Intermediate Polars Discovered inthe Palomar Transient FactoryBruce H. Margon1,2, David Levitan3, Thomas A. Prince3, and Gregg Halinan3 on behalf of the PTF Collaboration4

1. Department of Astronomy and Astrophysics, University of California,1156 High Street, Santa Cruz, CA 95064, USA

2. E­mail: margon@ucsc.edu3. Division of Physics, Mathematics, and Astronomy,

California Institute of Technology, Pasadena, CA 91125.4. http://www.astro.caltech.edu/ptf/5. Law, N. M., Kulkarni, S. R., Dekany, R. G. et al., 2009, PASP, 121, 13956. Rau, A., Kulkarni, S. R., Law, N. M. et al, 2009, PASP, 121, 13347. Mukai, K. 2011,

http://lheawww.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html8. Silber, A. 1992, PhD thesis, MIT

Figure 1. PTF photometry on six previously anonymousobjects, advanced here as new Intermediate Polars. Theextreme variability and its timescales made theseobjects natural candidates for spectroscopic followup.

Figure 3. Folded PTF data for those systems thatexhibit high confidence periodic modulation. Thebest­fit periods are indicated for each system.

IntroductionUsing the Palomar Transient Factory (PTF) synopticsurvey, we have discovered via large amplitude opticalvariability, followed by spectroscopy, six previouslyunreported Intermediate Polars (IPs). As most IPspreviously have been X­ray selected, the availability ofwide­area synoptic surveys such as the PTF marks thestart of a systematic technique to select IPs optically.Despite extremely strong HeII optical emission in allthe spectra, indicating the presence of very highexcitation gas, only two of the six are reported X­raysources. Thus the possibility of significant (X­ray)selection bias in previous samples is clear. Weanticipate that the PTF and related future surveys willinvigorate the study of IPs by providing a large,homogenously selected sample for further study.

Intermediate PolarsIntermediate Polars (hereafter "IPs") are close reddwarf/white dwarf binary systems that can provideunique insights on the late stage evolution of massexchange binaries with compact companions. Thesesystems typically have orbital periods of 1.5­6h, withthe majority clustered in the 4­5h range, and showsporadic optical outbursts typical of other subclassesof cataclysmic variables (CVs). As the name implies,the white dwarf, while strongly magnetic, with fieldstrengths of a few through tens of MG, lacks theintense fields of polars ("AM Her stars"). As in the non­magnetic CVs, there is strong accretion flow and largeamplitude photometric outbursts, but the magneticfield, while not intense enough to synchronize the spinand orbital periods of the system as is the case inpolars, dramatically disrupts and controls theaccretion flow. IPs therefore can be laboratories forthe detailed study of accretion into deep potentialwells in the presence of strong fields, a scenario whichcomes up again and again in a variety of astrophysicalobjects. Further, IPs typically exhibit a range ofunique observational signatures which allow thesystems to be characterized in great detail. Often boththe visible and X­ray light curves will show the binaryorbital period, the white dwarf spin period, eclipses, aswell as moderate and extreme outbursts, both roughlyperiodic and aperiodic, and variable opticalpolarization. There are currently several dozensystems suggested to be IPs, although not all thesystems simultaneously exhibit all of the observationaldiagnostics noted above, so the list of "highconfidence" IPs is considerably smaller.

Palomar Transient FactoryThe majority of IPs have thus far beendiscovered via their strong X­ray emission,although there are certainly a few prominentoptically­discovered exceptions, for example theold novae DQ Her and GK Per. However,recently the Palomar Transient Factory (PTF)[5,6] has provided the capability to opticallydiscover and characterize a very large number ofhighly variable objects. The PTF uses theOschin 48 inch Schmidt telescope at thePalomar Observatory to image 7.2 deg2 in eachexposure to a depth of R~20.6, with a typicalcadence of 1­5 days. The PTF is particularlysuited for the discovery of IPs due to its widesky coverage (~16,000 deg2), excellentphotometric calibration, and broad magnituderange, which covers both the brightest andfaintest currently known IPs. Subsets of variableobjects are then followed up spectroscopically todetermine their nature.

Figure 2. Spectra obtained at the Keck, Palomar5m and Lick 3m telescopes of six newlydiscovered Intermediate Polars, selected via largeamplitude photometric variability in the PTF. Allexhibit intense Balmer and HeII emission. Notealso the extreme strength of HeII λ4686 relative toHβ and Hα, a signature of Intermediate Polars.

New IP CandidatesIn this followup process we have recentlydiscovered a group of highly variable PTF objectswhich typically vary in the R~17­21 range andprove to have the distinctive spectra of IPs. InFigure 1 we show the PTF photometry of the sixcandidates discussed here; this behavior led totheir selection for spectroscopic followup andclassification.In Figure 2 we show the spectra of 6 of the sixblue objects. The spectra are dominated byintense Balmer and HeII emission lines, and,most remarkably and unusually, HeII λ4686equals or exceeds the strength of Hβ and Hα.This extraordinary condition is one ratherunambiguous spectral diagnostic of IPs [7,8].Further, for 5 of the 6 objects we have alsodiscovered photometric periods, using data fromPTF and other synoptic surveys, and also newlyobtained observations from the Lick Shanetelescope (Figures 3 and 4). In 4 of these 5 cases,our measured periods are in the 1.4­3.9h rangeand are undoubtedly orbital; the 5th case showsa 52m period, which is probably the spin period.Without the detection of polarization, and/orobservation of both orbital and white dwarf spinperiods simultaneously in the same object, theclassification as IPs must remain tentative; one ormore of these objects could prove to be full blownpolars. We do have very recent polarimetricobservations on hand, which when reduced mayaddress this issue.

X­Ray Emission??The lack of appearance of most of these PTFoptically­selected IPs in previous X­ray surveys isinteresting. We believe that two of the six stars,PTF 1217bb and PTF 1223g, are the previouslyunrecognized optical counterparts of otherwiseunremarkable X­ray sources in the ROSAT AllSky Survey and the XMM­Newton Slew Survey,respectively, as the positional coincidences areexcellent, but the remaining 4 objects are to ourknowledge undetected by any X­ray mission. Thiscannot simply be the result of distance, as all sixPTF objects are of comparable median opticalmagnitudes, nor interstellar photoelectricabsorption, as all the objects are at high galacticlatitude, and the X­ray spectra of IPs tend to behard in any case. If only one or two objects wereundetected, we could attribute this to chance, orthe source temporarily lying in an extreme lowstate. However, with 4 out of the 6 systemscurrently not known to be X­ray emitters, and thesample gathered from the first homogenous set ofoptically rather than X­ray selected sources,there is an obvious suspicion that the propertiesof optically and X­ray selected IPs might beintrinsically different.

Figure 4. Photometry of PTF 1221e, one of thenewly discovered IPs, obtained with the Lick 3mShane telescope in July 2012. Note the very deep(~5 mag) eclipse; there are only a handful of fullyeclipsing IPs known. From many observed orbitalcycles, we derive a period of 3.91682h.