Search for TeV γ -ray emission from AE Aqr coincident with high optical and X-ray states with the MAGIC telescopes
R. Lopez-Coto, O. Blanch Bigas, J. Cortina, D. Hadasch, L. Takalo, D. Torres, M.Bogosavljevic, Z.Ioannou, C.W.Mauche, E.V. Palaiologou, M.A. Perez-Torres, T. Tuominen
333 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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Search for TeV γ -ray emission from AE Aqr coincident with high optical and X-ray states with the MAGIC telescopes R. L ´
OPEZ -C OTO ∗ , O. B LANCH B IGAS , J. C ORTINA , D. H ADASCH , L. T AKALO , D. T ORRES , ON BEHALF OFTHE
MAGIC
COLLABORATION AND
M. B
OGOSAVLJEVIC , Z. I OANNOU , C.W. M AUCHE , E.V. P ALAIOLOGOU ,M.A. P ´ EREZ -T ORRES , T. T UOMINEN IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain Institut de Ci`encies de l’Espai (IEEC-CSIC), E-08193 Bellaterra, Spain Tuorla Observatory, University of Turku, FI-21500 Piikki¨o, Finland Astronomical Observatory Belgrade, 11060, Belgrade, Serbia Physics Department, College of Science, Sultan Qaboos University, P.O. Box 36, PC-123, Muscat, Oman Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 95125, USA Physics Department, University of Crete, P.O. Box 2208, GR-71003, Heraklion, Greece Instituto de Astrof´ısica de Andaluc´ıa (CSIC), E-18080 Granada, Spain *[email protected]
Abstract:
We report on observations of the nova-like cataclysmic variable AE Aqr performed by MAGIC. Theobservations were part of a quasi-simultaneous multi-wavelength campaign carried out between 2012 May andJune covering the optical, UV, X-ray and gamma-ray ranges. MAGIC conducted the campaign and observed thesource during 12 hours. The other instruments involved were KVA, Skinakas, and Vidojevica in the optic and Swiftin the X-ray. We also used optical data from the AAVSO. The goals were to: monitor the variability of the sourceat different wavelengths, perform gamma-ray studies coincident with the highest states of the source at the otherwavelengths, and confirm or rule out previous claims of detection of very-high-energy emission from this object.We report on a search for steady TeV emission during the whole observation, for variable TeV emission coincidentwith the highest optical and X-ray states and periodic TeV emission at the 33.08 s rotation period (30.23 mHzrotation frequency) of the white dwarf and its first harmonic (60.46 mHz rotation frequency). These are the firstobservations under good weather conditions performed by the present generation of IACTs for this object.
Keywords:
Cataclysmic variable stars, AE Aqr, gamma-rays.
Cataclysmic Variable stars (CVs) are binary systems com-prising a white dwarf (WD) and another companion (usu-ally a red dwarf) that transfers matter to the white dwar-f. They are classified depending on the type of variationthey manifest as novae, nova-like variables, dwarf novae,and magnetic CVs. They have outbursts observed at dif-ferent wavelength, from radio to X-rays. TeV γ -ray emis-sion from AE Aqr has also been reported by two differentgroups [3, 11] using the imaging atmospheric Cherenkovtechnique, but these detections could not be reproduced bythe following generations of instruments using the sametechnique [8].AE Aqr is a bright nova-like cataclysmic variable starconsisting of a magnetic white dwarf and a K4-5 V sec-ondary. The orbital period of the system is T o =9.88 hoursand the spin period of the white dwarf is T s =33.08 s [12],the shortest known. The system is located at a distanceof ∼
100 pc. It was originally classified as a DQ Her star[13], but it shows features that are not explained by thismodel, such as its violent activity at different wavelengths;the single-peaked Balmer emission lines, which produceDoppler tomograms that are not consistent with those of anaccretion disk, and its fast spin-down rate. Its spin downrate was firstly measured to be ˙P=5.64 × − s s − [6],although more recent X-ray measurements show that thespin-down rate is even larger [9]. It exhibits flares 50 % ofthe time, going in the visible regime from mag=12.5 (dur- ing quiescence) to mag=10 (during flares). These flares areemitted randomly all the time. Radio flares have been ob-served from this source, on timescales similar to the opti-cal flares. In [2] it is shown that the radio flares can be ex-plained by relativistic electrons, which provides evidencesof accelerated particles, radiating synchrotron emission inmagnetized clouds. Hard X-rays have also been detectedcoming from the target with a 33 s-modulation [9]. Due tothis fact and the fast rotating period of the primary, there areWDs with similar behaviour as pulsars and they have beenproposed to be contributors of low energy cosmic rays [14].It also has to be mentioned that the source was observedin 2005 in the context of a multiwavelength campaign byMAGIC and HESS, but due to bad weather conditions thedata are not of good quality [10].Moreover, after the discovery of γ -ray emission fromthe symbiotic nova V407 Cygni [1], and the later reporton emission from two additional novae [5], CVs have beenincluded amongst high energy emitters. Due to the rapidrotation of its white dwarf, AE Aqr is one of the bestcandidates amongst the population of CVs to be observedby ground-based Cherenkov telescopes. During the period between 2012 May 15 and June 19, amulti-wavelength campaign triggered by the MAGIC tele-scopes was carried out to observe AE Aqr. The purpose of a r X i v : . [ a s t r o - ph . H E ] S e p E Aqr observations with MAGIC33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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Phi_orb0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 M J D - MAGIC observationsKVA observationsXRT observationsSkinakas observationsVidojevica observations
Fig. 1 : Summary of the multiwavelength campaign. Theinstruments used, the Modified Julian Date and the orbitalphases are plotted.this campaign was to look for gamma-ray emission duringthe different states of the source at several wavelengths. InFigure 1 you can see a summary of the observations carriedout during the campaign.
MAGIC is an Imaging Atmospheric Cherenkov telescopesituated on the island of La Palma (28.8 ◦ N, 17.9 ◦ W at2225 m.a.s.l). It is a stereoscopic system that can achieve asensitivity of (0.76 ± ◦ and 50 ◦ . Swift [7] target-of-opportunity observations of AE Aqr wereobtained during 25 orbits during 2012 May 15-June 2 andJune 13-18. Data was obtained with the X-ray Telescope(XRT)[4], Ultraviolet/Optical Telescope (UVOT), and BurstAlert Telescope (BAT), although only the XRT data havebeen analyzed in support of the MAGIC observations. TheXRT nominal exposures ranged from 37 s to 1553 s, with ∼
950 s being typical. Excluding the shortest exposure and3 exposures when the source was placed on one of the XRTdetector dead stripes, there are 21 useful exposures for atotal of 19.94 ks on-source.
Three optical telescopes participated in addition to theAmerican Association of Variable Star Observers (AAVSO)in the campaign.
KVA telescopes
The KVA optical telescopes are located on La Palma, butare operated remotely from Finland. The two telescopes areattached to the same fork. The larger telescope has a mirrordiameter of 60 cm and the smaller 35 cm. The 35 cm tele- scope was used for simultaneous photometric observationswith MAGIC. The AE Aqr observations were performedin the B-band using 20 second exposures extending to ∼ Skinakas
The data from the Skinakas Observatory in Crete wereobtained with the 1.3-m Ritchey-Chr´etien telescope locatedon the Skinakas mountain at an altitude of 1750 meters. Thetelescope has a focal ratio of f/7.6. The data were acquiredwith an Andor Tech DZ436 2048x2048 water cooled CCD.The physical pixel size is 13.5 microns resulting in 0.28arcsec on the sky. The camera mode used was the 2 µ sper pixel readout mode. At this mode the camera exhibitsa readout noise of 8.14 electrons and its gain is 2.69electrons/ADU.The observations were taken using a Bessel B filter using10 second exposures, while the cycle time from the start ofone exposure to the next was 14 seconds. Due to variableweather conditions, as well as the relatively low altitudeof AE Aqr, the atmospheric seeing during the observationsvaried between 1-3 arcsec.The data from Skinakas were taken during ∼ Vidojevica
The Astronomical Station Vidojevica data have been ob-tained with the 60-cm Cassegrain telescope. The telescopeis used in the f/10 configuration with the Apogee Alta U42CCD camera (2048 x 2048 array, 13.5 micron pixels provid-ing a 0.46 arcsec/pix plate scale). The B filter from OptecInc. (Stock No. 17446) was used for all observations. Thefield centered on the target AE Aqr was observed contin-uously with 10 seconds of exposure time. Only a fractionof the full CCD chip field of view, roughly 5 arcmin on aside, was readout incurring for approximately 4 secondsof readout time, and 14 seconds total cycle time betweenexposures.The data were taken for periods between ∼ E Aqr observations with MAGIC33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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65 70 75 80 85 90 95 100 B M agn i t ude KVA dataSkinakas dataVidojevica data X R T [ c oun t s / s ] XRT data ] - s - T e V F l u x [ c m -12 -11 -10 Upper limit above 200GeVUpper limit above 1TeV
MJD-56000
Fig. 2 : Light curves of the multiwavelength campaign. Theplot includes MAGIC daily upper limits considering apower-law spectrum with a -2.6 slope (top), XRT flux (medi-um) and B magnitude measured by the optical telescopes(bottom). For the optical data, as the source variability isvery large, the error bars indicate the maximum and mini-mum state reached during that observation night. The shad-owed areas indicate the observations simultaneous withMAGIC.
AAVSO
A number of AAVSO observers provided us with additionalobservations. However, due to the low time resolution ofthese observations we did not include the AAVSO data intoour timing an AAVSO data into our timing analysis
The results of the campaign are shown in this section. Thelight curves of the multi-wavelength campaign can be seenin Fig. 2.
MAGIC data have been analyzed in different ways in orderto look for a steady signal or a periodic one. A summary ofthe MAGIC observations can be found in Table 1. Most ofthose observations were simultaneous to the optical and X-ray ones. Therefore, a study of the correlation of optical/X-ray flux with the possible γ -ray emission has been done. Steady emission
First of all, we stacked all the data together and look for a γ -ray significant signal. Since these observations took place atzenith angles > ◦ , the energy threshold of the telescope Date MAGIC observationtime window19/05/12 03:12 - 04:5126/05/12 02:47 - 03:3327/05/12 02:40 - 03:2528/05/12 02:40 - 03:2329/05/12 02:34 - 03:1730/05/12 02:28 - 03:1431/05/12 02:15 - 03:0901/06/12 03:37 - 04:5702/06/12 03:42 - 04:5414/06/12 01:32 - 02:3415/06/12 01:31 - 02:1016/06/12 01:23 - 02:0617/06/12 01:18 - 01:5818/06/12 01:13 - 02:0419/06/12 01:22 - 01:55 Table 1 : Observation time for every night (all the timesare given in UT) for the MAGIC observations of AE Aqrperformed during the 2012 multi-wavelength campaign.was 250 GeV. As the shape of the spectral distribution isnot known, we have considered power-laws with differentspectral indexes, as well as different energy thresholds inthe upper limits calculation. We did not find a significantsignal in any energy bin. The upper limits for the steadyemission of the source considering power-law spectra withdifferent slopes can be found in Table 2. We also computeddaily integral upper limits for the steady emission of thesource considering a power-law with a -2.6 slope (Crab-like) spectrum. Those upper limits are shown in Fig. 2.MAGIC Integral upper limitsSlope [ cm − s − ] above 250 GeV above 1TeV-2 4.165e-12 7.574e-13-2.6 6.391e-12 7.401e-13-3 8.011e-12 7.377e-13 Table 2 : MAGIC upper limits considering a power-lawspectrum with different slopes
Emission coincident with several optical states
According to the purpose of the multi-wavelength campaign,we have studied the behavior of the source coincident withdifferent optical states. We selected the events coincidentwith B magnitude <
12 and B magnitude < Pulsed emission
A search for pulsed emission at the spin frequency of thewhite dwarf and its first harmonic has also been performed.We used the ephemeris for the source presented in [9]and limit the signal region to the bin corresponding to themaximum of the XRT spin-phase-folded light curve (seeFig. 4). We did not find any hint of periodic signal for either
E Aqr observations with MAGIC33 RD I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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B Magnitude MAGIC Upper limits [ cm − s − ] above 250 GeV above 1TeVBrighter than 11.5 2.051e-11 1.624e-12Brighter than 12 7.299e-12 1.189e-12 Table 3 : MAGIC upper limits for different optical statesfrequencies.
Phase0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 E v en t s AE_Aqr
Entries 2129) s = 13.61 Prob = 8.50e-01 (0.2 Z ) s H Test = 0.55 Prob = 8.03e-01 (0.2 ) s /ndf = 1.77/9 Prob = 9.95e-01 (0.0 c s Nex = -13.33+-16.71 SigLiMa = 0.8
Fig. 3 : Phaseogram for the MAGIC data above 250 GeVfor a frequency of 30.23mHz
The XRT event data was used to produce the X-ray lightcurve shown in Fig. 2 and the spin-phase-folded light curveshown in Fig. 4.
Fig. 4 : XRT spin-phase-folded light curveThe spin-phase-folded light curve is fit with a cosine func-tion A( φ spin )=A +A cos[2 π ( φ spin - φ o f f )] with constants:A = . ± . = . ± . φ o f f = . ± . χ /dof=4.94/7=0.71. Hence, the relative pulse am-plitude A /A =16.8% ± φ o f f is not equal to zero, the [9] cubicephemeris is not exact and these and other data should beused to update and refine it. In the optical part we have plotted the results of all the ob-servations together in order to check the consistency be-tween the magnitudes measured by the different telescopes.The simultaneity of the observations let us determine thestate of the source at TeV energies at different optical e-mission levels. The highest optical state was achieved thenight of the 2nd of June, reaching B mag =11.08. The spin-phase-folded light curve could not be produced due to thelow time resolution of the observations.
We have carried out the most sensitive observations by anIACT of a cataclysmic variable star. The very-high-energyobservations go together with optical and X-ray observa-tions that help to characterize the behavior of the sourceat different states. With the X-ray data we will be able toupdate the ephemeris of the source. We have searched forsteady γ -ray emission during the whole observation peri-od, coincident with different optical states and pulsed γ -rayemission as well. We do not find any significant emissionfrom AE Aqr in any of the searches performed. Acknowledgment:
We would like to thank the Instituto deAstrof´ısica de Canarias for the excellent working conditions atthe Observatorio del Roque de los Muchachos in La Palma. Thesupport of the German BMBF and MPG, the Italian INFN, theSwiss National Fund SNF, and the Spanish MINECO is grateful-ly acknowledged. This work was also supported by the CPANCSD2007-00042 and MultiDark CSD2009-00064 projects of theSpanish Consolider-Ingenio 2010 programme, by grant 127740of the Academy of Finland, by the DFG Cluster of Excellence“Origin and Structure of the Universe”, by the DFG CollaborativeResearch Centers SFB823/C4 and SFB876/C3, and by the PolishMNiSzW grant 745/N-HESS-MAGIC/2010/0. CWM’s contribu-tion to this work was performed under the auspices of the U.S.Department of Energy by Lawrence Livermore National Labora-tory under Contract DE-AC52-07NA27344. MB acknowledgessupport of Serbian MESTD through grant ON176021. The au-thors thank N. Gehrels for approving our request for target-of-opportunity observations and the Swift Science Operations Teamfor scheduling them. We would also like to thank the AmericanAssociation of Variable Star Observers for their help during thecampaign.