , number of frames and total exposure time in each filter are given in the observing log which is table 1 .
we have carried out visual inspection and aperture photometry of each of the nine known cvs ( referred to by @xcite as cv1 through cv9 ) in every available archival hst image . while a modest level of variability ( up to a few tenths of a magnitude ) is detectable in all nine cvs , and two of the objects ( cv1 and cv6 )
now have measured photometric periods @xcite , none is yet reported to show a dwarf nova - like outburst .
the hst images of cv2 and cv3 are shown as photo - montages in figures 1 and 2 , respectively . the median brightness in every epoch , and in every available filter
is shown with mean filter wavelength running from blue ( left ) to red ( right ) .
we also show the 5-epoch light - curves of these two objects in figure 3 .
each cv has been imaged in the wfpc2 f814w filter in epochs 1 , 2 and 3 , and in the stis ccdclear filter in epochs 4 and 5 .
we have normalized epochs 1 - 3 with the first observation of each cv in epoch 1 , and epochs 4 - 5 with the first observation in epoch 4 .
the key result of this paper is that the putative magnetic cataclysmic variables cv2 and cv3 are clearly detected as erupting dwarf novae .
cv2 is seen to brighten ( in epoch 5 ) in the space telescope imaging spectrograph ( stis ) uv filter images .
it is 2.7 magnitudes brighter than it was in epoch 4 .
cv3 is seen in eruption in the second epoch in visible and near infrared pass - band images taken with the hst wide field and planetary camera ( wfpc2 ) .
it is 1.8 magnitudes brighter in eruption than in quiescence .
it is also seen in an intermediate brightness state ( between eruption and quiescence ) in blue and near - uv wfpc2 images in epoch 3 .
the rise time of cv3 in epoch 2 ( about 2 days ) is well in accord with field dwarf nova rise times .
the hst spectra of cv1 , cv2 and cv3 are presented in @xcite ; that of cv4 is given in @xcite .
heii emission with equivalent widths in the range 6 - 15   appear in the spectra of all four objects , as do strong balmer lines .
in particular , @xcite found for cv2 and cv3 , respectively , heii 4686 equivalent widths of 11 and 15  .
the h@xmath2 equivalent widths of these same two objects are 32 and 59  , respectively .
the disks in all four objects are faint ( @xmath3 ) .
@xcite has carefully compared the disks and heii line ratios of cvs 1 - 4 with those of other cataclysmic systems .
we refer the interested reader to that careful and exhaustive analysis , simply quoting here edmond et al.s conclusions regarding cvs 1 - 3 :    `` they do not appear to be recent novae or nova - likes because of their faint disks ( with extra evidence from their heii 4686   line ratios ) , nor do they appear to be dwarf novae because they have moderately strong heii lines .
the final option is magnetic systems . to conclude , cvs 1 - 3 do not appear to be dwarf novae , but they could be dq her - type systems .
a possible alternative to the dq her hypothesis is that some of the ngc 6397 cvs are old novae ( possibly in deep hibernation between outbursts ; see shara et al .
1986 ) . ''
figures 1 and 2 of this paper convincingly demonstrate that cv2 and cv3 are , in fact , erupting dwarf novae despite their prominent heii emission .
intensive monitoring with hst is essential to determine if all of the cv candidates in ngc 6397and other globular clusters  eventually erupt as dwarf novae .
the hibernation scenario of cvs @xcite postulates that old novae eventually display dwarf - nova like outbursts .
one old classical nova in the globular cluster m80 has been recovered @xcite at dwarf nova - like luminosity , and one recent erupting classical nova in a globular cluster ( of the giant elliptical galaxy m87 ) has been reported @xcite .
cv2 and cv3 could be hibernating old novae , but we have , at present , no observational proof that they once underwent nova eruptions .
edmonds suggestion that the ngc 6397 cvs might be dq her - like magnetic systems ( also known as ips or intermediate polars ) might seem to be disproved with the observed dwarf nova eruptions of cv2 and cv3 . however , there are a few well - documented cases of ips that undergo dwarf nova eruptions , including gk per , ex hya , xy ari , do dra , tv col , ht cam and v1223 sgr .
most dwarf novae show some heii emission ( see e.g. @xcite for a compilation ) , so cv2 and cv3 are certainly not unique in this respect .
a comparison of the heii and h@xmath2 line strengths for the cvs in ngc 6397 and for field ips is presented by @xcite .
both the line strengths and line ratios of heii and h@xmath2 for cv2 and cv3 are comparable to those of other low mass transfer rate ips , with ex hya being a particularly good match .
@xcite have presented a model for dwarf nova - like outbursts in magnetic cvs , which they apply to tv col and gk per . if the white dwarf in a cataclysmic variable has a strong magnetic field , the inner region of the accretion disk is disrupted .
they found that disruption of the inner disk region leads to shorter outbursts , and , for disk instabilities starting near the inner disk edge , to much longer intervals between outbursts .
the outbursts of most ips tend to be rare , short and of only moderate amplitude . as an example
, @xcite has noted that the ex hya outbursts occur at @xmath4 year intervals , last less than 4 days and are of only @xmath5 magnitude amplitude . if the many globular cluster cv candidates identified by chandra @xcite and hst @xcite are also intermediate polars then the remarkably small numbers of erupting dwarf novae found by hst in globular clusters @xcite can be explained . in particular , a population of even dozens of ips will only occasionally show a dwarf nova near maximum light . under this scenario
we can predict that more of these cvs will eventually be seen to erupt if individual clusters are searched on dozens or hundreds of occasions .
we have observed ngc 6397 with hst during 5 separate epochs .
two of the four spectrographically confirmed cvs are observed to undergo dwarf nova eruptions with amplitudes of at least 1.8 and 2.7 magnitudes .
these eruptions were somewhat unexpected , as both objects display moderately strong heii 4686 emission lines , generally associated with non - erupting , magnetic cvs .
the dwarf nova - like eruptions reported in this paper can occur in intermediate polars , suggesting that some of the ngc 6397 cvs ( and by implication some of the cvs in other globular clusters ) are indeed magnetic systems .
the apparent rarity of erupting dwarf novae in globular clusters with large cv candidate populations is reconciled if those cvs are infrequently erupting intermediate polars .
support for program # 8630 was provided by nasa through a grant from the space telescope science institute , which is operated by the association of universities for research in astronomy , inc .
, under nasa contract nas 5 - 26555 . "
l|l|c|c|c|c|c|c 1 : 3/6/96 & king & 700s,6@xmath6500s , & 4@xmath6500s , & 6@xmath640s,8s,1s & & & 2@xmath640s,8s,1s + & # 5929 & 28@xmath6400s,2@xmath680s , & 16@xmath6400s , & & & & + & & 2@xmath610s & 2@xmath680s,2@xmath610s & & & & + 2 : 4/3/99 & grindlay & & & 24@xmath640s,2@xmath68s & 1000s,4@xmath6900s & 38@xmath640s & 24@xmath640s + & # 7335 & & & 2@xmath61s & 18@xmath6800s,7@xmath6700s & 2@xmath68s,2@xmath61s & 2@xmath68s,2@xmath61s + & & & & & 2@xmath6140s & & + 3 : 11/4/01 & noll & 3@xmath6160s & 2@xmath6160s & & & & 3@xmath640s + & # 9313 & & & & & & + stis epochs & pi / prog .
i d & clear ccd & ccd lp & clear mama & f25qtz & f25srf2 & + 4 : 3/28/03 & shara & 3@xmath6180s & 3@xmath6180s & & 533s,865s & & + & # 8630 & & & & 6@xmath6600s & & + 5 : 5/16/03 & `` '' & 3@xmath6180s & 3@xmath6180s & 3@xmath61000s & & 2@xmath6600s,883s & + & & & & & & 2@xmath61021s & +
ngc 6397 is the closest globular cluster , and hence the ideal place to search for faint stellar populations such as cataclysmic variables ( cvs ) . 
 hst and chandra observers have identified nine certain and likely cvs in this nearby cluster , including several magnetic cv candidates . 
 we have combined our recent uv imagery with archival hst images of ngc 6397 to search for new cv candidates and especially to look for dwarf nova - like eruptive events . 
 we find remarkable and somewhat unexpected dwarf nova - like eruptions of the two well - known cataclysmic systems cv2 and cv3 . 
 these two objects have been claimed to be _ 
 magnetic _ cvs , as indicated by their helium emission - line spectra . 
 magnetic fields in cvs are usually expected to prevent the disk instability that leads to dwarf nova eruptions . 
 in fact , most field magnetic cvs are observed to _ not _ undergo eruptions . 
 our observations of the dwarf nova eruptions of cv2 and cv3 can be reconciled with these objects heii emission lines if both objects are infrequently - erupting intermediate polars , similar to ex hya . 
 if this is the case for most globular cluster cvs then we can reconcile the many x - ray and uv bright cv candidates seen by chandra and hst with the very small numbers of erupting dwarf novae observed in cluster cores .