arXiv:0907.0831v1 [astro-ph.HE] 5 Jul 2009 oeu o36Ca nt ( flux units integral Crab the 3.6 nights to some up In rose period. during whole activity prolonged the and intense The showed telescope. gamma-ray source MAGIC the by band GeV) nXryadVEbnsaentrlyepandby explained naturally are bands VHE and variabilit X-ray between correlations in instance, observa- For multi-band [21]. in parame- measured tions observational as they sources, of well these evolution of how ters the on for test relies important account models most can emission The of [1]. minutes validity ener- few of GeV-TeV of at be which can gies timescales, different on evidence. band, strong with discarded be none [15]) still can (e.g. but these models, leptonic of different and by invoked [17]) are which [14], processes for bump, (e.g. energy More hadronic high jet. the both of the nature emis- within the synchrotron is electrons debated relativistic as the regarded from is sion bump energy low the twsitnieyosre nteVE( VHE the in observed 2008 intensively the June was until of 2007 it sources December gamma-ray From TeV sky. brightest Northern the of one netgtn h orltosbtendfeeten- different bands. between ergy data, correlations VHE MAGIC the the investigating with data Swift-XRT) ASM, observed. were variations flux rapid night hra opnns htpouete”double-bumped” non- the νF broad, produce by that the components, dominated wavebands, thermal clearly all is In jet sight. spectrum of relativistic radiation line with the AGN, to aligned ellipti- radio-loud closely giant a detected hosts model optically galaxy the AGN cal that unified means accepted this widely [23], the to studied. According best the first of the one and therefore [19] sources; detected TeV extragalactic brightest RCEIG FTE31 THE OF PROCEEDINGS icm Bonnoli Giacomo Moreover Abstract k 2,i n ftecoet( closest the of one is 421, Mkn Keywords (RXTE- X-ray and (KVA) optical the compared We tblnst h High-peaked the to belongs It ν § rfiewihi hrceitcof characteristic is which profile uraOsraoy eateto hsc dAtooy Un Astronomy, ad Physics of Department Observatory, Tuorla osacaSatalecka Konstancja AI bevtoso k 2 n20,adrelated and 2008, in 421 Mkn of observations MAGIC h B-yebaa akra 2 is 421 Markarian blazar HBL-type The . ∗ AI k41multiwavelenght Mkn421 MAGIC : iatmnod iia Universit Fisica, di Dipartimento blazars k NF sevtroAtooiod rr,vaE inh 46, Bianchi E. via Brera, di Astronomico Osservatorio INAF, † .I I. a-lnkIsiu u hsk F Physik, fur Max-Planck-Institut hwitnevraiiyi every in variability intense show ∗ NTRODUCTION hn hn Hsu Cheng Ching , st CC Ł ICRC, > E pia/-a/e W study MWL optical/X-ray/TeV ¶ ¶ noi Stamerra Antonio , EY ltnnle ,178Zuhn Germany Zeuthen, 15738 6, Platanenallee DESY, blazar z 200 OD ´ blazars 0 = 2009 Z ´ . h AI Collaboration MAGIC the e) Intra- GeV). 031 HL class. (HBL) Currently, . > E † 5)and [5]) , el td iSea i oa5,510Sea Italy Siena, 53100 56, Roma Via Siena, di Studi degli a ` lra Goebel Florian , 100 ‡ higrRn ,885M 80805 6, Ring ohringer ¨ deceased y ∗ for arzoTavecchio Fabrizio , mns AT:tetigrtrsodo h eecp is telescope the threshold of energy currently threshold lowest trigger the in the IACTs: astronomy for amongst VHE allows for This telescope operation. dish single largest the rsn td r usqett ao adaeup- (from hardware enhanced that major 2007, a February in to completed subsequent grade, are study present (e.g.[4]). papers dedicated detail and in More FADCs found fulfilled. are be of criteria can trigger means if by disk to digitized stored then fibers, optical ragdi nhxgnlsae h bevdfil of field observed the (PMT), shape; photomultipliers is hexagonal view 576 an of in made arranged is mirror, the of eslae aaoi irr( mirror parabolic tessellated fL am,a h bevtr fRqed Los de Roque of Observatory the at ( Palma, Muchachos La of osrn,epcal thge nris[] htthe active. constant that when exceptionally as especially [8], assumed windows, safely observation energies be different between cannot higher source is the at of variability state especially time strong, as careful observations, so imposes the source the the of in of coordination processes variability relevant rapid the The on jets. insight there- deeper bright can cast instruments fore this VHE and of Telescopes X-ray involving source, Cherenkov observations Multiwavelenght Air (IACT). Imaging with energy soft observation at high current The peaks with Swift-XRT. component as observed such be telescopes, can X-ray and subclass, HBL at these peaks in bump [11] flares ”orphan” for contexts[12]. account Compton more to is External while difficult as (SSC), Compton such Synchrotron-Self or [16]) (EC) (e.g. models leptonic 1 l h AI bevtoscniee o the for considered observations MAGIC the All h AI aea ntle ttepiayfocus primary the at installed camera, MAGIC The MAGIC ntecs fMn41telweeg (synchrotron) energy low the 421 Mkn of case the In ao topei am-a mgn Telescope Imaging Gamma-ray Atmospheric Major 3 ‡ ln Lindfors Elina , . vriyo uk,F-01 uk,Finland Turku, FI-20014 Turku, of iversity 5 60 1 ◦ h M nlgsgasaerue through routed are signals analog PMT The . slctdo h etr aainIsland Canarian western the on located is I T II. 28 e o bevtoscoet h zenith. the to close observations for GeV . 75 keV HE ◦ N, -30 eae Italy Merate, I-23807 nhn Germany unchen, ¨ AI T MAGIC k nris hc stpclo the of typical is which energies, ∼ 18 n oetWagner Robert and , . 76 10 ◦ § D − W rtkMajumdar Pratik , , 17 = 100 ELESCOPE 2225 GeV m, ....Wt its With a.s.l.). m f/D loigfor allowing , 1 = † , ,i is it ), ¶ , 1 s 2 BONNOLI, G. et al. MAGIC MKN 421 MWL OBSERVATIONS IN 2008

−10 −2 −1 300 MHz to 2 GHz) the time sampling capability of 0.05stat ×10 cm s [2]) all along the period, con- the DAQ. This allowed for better rejection of the Night firming an intense and persistent active state. The max- Sky Background (NSB) and introduction of new refined imum observed flux (FE>200 GeV = 6.99 ± 0.15stat × analysis techniques [22] based on the time properties of 10−10cm−2s−1) was on March 2008, the 30th. Cherenkov signals. In particular, on February the 6th and March the 30th long observations could be taken of very high flux states, III. MAGIC DATASET under good weather conditions. Lightcurves in 8 min. time bins are shown for these nights (figures 2 and MAGIC observed Mnk 421 in 81 nights since De- cember 2007 until June 2008, which is the full visibility 3 respectively) with a softer (E > 200 GeV , upper window for this source. Data were taken in “wobble” panels) and harder (E > 400 GeV , lower panels) energy threshhold. Flux variability with doubling times down to mode, originally suggested by Fomin [6], which allows simultaneous signal and background measurement. The 16 min can be seen in the first night, while in the second night no rapid evolution is visible. method avoids the need of devoting observing time to the measurement of background. This is of paramount IV. MWL DATA importance here, as the source is variable and full time For the multiwavelenght study optical (R-band) and coverage of it’s evolution is highly desirable. After soft-X (0.2 − 10 keV ) data have been collected. quality cuts, ≈ 60 hours of observations in 66 nights were available for further analysis. The zenith distance A. Optical Data ◦ ◦ of the source ranged from ∼ 5 at culmination to 46 . The Tuorla Observatory constantly monitors the MAGIC VHE (known or potential) sources, by means A. Data analysis of the 35 cm remotely operated KVA optical telescope The data were analyzed with the standard MARS2 also located at Roque de los Muchachos and of a package. The suppression of NSB residuals survived 103 cm telescope located at Tuorla, Finland. Along the to the trigger selection and the calculation of image period of MAGIC observations, 117 flux measurement parameters [9] were performed with the new timing of Mkn 421 were performed in the Johnson R band. techniques. The rejection of hadronic background and The light contribution from the host galaxy and nearby the estimation of the energy for the primary VHE γ- companion galaxy (Fh+cg =8.07 ± 0.47 mJy [18]) has rays were performed by means of an implementation of been subtracted from the measured fluxes. The optical the Random Forest classification method [3]. Two new lightcurve is reported in the upper panel of figure 1. parameters were calculated, from the image and time B. X-ray data from Swift-XRT parameters, for each event: Hadronness and Estimated Energy. The X-Ray Telescope (XRT) onboard of the NASA The signal extraction was performed by applying Swift mission has observed Mkn 421 43 times along the − cuts in the Size, Hadronness and Alpha parameters. In period, mainly with short (1 2 ks) exposures. Swift- particular the Size cut, rejecting events with less than XRT data were reduced using the xrtpipeline software 150 photoelectrons of total charge, leads to an energy distributed with the heasoft 6.3.2 package by the NASA threshold ∼ 140 GeV in the present analysis. A total High Energy Astrophysics Archive Research Center − of ∼ 48 × 103 excess events from the selected 60 hours (HEASARC). The observed count rates in the 0.2 10 of observation were detected; the excess was statistically keV band are reported in the second panel of figure 1. significant (above 5σ according to Li&Ma statistics [13]) C. X-ray data from RXTE-ASM in each energy bin from 90 GeV to 8 TeV. Although with worse sensitivity and precision w.r.t. The whole analysis procedure was step by step val- Swift-XRT the All-Sky Monitor onboard of the Rossi idated on compatible datasets from observations of the X-ray Timing Explorer is still able to set one point per Crab Nebula, widely regarded as the standard candle for day from Mkn 421. VHE astronomy. The results were in agreement with the Mkn 421 ASM data are taken from the results pro- known properties of the source. Double cross-checks of vided by the ASM/RXTE teams at MIT and at the the analysis gave compatible results. RXTE SOF and GOF at NASA’s GSFC. The count rates observed in the 2 − 10 keV band are shown in the third B. MAGIC VHE lightcurves panel of figure 1. The night-averaged integral flux above 200 GeV was calculated for the 66 nights with datasets surviving V. CORRELATIONS the quality cuts. The lightcurve is shown in figure 1. VHE data have been correlated to the available data in It’s worth noticing that even if Mkn 421 is believed the other bands. Due to the general lack of tightly simul- to emit a low VHE flux baseline [20], the flux was taneous observations, points in the different bands taken seldom below the Crab level (FE>200 GeV = 1.96 ± within a time window of 0.5 days have been accepted as matching. This is quite acceptable for the optical, where 2MAGIC Analysis and Reconstruction Software variations can be assumed small on these timescale. As PROCEEDINGS OF THE 31st ICRC, ŁOD´ Z´ 2009 3

Fig. 1. Upper panel: R-band optical lightcurve of Mkn 421 from the Tuorla Observatory along the MAGIC observation period. Middle-upper panel: soft X-ray count rates measured by Swift-XRT along the same period. Middle-lower panel: soft X-ray count rates measured by RXTE- ASM. Lower panel: MAGIC VHE lightcurve above 200 GeV, for the 66 observation nights that passed quality cuts. MAGIC has detected the source clearly in all the nights; the integral flux level is lower than 2.0 × 10−10 (∼ 1 C.U., represented by the dashed horizontal line) only in a few nights. A maximum flux of ∼ 3.6 C.U. has been observed on March 2008, the 30th. far as RXTE-ASM measurements are concerned, these due to the variation of the source state between time- come from a 24-hours average, so the assumption is separated measurements. fully valid. For the correlation between Swift-XRT and MAGIC observations, the assumption is no longer valid, VI. CONCLUSIONS so the correlation has been restricted to the 10 tightly The MAGIC Telescope has extensively monitored the simultaneous observation slots alone. The results are that TeV HBL Mkn 421 since December 2007 until June optical and VHE (figure 4, upper panel) are partially 2008. correlated (Spearman’s rank [10] ρ = 0.26, probability The observed X-ray vs. VHE correlation is in full that the datasets are independent P (null)= 3.88%) agreement with previous observations (e.g. [7]) and fits Stronger correlation is found between RXTE-ASM and naturally the SSC scenario, where the same population MAGIC data (figure 4, middle panel): ρ = 0.49, of relativistic electrons is involved in both the syn- P (null)=0.17%. The result of correlation between chrotron and the Inverse Compton emission, although Swift-XRT and MAGIC data is clearly suffering from hadronic models cannot be ruled out. The weaker cor- low statistics: ρ =0.32, P (null)=18.2%. No dramatic relation between optical and VHE state can be regarded improvement is seen by relaxing the request on tight as due to a contribution to the optical flux from regions simultaneity, in spite of the greater statistics. This issue of the jet that are different from the one producing VHE cannot be satisfactorily explained here, but could be γ-rays. 4 BONNOLI, G. et al. MAGIC MKN 421 MWL OBSERVATIONS IN 2008

Fig. 2. Mkn 421 VHE lightcurves in 8 min. time bins, from the observations taken on 2008, February the 6th. Integral flux of excess (filled circles) and background (thin crosses) events is plotted. The Energy threshold is 200 GeV (upper panel) and 400 GeV (lower panel).

Fig. 3. Mkn 421 VHE lightcurves in 8 min. time bins, from the observations taken on 2008, March the 30th. Integral flux of excess (filled circles) and background (thin crosses) events is plotted. The Energy threshold is 200 GeV (upper panel) and 400 GeV (lower panel).

VII. ACKNOWLEDGMENT Fig. 4. Correlation plots for the MWL observations of Mkn 421. We would like to thank the Instituto de Astrofisica Upper panel: optical (R-band) vs. MAGIC VHE data. Within a time de Canarias for the excellent working conditions at window of 0.5 days, 48 observations have been matched. The prob- ability of independence of data is 3.88% (ρ = 0.26). Middle panel: the Observatorio del Roque de los Muchachos in La RXTE-ASM X-ray data vs. MAGIC VHE data. Within a time window Palma. The support of the German BMBF and MPG, of 0.5 days, 33 couples of points were matched. The probability of the Italian INFN and Spanish MICINN are gratefully independence of data is 0.17%, ρ = 0.49. Lower panel: correlation plot for the 10 tightly simultaneous observations from MAGIC and acknowledged. Swift-XRT. The probability of independence is 18.2%, ρ = 0.32. REFERENCES

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