You are an expert at summarizing long articles. Proceed to summarize the following text: 
the temporal flux variability from active galactic nuclei ( agn ) , detectable in nearly all wavelength regimes , contains information on the underlying emission processes and source geometry that is otherwise difficult to probe @xcite . however , precise details of the physical mechanism generating the observed nuclear variability in agn remain unclear @xcite . current and future large - scale photometric time - domain surveys have motivated many recent studies of the optical broadband variability properties of various agn subclasses using large numbers of well - sampled light curves . this has been especially useful for agn identification and selection @xcite .    beyond the optical , large - scale surveys of agn variability have been pursued at many other wavelengths , including the radio @xcite , ultraviolet @xcite , and @xmath2-ray regimes @xcite . fewer studies of agn variability have focused on the infrared ( ir ) , due in part to the expectation that non - variable dust and the host galaxy dominate the emission at these wavelengths ( e.g. see * ? ? ? explanations of agn variability in the optical as due to localized temperature fluctuations in inhomogeneous accretion disks @xcite or global accretion rate changes @xcite predict little flux variability from disk emission in the ir . reprocessing of variable optical emission by a dusty torus has been suggested by @xcite as a possible cause for the nir variability , although the time - lags between the optical and nir are difficult to detect . for example @xcite have shown small time lags between nir and x - ray data for the agn 3c273 , with the nir variability leading the x - ray by @xmath41.5 days , implying this nir variability originates near the nucleus .    a study of nir variability in the spitzer deep wide - field survey botes field by @xcite showed that although only 1.1% of objects appear to be variable in the near - ir ( nir ) , the vast majority of the variable objects were agn . small samples of agn monitored in both the optical and nir have shown that while nuclear variability is prevalent , the amplitude of variations decreases towards longer wavelengths . for example , @xcite have produced realistic models of the optical to nir variability for ngc 4151 , which show a small amplitude time lag and decreasing amplitude variability as a function of wavelength . however , this is not true for blazars , whose nir flux is dominated by non - thermal emission from a relativistic jet , rather than circum - nuclear dust which is not expected to be non - variable @xcite . recent results from optical / nir monitoring of @xmath5 @xmath2-ray blazars have shown that flat - spectrum radio quasars are more variable in the nir than in the optical @xcite . @xcite have used 7 year photometric light curves of 7 blazars , spanning the optical to nir bands ( @xmath6 ) , to find that variability amplitudes increase with wavelength for these objects .    in this study we search for nir variability of luminous agn using well sampled light curves from the two micron all sky survey ( 2mass ) calibration data , finding one highly variable blazar ( sdssj14584479 + 3720215 ) . we explore the utility of nir variability - based selection of agn , including its use in identifying counterparts to @xmath5 @xmath2-ray sources . we also study the nature of non - agn contaminants with similar nir colors and variability properties , laying groundwork for the agn variability science possible with current and future multi - epoch ir surveys . the 2mass survey observed the full sky in the near - ir over the timespan of june 1997 to february 2001 using the @xmath7 , @xmath8 , and @xmath9 bands @xcite . photometric zero - points for calibrating the survey were based on hourly observations of a set of 40 standard fields , each @xmath10 in size , and spaced evenly throughout the sky . these fields were visited repeatedly over the course of the survey , resulting in 562 to 3692 epochs of observation per field . this produced @xmath0 light curves for 113,030 individual objects , known as the 2mass calibration point source working database ( hereafter cal - pswdb , * ? ? ? * ; * ? ? ? these light curves are the most precise nir time domain survey to date and provide a novel dataset in which to study the infrared variability properties for a wealth of astrophysical phenomena . this dataset has produced the best sampled nir light curve for a rr lyr star @xcite , a hunt for stellar flares at long wavelengths @xcite , a large sample of young stellar objects ( yso s , * ? ? ? * ; * ? ? ? * ) , and many well characterized binary star systems ( * ? ? ? * ; * ? ? ? * davenport 2014 in prep ) . here we make use of these data to search for nir variability of agn with unprecedented precision .    the wide - field infrared survey explorer ( wise , * ? ? ? * ) has created the deepest mid - ir survey of the entire sky in four photometric bands , from 3.4 @xmath11 m to 22 @xmath11 m . several recent studies have demonstrated the utility of wise colors for effectively separating agn from stars and unresolved galaxies . we spatially matched the cal - pswdb objects to the wise all sky data release using a match radius of 15 . this resulted in 52,148 objects with @xmath0 light curves and a detection in at least one wise filter . the distribution of these point sources in wise color  color space is shown in figure [ wcolor ] . for comparison , we show the wise `` blazar strip '' , defined in @xcite as the gaussian kernel density estimate of the wise colors of blazars from @xcite . to find previously known agn in our data , we spatially matched these 52k objects to the `` milliquas catalog , '' version 3.3 . we recovered 27 known agn with light curves in cal - pswdb , 25 of which had wise colors shown in figure [ wcolor ] . these agn had between 26 and 3482 epochs of cal - pswdb photometry , with an average of 924 epochs . of these , 19 have been spectroscopically confirmed by the sloan digital sky survey ( sdss ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) , one of which has been identified as a blazar ( sdssj14584479 + 3720215 ) . -band ( top ) and @xmath12 color ( bottom ) light curves for the known blazar , sdssj14584479 + 3720215 . the median photometric error is shown for comparison ( vertical bar).,width=336 ]    this set of known agn with well sampled cal - pswdb light curves creates a unique dataset in which to search for agn variability in the nir . to determine if these objects were variable in the 2mass bands , we computed the error - weighted root mean square variability for each light curve , @xmath13 , following the prescription in @xcite . objects were classified as variable if they had @xmath14 mag . of the `` milliquas '' agn in our dataset , only the known blazar , sdssj14584479 + 3720215 , exhibited variability above this threshold . the @xmath7-band and @xmath12 color light curves for this blazar , with 1972 epochs of cal - pswdb photometry spanning over three years , are presented in figure [ lc ] . the remaining 26 known agn in our sample displayed no significant nir variability using this metric . we found strong variability in sdssj14584479 + 3720215 in all three 2mass bands with our densely sampled light curves . however , as seen in figure [ lc ] no significant variation in @xmath12 color was found for this object , indicating the underlying variability was `` grey '' over this wavelength regime . a larger sample of such well studied variable objects is needed to test if this is truly a generic property of blazars in the nir . characterizing photometric variability timescales has proven to be a very efficient means of selecting a clean sample of agn from time domain surveys ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? the so - called `` damped random walk '' ( drw ) model provides a robust fit to agn light curves as a function of two observable free parameters : the characteristic timescale @xmath15 , and the variability amplitude sf@xmath16 . stars are easily removed when fit with this model , as their primary sources of variability are typically stochastic ( e.g. flares or flickering ) or strongly periodic ( such as pulsations or rotation ) in nature . combining drw variability models with multi - wavelength photometry provides even greater power in separating blazars from `` normal '' agn @xcite .    from our cal - pswdb wise matched sample we selected all light curves having at least 50 good @xmath7-band epochs ( ph_qual = a , b or c ) , median @xmath7-band magnitudes brighter than 16.5 mag , and wise colors of @xmath17 to remove main sequence stars @xcite . we also required sources to be detected as variable using the @xmath18 cut described above , and removed objects known to be periodic ( binaries and pulsating variables ) or long period variable stars ( * ? ? ? * davenport 2014 in prep ) . this yielded 22 variable cal - pswdb targets to search for drw timescales , which included our known blazar object , but did not select any other previously identified agn . for completeness we also analyzed the other 26 known `` milliquas '' sources for drw variability . for each object we down - sampled the light curves to a single data point per night , using the median of the @xmath7-band photometry , which reduced the light curves to an average of 144 epochs . we then ran the drw fitting code from @xcite and @xcite on the reduced @xmath7-band light curves for all 47 objects . we used the probability thresholds outlined in @xcite to determine which light curves had significant drw variability . the known blazar had a weakly constrained drw timescale of @xmath19 days , and a significant driving amplitude of @xmath20 mag year@xmath21 . the relative likelihood for the drw versus an infinite or unconstrained timescale was log(l@xmath22/l@xmath23 ) = 1.4 , where values greater than  0.05 are considered significant . similarly , the relative likelihood for the drw model versus pure noise was log(l@xmath22/l@xmath24 ) = 153 , which was much greater than the threshold for significance of 0 . none of the other 26 previously identified agn showed any signs of significant drw variability . however , 7 of the variability - selected objects did show significant drw variability . we discuss the origin of these objects in the following section . we additionally fit these light curves using the continuous auto - regressive moving average , or carma(p , q ) , models of @xcite , where @xmath25 is the auto - regressive order and @xmath26 the moving average order . the drw is a special case of the carma models , equivalent to a carma(1,0 ) , or a first - order continuous auto - regressive process with no moving average . in the carma formalism , a drw model s power spectral density is described as a single lorentzian function with a characteristic break frequency . as mentioned above , the drw has been shown to empirically model the behavior of qso light curves well , but is otherwise inflexible . by looking at higher order carma models we search for more complex behavior in the light curve power spectral density , such as stochastic variability on multiple timescales or quasi periodic oscillations . for sdssj14584479 + 3720215 we found a moderate preference for the carma(2,0 ) model compared to a drw when using the deviance information criterion for model selection . the drw model provides a sufficient fit to the data , in that the sequence of residuals is independently and normally distributed . a markov chain monte carlo analysis of the carma(1,0 ) model provided 1-@xmath3 confidence levels on the timescale of @xmath27 days , and on the standard deviation of the driving noise of between 0.14 and 0.20 magnitudes . the recovered drw timescales for the seven variability - selected objects we found in the previous section had an average timescale of @xmath28 days , significantly shorter than the typical timescale seen in the optical for agn @xcite , as well as for our blazar target . this was reproduced in the carma(1,0 ) models as well . these seven objects all reside in field # 90009 , which was centered on the star forming region @xmath1 ophiuchus , making these likely ysos . of these ysos , five had x - ray detections in the literature , one coming from a targeted x - ray monitoring campaign of the field @xcite , and four from the chandra x - ray source catalog @xcite . the other two ysos in our sample did not have a published x - ray counterpart within 0.1@xmath29 . nir light curve properties for ysos within this field in the cal - pswdb have been studied in great detail by @xcite and @xcite . these seven ysos had mid - ir colors that placed them within the blazar strip , nir variability amplitudes comparable to our known blazar , and light curves best parameterized by a carma(1,0 ) model . we thus highlight ysos as a potentially important source of contamination for classifying agn from future multi - wavelength time domain searches . while using longer wavelength filters can improve the efficiency of selecting agn from normal stars ( e.g. * ? ? ? * ) , ysos remain a potential contaminant for object classification ( see also color spaces in fig 7 of * ? ? ? . however , these objects may be distinguished from bona fide agn based on their @xmath410x shorter characteristic timescales under a carma(1,0 ) model . ysos are also mostly confined to the galactic disk and dense star forming regions , and as such can be avoided by surveys . the _ fermi gamma - ray space telescope _ provides the deepest survey to date in the 100 mev to 100 gev regime , and has discovered 886 @xmath2-ray agn in the 2nd _ fermi _ agn catalog @xcite . the vast majority of these agn are blazars . however , @xmath430% of _ fermi _ sources in the _ fermi _ 2nd point source catalog ( 2fgl ; * ? ? ? * ) are unidentified , largely due to the poor angular resolution of _ fermi _ s pair - conversion large area telescope ( lat ; * ? ? all reliably - associated agn in the 2nd _ fermi _ agn catalog are confidently detected by _ fermi _ with test statistic @xmath30 , corresponding to approximately @xmath31 detections . for a _ fermi _ counts map of a region of interest ( roi ) centered on a new possible source , the ts detection significance is the defined to be the ratio of likelihoods ts = 2(log@xmath32(source ) - log@xmath32(nosource ) ) , where @xmath32(source ) is the likelihood of a model of all known sources in the roi and a new source at the central location , while @xmath32(nosource ) is the likelihood for the same model but no new source at the central location @xcite . the detection significance is approximately @xmath33 . thousands of potential faint @xmath2-ray blazars lie below the @xmath30 ( @xmath31 ) detection threshold , but are difficult to localize due to extremely low photon counts . these faint @xmath2-ray blazars have large positional uncertainties , with error ellipses with radius @xmath3420  for 3@xmath3 fermi detections using 4 years of photon data @xcite . the strong @xmath0 variability exhibited by the blazar sdssj14584479 + 3720215 relative to the quasars in our sample suggests that this nir variability is likely to be indicative of strongly beamed jets , which are characteristic of blazars , and are known to produce strong @xmath2-ray emission . this allows for nir - variability based identification of previously unidentified or undetected @xmath2-ray blazars . sdssj14584479 + 3720215 was not included in the _ fermi _ 2fgl catalog , and thus had @xmath35 is 2 years of photon data . however , motivated by the strong nir variability we observe , we investigated whether this blazar can be detected at high significance using more photon data . we downloaded approximately 4.5 years of available pass 7 _ fermi _ photon data of the source event class , spanning mission elapsed time 239557417 to 383570220 seconds . we selected photons in the 200 mev to 100 gev energy range , within a 10@xmath29 region of interest ( roi ) centered on the blazar .    using the standard binned likelihood analysis scripts provided in the fermi science tools package , we performed photon event selection with a roi - based zenith angle cut of @xmath36100@xmath29 and a rock angle cut of @xmath3652@xmath29 . we produced a spatial model of the likely @xmath2-ray sources within 15@xmath29 of the blazar based on the 2fgl catalog , using the 2fgl best fit values for the template spectral model of these sources . we included the blazar sdssj14584479 + 3720215 as an additional point source , modeled as a simple power - law spectrum . the spectral normalization for all sources within 15@xmath29 was allowed to vary , and all spectral parameters were set as free parameters within the roi . we included the appropriate 2-year pass 7 galactic diffuse emission and extragalactic isotropic diffuse emission models in our source model fit to the observed counts map . the blazar sdssj14584479 + 3720215 is detected with @xmath37 , approximately a 5@xmath3 detection , with a @xmath2-ray power - law spectral slope of @xmath38 , typical of _ fermi _ blazars @xcite . this detection significance is consistent with the @xmath39 found in a preliminary version of the _ fermi _ 4-year point source catalog by @xcite , produced using 4 years of photon data . we note minor discrepancies in our modeling of the brightest object in the field , noticeable in the lower - left corner of the difference image . these are well - known issues in _ fermi _ source analysis caused by the finite resolution of the counts maps , and do not affect our detection . we have conducted a search for nir variability from agn using the 2mass c al - pswdb light curves . no significant variability was found for 26 of the 27 known agn in our sample . one previously known agn , showed strong variations in the 2mass filters , and we have presented the 1972 epoch nir photometric light curve for this blazar ( sdssj14584479 + 3720215 ) . this previously known blazar is a benchmark object , displaying almost 1 magnitude of variability in the @xmath0 bands , but with no significant variation in @xmath12 color . this is the best sampled nir light curve of a blazar ever measured . we note a handful of other agn candidates from the 2mass cal - pswdb were recently identified by @xcite , including the blazar sdssj14584479 + 3720215 .    our detection of sdssj14584479 + 3720215 as a @xmath2-ray source , selected by infrared variability , has implications on the association and classification of multi - wavelength counterparts of _ fermi _ sources . current selection methods such as mid - ir color @xcite and optical variability @xcite suffer from low efficiency . this is often due to `` normal '' type 1 quasars , which have similar colors in the mid - ir and are also variable in the optical . as this pilot study demonstrates , these issues may be surmounted by the inclusion of ir variability information , since normal agn are not significantly variable at this wavelength regime . a search for other such light curves in the 2mass cal - pswdb reveals a large number of contaminating ysos . this study establishes a baseline for the agn variability science possible with current and future multi - epoch ir missions such as the wise , the wide field infrared survey telescope ( wfirst ; * ? ? ? * ) , and vista variables in the va lctea ( vvv ; * ? ? ? * ) surveys . we thank p. plavchan and j. parks for helpful discussions of yso variability and sharing an early version of their manuscript . the authors acknowledge support from nasa adp grant nnx09ac77 g and nasa nnx14ak26 g . jjr is supported by nasa fermi grant nnx14aq23 g . this publication makes use of data products from the two micron all sky survey , which is a joint project of the university of massachusetts and the infrared processing and analysis center / california institute of technology , funded by the national aeronautics and space administration and the national science foundation . this research has made use of the nasa / ipac extragalactic database ( ned ) which is operated by the jet propulsion laboratory , california institute of technology , under contract with the national aeronautics and space administration . this publication makes use of data products from the wide - field infrared survey explorer , which is a joint project of the university of california , los angeles , and the jet propulsion laboratory / california institute of technology , funded by the national aeronautics and space administration . lcccccc j@xmath40 & 1063 & 16.86 & 1.45 & 14.91 & 1.33 & 3.55 + j@xmath41 & 30 & 16.96 & 1.43 & 15.80 & 1.41 & 3.05 + j@xmath42 & 339 & 17.00 & 1.51 & 15.29 & 0.42 & 2.71 + j@xmath43 & 86 & 17.03 & 1.52 & 15.61 & 1.27 & 3.38 + j@xmath44 & 11 & 17.08 & 1.18 & 15.39 & 1.48 & 2.39 + j@xmath45 & 254 & 16.99 & 1.48 &  &  &  + j@xmath46 & 2580 & 16.72 & 1.75 & 13.58 & 0.97 & 2.85 + j@xmath47 & 68 & 17.08 & 1.49 & 15.22 & 1.49 & 3.27 + j@xmath48 & 778 & 16.23 & 1.56 & 13.39 & 0.87 & 2.58 + j@xmath49 & 1873 & 16.68 & 1.41 & 14.89 & 0.51 & 2.91 + j@xmath50 & 9 & 17.01 &  & 15.20 & 1.32 & 3.20 + j@xmath51 & 19 & 17.09 & 1.76 & 15.78 & 1.43 & 3.44 + j@xmath52 & 49 & 17.02 & 1.41 & 15.77 & 0.59 & 3.18 + j@xmath53 & 2737 & 16.29 & 1.10 & 13.38 & 1.24 & 2.85 + j@xmath54 & 18 & 17.10 &  & 16.50 & 1.20 & 3.84 + j@xmath55 & 1212 & 16.91 & 1.56 & 14.63 & 0.20 & 2.64 + j@xmath56 & 916 & 16.40 & 1.13 & 13.80 & 1.59 & 2.97 + j@xmath57 & 1568 & 16.79 & 1.64 & 14.16 & 1.18 & 3.09 + j@xmath58 & 1500 & 16.50 & 1.48 & 13.88 & 0.74 & 2.92 + j@xmath59 & 58 & 17.06 & 1.38 & 15.32 & 0.98 & 3.56 + j@xmath60 & 1927 & 16.33 & 1.77 & 14.03 & 1.10 & 2.85 + j@xmath61 & 1191 & 15.51 & 1.73 & 12.46 & 1.07 & 2.52 + j@xmath62 & 21 & 16.99 & 1.49 &  &  & + j@xmath63 & 888 & 16.75 & 1.32 & 14.34 & 1.45 & 2.90 + j@xmath64 & 15 & 16.88 &  & 15.22 & 1.20 & 2.36 + j@xmath65 & 7 & 16.99 & 1.46 & 15.06 & 1.39 & 2.80 + j@xmath66 & 358 & 16.95 & 1.52 & 14.62 & 0.86 & 2.41 [ agntable ]                                                                                                  , d.  j. , ballet , j. , burnett , t. , & fermi large area telescope collaboration . 2014 , in american astronomical society meeting abstracts , vol . 223 , american astronomical society meeting abstracts 223 , 256.01
active galactic nuclei ( agns ) are well - known to exhibit flux variability across a wide range of wavelength regimes , but the precise origin of the variability at different wavelengths remains unclear . to investigate the relatively unexplored near - ir variability of the most luminous agns , we conduct a search for variability using well sampled @xmath0-band light curves from the 2mass survey calibration fields . 
 our sample includes 27 known quasars with an average of 924 epochs of observation over three years , as well as one spectroscopically confirmed blazar ( sdssj14584479 + 3720215 ) with 1972 epochs of data . 
 this is the best - sampled nir photometric blazar light curve to date , and it exhibits correlated , stochastic variability that we characterize with continuous auto - regressive moving average ( carma ) models . 
 none of the other 26 known quasars had detectable variability in the 2mass bands above the photometric uncertainty . 
 a blind search of the 2mass calibration field light curves for agn candidates based on fitting carma(1,0 ) models ( damped - random walk ) uncovered only 7 candidates . 
 all 7 were young stellar objects within the @xmath1 ophiuchus star forming region , five with previous x - ray detections 
 . a significant @xmath2-ray detection ( 5@xmath3 ) for the known blazar using 4.5 years of fermi photon data is also found . 
 we suggest that strong nir variability of blazars , such as seen for sdssj14584479 + 3720215 , can be used as an efficient method of identifying previously - unidentified @xmath2-ray blazars , with low contamination from other agn .