TeV variability and flaring of the Blazar Markarian 421

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Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

TeV VARIABILITY

AND FLARING

OF THE BLAZAR

PROCEEDINGS SUPPLEMENTS

MARKARIAN

421.

David J. Fegan Physics Department, University College Dublin, Dublin 4, Ireland. Four years of TeV observations of the Blazar Markarlan 421 (Mrk 421) by the Whipple collaboration are reviewed and long term variability patterns presented. The dramatic short term flaring of May 1996 is discussed in some detail. Based on the brevity of these flares, estimates are made of the Doppler boost factor and the size of the emission region within the source.

TeV OBSERVATIONS OF BLAZARS. Flat spectrum radio sources with highly variable polarised non-thermal continuum emission extending to X or T-ray frequencies are termed blazars. Blazars are a sub-category of Active Galactic Nuclei (AGN). A sub-class of blazars, BL Lacertae objects, are characterised by either very weak or absent optical emission lines. The broad continuum emission characteristic of blazars is thought to arise from the relativistic motion of plasma in the "jets" associated with suspected compact objects at the cores of active galaxies. If blazars are to be observed at TeV energies they probably have to be located at a redshift of less than 0.5, otherwise pair production interactions with the intergalactic infrared background will cause absorption of the 7-ray beam (Stecker et al., 1992; Biller, 1995). The giant elliptical galaxy Mrk 421 has a nucleus of the BL Lacertae type (Ulrich et al., 1975) and has been observed to emit at radio (Owen et al 1978; 7.hang et al. 1990; Mufson et al. 1990), optical (Xie et al. 1988; Mufson et al. 1990), X-ray (Mushotzky et al. 1979; George, et al., 1988) and both MeV (Lin et al., 1992) and TeV 7-ray frequencies (Punch et al., 1992). With a redshift of 0.031, Mrk 421 was the first blazar detected at TeV energies (500 GeV threshold). A similar BL Lac object, Mrk 501, with a redshift of 0.034 has recently been detected (Quinn et al. 1996) at TeV energies, as has another BL Lac object, 1ES 2344 + 514 (Schubnell et al., 1996a). All these nearby blazars have been detected by the Whipple TeV Cerenkov 0920-5632/98/$19.00 © 1998 Elsevier Science B.V All rights regerved. PII S0920-5632(97)00500-8

imaging telescope (Cawley et al. 1990). TeV upper limits on many other AGNs have been reported by Kerrick et al.(1996a). Urry and Padovani (1995) have proposed unification schemes for Radio Loud AGNs, one linking high luminosity sources (quasars and luminous radio galaxies) and one linking low luminosity sources (BL Lacertae objects and less luminous radio galaxies). Features which determine the spectral energy distributions (SED) of blazars, have been discussed in detail by Sambruna, Maraschi and Urry (1996). They find that the three selection categories of radio selected BL Lacs (RBLs), X-ray selected BL Lacs (XBLs) and fiat spectrum radio quasars (FSRQs) are essentially continuous. While XBLs and FSRQs occupy separate regions in broadband colour-colour diagrams, RBLs bridge the gap between the populations in question. Within the sequence FSRQs, RBLs, XBLs a decreasing bolometric luminosity is observed in the radio to Xray region together with an increase in the frequency of the peak SED. This sequence, as applied to BL Lacs at least, may be due to relativistic beaming effects with XBL jets observed at larger angle lineof-sight than RBLs. Alternatively, the differences might be attributed, at least in part, to underlying physical differences operating in the various source classes. TeV observations of BL Lacs will have an important role to play in unravelling the connections between XBLs, RBLs, and FSRQs in order to advance our understanding as to how blazars exhibit such a diversity and richness of radiation emitted

38

D../. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

over such a broad region of the electromagnetic spectrum. Recently, Stecker, de Jager and Salamon (1996) have re-emphasised the RBL / XBL separation of Sambruna. Maraschi and Urry (1996), made on the basis of the SEDs, by defining objects as low-frequency peaked BL Lacs (LBLs) if the ratio of X-ray flux in the 0.3-3.5 keV to radio flux at 5 GHz satisfies the condition log(Fx/Fr) < -5.5. Such objects fall in the observational category of radio selected BL Lac (RBL) objects. Conversely, if log(Fx/Fr) > -5.5 the objects can be classified as high frequency peaked BL Lacs (HBLs) and fall in the observational category of X-ray selected BL Lac(XBL) objects. To date, only X-ray selected objects have been detected at TeV energies but since this sample is modest, it may be an artefact of technique sensitivity rather than any underlying physical trend. Of the 14 BL Lacs detected by EGRET, 12 are RBLs and 2 are XBLs. Again, this may be a selection effect, rather than any inherent underlying trend. Clearly a much deeper and exhaustive program of TeV observations is required.

of energies required to explain incoherent synchrotron emission from giant radiogalaxies is in excess of 1060 ergs and if it is assumed that the efficiency of conversion of energy into relativistic particles is about 0.1% then really the true factor is about 1063 ergs. There is now

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J E T S AND J E T F O R M A T I O N Jets appear to be associated with AGNs, the violent and luminous cores found in about 10% of galaxies. The jets may be extremely well collimated with opening angles of a few degrees and may extend for hundreds of kpc on either side of the galactic nucleus. This implies some form of ordering mechanism which maintains the stability of the jet over periods of time in excess of 106 yr. Convincing evidence as to what might lie at the base of the jet is a matter of conjecture, beyond the resolution of existing optical or radio telescopes, but whatever the true nature of the jet source, it is probably considerably smaller than 1 pc. The scale

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Given the pivotal role of Mrk 421 in establishing extragalactic TeV astrophysics this paper will summarise in a comprehensive manner, the Whipple collaboration observations of this source. In particular, two dramatic outbursts of photons detected during May 1996 will be discussed in detail and estimates made of the size of the emission region, the Doppler jet boost factor and the cooling times associated with the electrons in the jet.

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Fig.O) Schematic model of an AGNjet system. compelling evidence for the continuous accretion fuelling of extragalactic radio sources, based on observations of moving structures or knots of plasma emitted from the underlying nucleus and ejected along the jet (Fig. 1). The discovery of galactic "microquasars" (Mirabel et al. 1992; Rodriguez et al. 1994) may imply an important new class of very local astrophysical objects with the three essential components of quasars: a black hole (stellar mass), an accretion disk with temperatures in excess of a few tens of keV and collimated jets of high energy particles extending over a few light years and exhibiting superluminal motion (Mirabel and Rodriguez, 1995).

39

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

A favoured model for AGNs has evolved, whereby the ultimate source of energy is a supermassive black hole (108 Mo ) at the galactic center, which accretes gas and dust from its surrounding accretion disk. In the process, it converts up to 10% of the rest mass of accreted material into relativistic particle beams (Begelman et al. 1984; Marscher and Gear, 1985; Blandford et al. 1990). In most models a magnetic field threads the accretion disk which gets wound up by the rotating disk. The density of the disk is typically about 100 times that of the corona which lies above and below Fig. (1). As the disk sheds matter into the gravitational potential well of the black hole it loses angular momentum. The magnetic field threads the disk connecting to the surrounding corona and differential rotation of the disk and the corona generates an azimuthal magnetic field. The field acts like a stretched wire, dragging the corona around with the disk and increasing its angular momentum at the expense of that of the disk. The azimuthal magnetic field component in the corona pinches the plasma onto the axis of rotation. Pressure grows close to the centre of the potential well. The thermally driven plasma wind blowing outwards from the disk surface is therefore collimated in this process to form the jet. The magnetic field component parallel to the rotation axis is compressed and amplified by the infall process, playing a key role in the collimation of the jet. Recent development of time dependent relativistic hydrodynamical software (van Putten 1993; Eulderink and Mellema 1994; Marti et al; Duncan and Hughes 1994; Bell and Lucek 1995; Lucek and Bell 1996) facilitates realistic simulation of the emission characteristics of jets. Models show that by temporally increasing the flow velocity at the base of the jet, superlaminal features of enhanced emission may be reproduced (Gomez et al. 1996 ). The lower frequency SED component of blazars is probably due to synchrotron radiation emitted by relativistic electrons in the jet. Relativistic motion of hot spots in the jet implies that the spectra will be boosted with a Doppler factor of 5 - 3 - 10 in frequency and by a factor 54 in luminosity (Urry and Padovani 1995). This relativistic beaming also implies strong anisotropy in the emission and leads to relativistically

contracted timescales in the rest flame of the observer. For acceleration models based on relativistic beaming of electrons, RBLs exhibit peak synchrotron emission at radio frequencies with the corresponding inverse Compton peak occuring in the GeV range. However, XBLs exhibit peak synchrotron emission at X-ray energies with the corresponding inverse Compton peak occuring at TeV energies. Models for emission of T-rays from AGNs have been discussed by K6nigl (1981) Dermer and Schlickeiser (1993) Sikora et a1.(1987) and Blandford and Levinson (1995) while the role of protons in blazar jets has been examined by Mannheim (1993, 1996). T-ray Selection. Data is acquired by the Whipple 10In imaging telescope (Cawley et al. 1990) in either of two principal modes of operation, (a) a source is tracked for 28 minutes (ON scan) followed after two minutes by another 28 minute exposure of a comparison region of sky (OFF scan) which may be used as a control, or (b) a source may be tracked indefinitely in contiguous 28 minute scans. Individual images captured by the high resolution camera are subjected to extensive image calibration, normalisation, noise reduction and possible software padding as described in detail by Fegan (1996). 4

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Cleaned images are then passed through a parameterization process which, in effect, fits an

40

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

ellipse to each image, on tile basis of principal moment analysis, Fig (2). While a number of alternative methods of image selection have been developed (Danaher et al. 1993; Reynolds et al. 1993; Fegan et al. 1994; Reynolds and Fegan 1995) the favoured method of background rejection is the Supercuts approach (Punch et al. 1991) which selects candidate y-ray events on the basis of both image shape and pointing characteristics, determined mainly by the parameters Width, Length and Alpha. y-ray events tend to be more compact than hadronic background events and tend to have small values of Alpha as the major axes of the image ellipse point towards the centre of the camera field of view (Fegan 1992). This is because the y-ray source is located at the centre of the field of view of the telescope and resulting shower axes are always parallel to the optic axis. On the other hand, hadronic showers can have their points of origin associated with any location within the field and the hadronic shower axes need not be parallel to the reflector's optic axis. Hence major axes of hadronic images show no preferential pointing characteristics. Supercuts rejects more than 99.97% of the background while retaining in excess of 50% of the incident y-rays. Almost all of the results which follow and relate to observations of Mrk 421 are based on a Supercuts selection which generally incorporates a minimum shower Size selection based on total detected light content (400 digital counts (dc)). This process rejects an appreciable fraction of the very small events which are more problematic to handle, due to low light level content. The operative selection generally is as follows: Supercuts

Size Frac 3

Alpha

> < 0.07 < 0.16 < 0.51 < <

400dc 0.98 Width < 0.15 Length < 0.30 Distance < 1.1 1 5 °.

The imposition of the size and trigger selection cuts raises the effective energy threshold of the detector to about 300 GeV. During 1995-1996,

significant progress has been made on analysis strategies which might facilitate the selection of small y-ray events with Sizes < 400 dc, (Moriarty et al 1997 in preparation). Small events tend to lie much closer to the trigger threshold of the instrument and for that reason are more susceptible to the influences of night-sky background fluctuations, camera response systematics, the presence of individual stars imaged in the focal plane and the insidious presence of small arcs or partial rings of Cerenkov light from individual charged muons passing close to the reflector. Small background events tend therefore to have an appearance which mimics that of small genuine T-ray images. Given the relatively coarse pixellation of the 109 tube camera, the pointing characteristics of small showers (< 400 dc) are worse than large ones and the Alpha parameter has to be relaxed to 21 ° to avoid discriminating against small genuine y-ray images Two strategies have been developed, either of which may be employed, depending upon the particular requirement. The first, (Option A), is based on a neural network which has been trained on muon and small y-ray simulations to maximise the rejection of the muon images. The overall rejection of events is high and the selection therefore harsh. The second, (Option B), is based on use of the Length/Size parameter (Catanese et al. 1996). Muon arcs contain low light levels per pixel and their light distribution is flat rather than concentrated as it is for y-rays and hadrons. Muon images tend to be long in proportion to their light content and a Length/Size cut efficiently eliminates them without very significant elimination of y-rays. This latter selection strategy is more gentle than the first. Application of such a cut to events with size > 400 does not help in the discrimination process since Supercuts itself very efficient at rejecting muons. Both selections are summarised below and have been applied to some of the 1995-96 data. Small Events (Option A-Neural network, harsh).

150 < Size < 400 Fr2 < 0.975; 0.51 < Dist< 1.1 0.16 < Length < 0.3; 0.073 < Width < 0.15 Plus, Neural network muon rejection Plus, ex < 21 °

D.J. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

Small Events (Option B - Length~Size cut, gentle). 150< Size < 400 Fr2 < 0.975; 0.51 < Dist < 1.1 0.16 < Length < 0.3; 0.073 < Width < 0.15 Plus, Length/Size < 0.000839 Plus, 0t<21 ° TeV O B S E R V A T I O N S of M r k 421 Observations during 1992-94. The first Whipple observations of Mrk 421 were conducted during 1984-85 with a 37 element imaging system, as part of a campaign to look for TeV "/-rays from extragalactic objects. No detection was made and an upper limit of 6.3x10 u photons cm 2 s t was reported at energies in excess of 400 GeV (Cawley et al. 1985). During the period March-June 1992, Mrk 421 was again observed but this time with the Whipple 109 element high resolution camera. Based on 7.5 hr of ON source observations (and an equivalent amount of OFF source observation) a 6.3 c excess of events was recorded after application of Supercuts, for Alpha values <15 ° (Punch et al 1992). This excess corresponded to an average flux of 1.5x10 1~ photons cm 2 s t above 500 GeV, corresponding to 30% of that detectable from the Crab Nebula. This first detection of TeV photons from any extragalactic object using the imaging Cerenkov technique has recently been independently confirmed by the HEGRA group at 1 TeV (Petry et al. 1996). Although some observations of Mrk 421 were made during 1992-93 which confirmed the findings of the previous year, major Whipple follow up observations were made during the period from December 23rd 1993 to May 10th 1994, yielding 130 hours of ON source data with an average source flux only half the 1992 discovery level (Kerrick et al. 1995), after application of Supercuts. However, subsequent observations (May 15.25 UT 1994) indicated a strong flare with a peak flux level approximately 10 times greater than the average 1993-94 Mrk 421 flux (Kerrick et al. 1995). The observational record for the period May 10th 1994

41

(day 130) through June 12th 1994 (day 163) indicates a rising T-ray signal consistent with an efolding rate of = 2 days, reaching a peak value on May 15th and followed subsequently by a second region of enhanced emission, of lesser significance, Fig.(3a). In Fig.(3b), the shower rate per 5" of the Alpha plot is presented without a size cut on the Supercut data and in Fig.(3c) the rate is shown per 10° of the Alpha plot and with a size cut of 500 dc. Threshold energies for these selections are 250 and 500 GeV respectively. During peak observation the T-ray rate was 4.5+ 0.8 photons rain ~ corresponding to a maximum flux of 2.1 x 101°photons cm 2 sl at a threshold of 250 GeV. Mrk 421 is one of more than 50 AGN sources detected at energies above 100 GeV by EGRET (Lin et al. 1992). Multiwavelength observations of AGN's provide important insights into the nature of these enigmatic objects. Simultaneous observations, where feasible, offer the unique possibility of assessing whether a single or multi-component emission region more properly characterises these objects. Observations of possible time-correlated emission across a broad multiwavelength spectrum could help resolve this issue. Simultaneous observations also provide perhaps the only real means of testing the hypothesis that T-ray emission might be described by a single power law over many decades of energy. During the period April/May 1994 Mrk 421 was simultaneously observed by Whipple (TeV), EGRET (GeV), ASCA (KeV), IUE (1300 A), UKIRT (1.48-1.78 ~lm), JCMT (1.1 ram) and UMRAO (14.5 Ghz). Many of the observations made by these instruments either coincide or almost coincide with the TeV flare of May 14/15. The ASCA X-ray observational program began approximately one day after the TeV flare and observed a possible correlated flare with the source in a high flux state (Takahashi et al. 1994). However, there was no evidence from EGRET at 100 MeV, of the emission enhancements observed in the TeV energy range. Mrk 421 at EGRET energies appeared not to exhibit any change in the state of it's output (Macomb et a1.1995).

42

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

Mrk 421 Variability 1994 i

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D.J. Fegan/Nuclear Physics B (Prec. Suppl.) 60B (1998) 37-58

In observations made of other BL Lac objects X-ray flaring is observed to precede variability at lower energies and this has been attributed by Marscher and Gear (1985) to shock propagation along the jet. Electrons with energies of about 1 TeV are required to produce X-ray synchrotron emission. Such relativistic electrons may be responsible for the production of TeV T-rays through the synchrotron self-Compton (SSC) scattering of the synchrotron photons to much higher energies or by scattering of an external soft photon field arising perhaps from the surface of an accretion disk. Any increase in the upper cut-off energy of the electron distribution might simultaneously result in the emission of an X-ray synchrotron flare and a burst of TeV photons. Activity at lower photon energies would be either marginal or non-existant. The observed May 15th X-ray flare from Mrk 421 may thus have been caused by a shift in the maximum cut-off energy of the electron population in the jet. Whipple TeV observations of Mrk 421 for the entire period April 1992 to June 1994 are summarised in detail in Schubnell et al. (1996b).

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Mrk 421 was extensively observed by Whipple during 1995. As part of a multiwavelength campaign spanning late April and early May, 26 hours of ON/OFF is summarised as a 2D plot of ~'ray emission Fig.(4a), together with the corresponding Alpha distribution, for selected events Fig.(4b). The EGRET error box is drawn as an ellipse in Fig.(4a). The method of selection is derived from that described in Akerlof et al. (1991) with minor modifications as described in Lessard and Buckley (1997). The 2o individual contours represent the statistic S = ~/(-2 In ~,) where ~, is the likelihood ratio derived from the ON and OFF counts at each grid point of the focal plane detector. The overall significance of the detection is 236.

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44

DJ. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

gives an average rate of 0.71 7-rays m x. Month by month and day by day variability are shown in Fig. (5 a,b). The latter plot shows the "/-ray rate, scan by scan. The limited duty cycle of the Cerenkov technique probably undersamples the true TeV 7-ray light curve. The pattern manifested in the 300 GeV data indicates nevertheless, that emission is not quiescent but is characterised by significant variability, indicative perhaps of rapid flaring of the source over a variety of amplitude and time scales, with the baseline level of emission close to the sensitivity limit of the instrument and consistent with barely observable steady emission. Occasional emission doubling times of less than one day are evident. During the period from April 20th to May 17th 1995, multiwavelength observations were again taken on Mrk 421. The Whipple (>300 GeV), ASCA (0.7 to 7.5 KeV), EUVE - DS/S ( 58 to 174 A) and Optical (650rim R band) overlapping observations of Mrk 421 are shown in Fig.(6). Significant cross correlation of the averaged daily TeV 7-ray rate and the ASCA X-ray rate is evident at zero phase shift while a maximum correlation of the X-ray/7-ray data with the EUV and optical plots occurs when a time lag of one day is allowed for, at these latter wavelengths. The ASCA flux drops from a peak value of 2.3 x 10"t° erg cm -2 s 1 to 0.4 x l f f 1° erg cm "2 s"l during the course of the observation. The amplitude of variability is similar at X-ray and "/-ray wavelengths ( - 400% ) compared with a 200% change in the EUV band and a 20% change at optical wavelengths. EGRET observations of Mrk 421 during this period were made in low sensitivity mode and failed to detect any flaring, providing an upper limit of 1.2 x 10"7 photons cm -2 s "~. The ASCA X-ray observations are indicative of amplitude variations on timescales considerably less than 1 day. The TeV data shown in Fig.(6) represents the average daily flux rate in 7-rays per minute. Examination of individual 30 rain ON scans throughout the period in question, fails to show any evidence for variability on such time scales. Fig.(7) shows a multiwavelength overlay plot of the Mrk 421 emission from radio to TeV 7rays, for observations made on days banding the two X-ray/TeV 7-ray correlated flares, May 15th 1994 to

April 26th 1995, together with some archival spectral measurements covering the period 19771994, for comparison. The star symbols show the d a t a taken within two days of the flare on April 26th 1995. The TeV flux from this day is calculated using an effective area of 3.0 x 10gcm2 and median 7-ray energy of 330 GeV, and is converted to a differential point assuming an E "27 spectrum. The X-ray flux point at 1.8x10 ~° erg cm -2 s ~ is the average value for observations taken over 1995 April 25.8-26.1 UT (Takahashi et al., 1995). Also shown is the average EUVE flux on the day of the flare, of approximately 2.8x10 "1° erg cm 2 s -1 at ~. = 80 A, assuming a value of NH = 1.45 x 1020 cm -2 for the Galactic hydrogen column density (Elvis et a1.,1989), with a spectral slope of ~t -- 1.3 joining the EUVE and 1.5 KeV ASCA points (Kartje et a1.,1996). Radio observations taken on April 29th 1995 (at 14.5 Ghz), May 16th (at 8.0 Ghz) and May 20th (at 4.8 Ghz) using the University of Michigan 26 m telescope are also shown. The filled circle symbols show data for days around the TeV flare on May 15th 1994 with the radio to soft UV data as well as the EGRET points taken from Macomb et al. (Macomb et al. 1995), and the X-ray spectrum from Macomb et al. (1996). The TeV point is derived from the integral flux which is recalculated using the same (high threshold) cuts as applied to the 1995 data and resulting in a higher median energy of 390 GeV (due to the differences in the PMT gains in the 1994 season) with the same effective area of 3.0 x 108 cm 2. The archival data represented by the open circle symbols is drawn from a variety of sources which are summarised by Buckley et al. (1996). Observations of multiwavelength flaring of Mrk 421, together with a possible time lag of the optical/UVE emission relative to the temporally coincident X-ray/T-ray flare, are the main conclusions of the 1995 observations.

TeV ),-ray variability 1996. Extensive observations of Mrk 421 were again made during 1996, with monitoring of possible variability and flaring being of prime

45

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

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D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

46

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D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

47

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Fig.(7) Multi wavelength observations of Mrk 421 corresponding to 1994 and 1995 flares. interest. Two dramatic episodes of flaring were observed. Large event (> 400dc) selection was again made on the basis of Supercuts but with an Alpha cut of 10°, due to improvements made in the tracking accuracy of the 10m mount. The database was extensive, consisting of 150 ON and tracking scans together with 73 OFF scans. The OFF data was used to estimate the effective Alpha ratio for the complete database and to characterise the data and look for possible sources of systematic error. The observing strategy was based on a mix of ON/OFF and tracking scans taken on Mrk 421. Rapid data analysis (almost realtime) facilitates the continuous tracking of the source, once the flux has been observed to be high. The average T-ray rate

throughout the year was 0.54 T min4 (0.45 T rain4 excluding the night of May 7th 1996). If the Alpha cut is relaxed to 15 ° a slight increase is observed, 0.47 Train 4, which can be compared with the 0.71 T rain 4 observed during 1995, with a 15 ° Alpha cut. As a check by month, of the day to day variability, a Z 2 test was performed to test if the daily flux, based on the three highest elevation Mrk 421 scans, was constant about appropriate monthly means. Table (1) indicates quite improbable values of ~ signalling significant day-scale variability as implied in Fig.(8a), which shows the daily T-ray rate for observations taken between December 1995 and May 1996. The rate is estimated on the basis of

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

48

10

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Fig.(8a - top) : Mrk421 daily gamma-ray rate December 1995 to May 1996. Fig.(8b - bottom) • The same data but with the very large flare of May 7th 1996 removed.

D J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

Table ( 1 ) 1996 Month bz Month Variability( Z 2 prob Month Average Rate i Dec i 1.35 x 10 -2 0.70 + 0.08 i 1.15 x 10 -5 Jan 0.56 + 0.04 Feb 0.46 + 0.06 : 0.792 Mar 0.71 + 0.05 i 2.6 x 10 14 Apr 0.30 + 0.03 i 2.09 x 10.3 May 1.12 + 0.04 ! < 1.0 x 10 -s° May 0.45 + 0.03 i 2.43 x 10 "13 exc. flares i selecting individual events of shower size greater than 400 dc. The blank gaps represent periods of observation precluded by moonlight. A very large flare is evident on May 7th 1996 which has been removed in Fig.(8b) where the scale of the variability is more readily discernible. A second smaller flare is evident on May 15th 1996. The flares in question are more powerful than anything previously detected at TeV energies, the first being the most dramatic in terms of detected flux, higher than any source in the current TeV source catalog by a factor of about 20. The second flare is remarkable in terms of its combined intensity and brevity. Both are worthy of further analysis and interpretation. T e V flares f r o m M r k 421, M a y 1996.

As stated earlier, the selection of events during the May flares has been made on the basis of the conventional Supercut selection of shower images with total light >400 dc, together with one or other of two possible alternative approaches to usefully exploiting smaller events, with sizes <400 dc. (i)

FLARE 1 : the flare of May 7th 1996.

A combination of ON, OFF and tracking observations covering the interval from 03:35 to 05:50 UTC were made. Observations were terminated by moonrise. In Fig.(9a) the Alpha plot distributions are shown for both the small (Option A - harsh Neural network selection) and large event

49

populations for combined ON and tracking Mrk 421 observations made on May 7th. T-rays are defined as having Alpha values <21 ° for the small events and < 15 ° for the large events, the "hot" regions of the distributions. The "cold" regions represent shape selected events picked out by the respective selection strategies applied to both small and large event populations which fail to "point" towards the centre of the camera and are therefore almost exclusively hadronic or muonic background events which may be used as a control sample if required. The small and large event rate populations are presented in Fig.(9b), in seven-minute sampling bins, for hot regions of the Alpha plots shown in Fig.(9a). Moonrise terminated observations after close to 140 minutes of elapsed time, due to gradual brightening of the sky. By fitting a polynomial to the events in the "cold" regions an estimate can be made of the percentage or rate of events in the "hot" Alpha region population, which may be due to background accumulated over the entire observational span. The estimated background contamination (assumed to be constant in rate over the duration of the flare) may be subtracted from rate plots of T-ray selected events to give a more realistic indicator of the true rate profile of the flare. Following estimation and subtraction of the background, the seven-minute populations for the hot Alpha region events are shown for both the small and large event populations, in Fig.(10a). The two populations correlate quite strongly (correlation coefficient 0.81) and are shown combined in Fig.(10b). During the course of the flare, the rate appears to have increased rather uniformly from an onset value of 9 T-rays rain ~ to a peak intensity around minute 115, of about 23 T-rays rain t . For comparative purposes, the flare profile has been reanalysed using the Option B (gentle Length/Size ) selection of small events. Fig.(lla) reflects the fact that the Alpha plot for small showers contains almost twice as many small T-ray candidates as with Option A, but at the expense of an obviously greater background contribution, as evident from the number of events in the cold region of the Alpha plot. The combined small and large events with estimated background subtracted, is

50

D.J. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58 FLARE 1 Mrk421 LARGE events

FLARE 1 Mrk421 SMALL events 700 r

250

600 B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 500 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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0

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50 100 FLARE 1 Mrk421 SMALL events

70

140

60

120

50

10o

4O

8O [a. o

e~ O

n

a. 30

60

20

40

10

20

0

0

50 100 Elapsed time in minutes

150

50 100 FLARE 1 Mrk421 LARGE events

0

0

50 100 Elapsed time in minutes

150

F L A R E 1 Mrk 421, Alpha plot distributions for S M A L L and L A R G E events. Fig.(9a - top): Fig.(9b - bottom): Corresponding event rate plots in seven-minute bins.

51

D.,L. Fegan/Nuclear Physics B (Prec. Suppl.) 60B (1998) 37-58 FLARE 1 Mrk421 LARGE events

FLARE 1 Mrk421 SMALL events 60

120

50

100

40

80

(,-

t-

O

°O ~ ¢0

-5 ¢3L ¢-,.

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nn

nn

60

t.m

20

40

10

20

0

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0 50 1O0 150 0 50 100 Elapsed time in minutes Elapsed time in minutes FLARE 1 Mrk421 Corr bined SMALL and LARGE events

150

18°I 160 140 120 ¢.-

.o

-.=

loo

O. O 0.-

._= 80 133

60 40 20 0

0

20

40

60 80 100 120 140 Elapsed time in minutes Fig.(10a - top): FLARE 1 Mrk421, rate plots with estimated background subtracted. Fig.(10b - bottom): Combined SMALL and LARGE event rate in seven-minute sampling bins.

D.J. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

52

FLARE 1 Mrk421 SMALL events 450

FLARE 1 Mrk421 LARGE events I

700

400 600 350 500 30( ._~ 25O

._~ 400

~_200

a. 300

Q.. 0

150 200 100

100

50 0

1

0

2o° I

50 100 0 50 Alpha Parameter Alpha Parameter FLARE 1 Mrk421 Combined SMALL and LARGE events ! ! ! ! ! !

100

18(3 16(3 14C ._~ 120 ..~ "5

~ioo

._ m 8O 60 40

20 0

0

20

40

60 80 100 120 140 Elapsed time in minutes Fig.(1 la - top): F L A R E 1 Mrk421, Alpha plot distributions for S M A L L and L A R G E events. Fig(1 l b - bottom): Combined S M A L L and L A R G E event rate in seven-minute sampling bins.

D.J. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58 shown in seven-minute bins in Fig.(11b). While exhibiting the same general shape as shown in Fig.(10b), this selection rises from an onset value of about 11 T-rays mins -1 to a peak intensity of about 26 T-rays min t . A total excess of 2500 y-ray events were detected during this flare, at an average rate of 22.1 per minute. This exceedingly large flux, unsurpassed at these energies, reflects the exceptional flux sensitivity of the Atmospheric Cerenkov technique and really demonstrates the power of the method to make fundamental contributions towards our understanding of Active Galactic Nuclei. (ii)

FLARE 2: the flare ofMay 14th 1996.

A combination of ON, OFF and tracking observations were made spanning the interval from 03:45 to 16:45 UTC. Figures (12a, b) show, respectively, the Alpha plot distributions based on Option B (gentle length/size) analysis of the small events and the combined small plus large event distributions, in three and a half minute bins, with estimated background subtracted. What is remarkable about this flare is the brevity of the emission. Allowing for the sampling gaps on either side of the main peak, it is evident that both rise and fall times are of the order of 30 minutes, remarkably brief in terms of whatever turn-on turn-off mechanism might be responsible for the phenomenon. The average rate during the central 28 minutes was about 7 T-rays win -1.

Multiwavelength observations and short time variability. The Mrk 421 one day time lag between X/T-ray and optical/EUVE multiwavelength observations of April 1995 appears difficult to explain on the basis of a single component SSC model, where the broad spectrum emission arises from a common localised region. A model involving shock driven inhomogeneous jets may qualitatively explain the phenomenon. Marscher and Gear (1985) have also examined the role of shocks in explaining jet emission variability associated with flares. Recently, Ghisellini et al (1996) have pointed out the importance within the

53

SSC model, of the break energy value of the electrons corresponding to the spectral power law break point. Within the framework of the SSC model, the two very different sources Mrk 421 and 3C 279 may be accommodated, depending upon the break point energy and the magnetic field. The observational consequence is a shift of the synchrotron and Compton peaks to higher frequencies. Well sampled, simultaneous multiwavelength observation of both sources may, in the future, help to resolve the issue. With regard to the TeV flares observed during May 1996, detection was somewhat serendipitous, not forming part of any organised multiwavelength observational campaign. The character of the two bursts appears to be different. The earlier flare (May 7th) had possibly peaked by the time observations were concluded, indicating a rather linear rise in the rate of detected events over a period of about 140 minutes. The doubling time of this flare is at least one hour, possibly very considerably longer. Follow-up observations made the next day indicated that the flux level had returned to a value = 30% of the Crab Nebula, indicating a decay timescale of < 1 day. The second flare (May 14th) is the more interesting due to the relative brevity. Most of the intense portion of this flare is contained with a 30 rain observation which unfortunately is bounded by OFF source observations. However, it seems not unreasonable to suggest a rise and fall time each of about 30 mins duration. In Table (2), estimates of the Doppler boost factor, 8, the dimension of the emission region, IL and the cooling times for the electrons, x e, coot,are estimated, on the basis of the risetime of the second flare and making the assumption that the TeV flare and the optical flux originate from a common region of the source. The implications are that the emission may occur very close to the nucleus of the galaxy, or in a flat emission region (like a shock) which is observed at a very small angle to the jet axis. However, it should be emphasised, that in these calculations (Table 2) it is assumed that the optical flux comes from the same physical region of the jet as the T-ray emission during the flare. Optical measurements made on the night of the 15th of May suggest that there were no

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

54

FLARE 2 Mrk421 LARGE events

FLARE 2 Mrk421 SMALL events

90 I

90

80

80 ................... ~...................

70

0

60

60

.o_ = so

o so

...................

i ...................

..................

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..................

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.

.

.

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.

.

.

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.

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.

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.

.

.

.

.

.

.

.

.

.

.

.

.

.

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.

.

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.

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.

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.

.

.

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.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

10

0

0 50 100 0 50 Alpha Parameter Alpha Parameter FLARE 2 Mrk421 Combined SMALL and LARGE events

100

3oI 25 i[

2O E o

t~

oO-15 a_

._~ m

10

0

0

20

40

60 80 100 120 140 Elapsed time in minutes Fig.(12a - top): FLARE 2 Mrk 421 Alpha plots for SMALL and LARGE events. Fig.(12b - bottom): Combined SMALL and lARGE flare populations in three and a half min. bins.

55

19J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

TABLE 2: Estimation of Doppler boost factor, size of the emission region and electron cooling time.

(a) Doppler boost factor 8 The relationship between 8 and y is given by 5 -1

then f o r a h o l e of mass l0 s Mo, Rs

)'(1-13 CosO)

=

where y and ~ respectively are the bulk Lorentz factor and jet velocity and 0 is the viewing angle. If the correlations of Optical and UV with )'-rays are valid then all the emission may have a common spherical emission region in the jet. A lower limit on 8 is then given by

.'.

8 > 4.9

--l/5

x

4/

'

2.95x1013cm

Limit on R

So, the implications are that the emission region is very small and that emission occurs within the jet and probably close to the nucleus of the Galaxy.

Electron cooling times

~, +1/5 4

Fu

Expressions for the Synchrotron cut off energy Esy,,m= and the Inverse Compton maximum energy E o ~ are given by

X = Doubling time; Fu = Optical U band flux, assuming a spectral index Ix = 0.7

E,~,~

~ 82)'2,,,,,~ B ; 2

From measurements made on the 1.2 m C f A optical telescope on May 7 (night of fast flare) Fu = 15.8 mJy. If x = 30 m = 1800 s, then 5=9.9

2

For the Comptonization of external photons of energy ~,a these equations yield a value of magnetic field of

This is twice the value of 8 = 4.9, estimated on the basis of the April 1995 TeV flare.

(b)

--17

Rs

(c) "

=

B < 0.2G for Ee, max = 1 TeV,

Esyn,n~ax = 1 kev,

6 -- 10.

Size of the emission region.

By causality, the size of the emission region is constrained to satisfy

The cooling time for electrons is given by te, coo,-- 3x107 (1+ z)8-1y ;1 [(I+TI)B 2 / 8rc]-'s

If 5 = 9 . 9 ; x = 3 0 x 6 0 = 1 8 0 0 s ;

z=0.031

Relative contributions of Synchotron and IC losses are determined by an efficency parameter I]. For 11 = 1 and B -- 0.2G, then

.'. R _< 5.18

x

1014 cm _ 35 AU t e,cool ~ 2hr

The emission region is very compact. In terms of the orbit of Pluto ( -- 39.5 ALl), or, in the context of a supermassive Black Hole of radius Rs, Rs = 2 G ~ c 2

This is marginally consistent with the observed rise and fail times of Flare 2..

56

D.J. Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

dramatic increases in the optical flux above the continuum level either at the time of the TeV flare or within the couple of hours immediately afterwards.

Blandford, R.D. and Levinson, A. Ap.J. 441, 79 (1995). Buckley, J.H. et al. Ap.J Letters (in press) (1996).

Observation of TeV photons from AGNs impose severe constraints on the radiation density fields surrounding such objects. Models which demand high densities of unbeamed photons from either accretion disks, or from broadline regions close to the nucleus, to initiate electron positron cascades, may be totally inadequate in accounting for the TeV emission due to the high opacity arising from the external photon fields (Coppi et al. 1993). Similarly, the model of Sikora et al. (1987) is strongly constrained by the TeV observations. As noted earlier, XBLs exhibit peak synchrotron emission at X-ray energies with a corresponding inverse Compton peak at TeV energies. Since no strong X-ray flares were seen in conjunction with the May 1996 TeV flares, then these hourscale flares may be entirely different phenomena to the dayscale flares detected in 1994 and 1995.

Acknowledgements This program is supported by the U.S Department of Energy, NASA, PPARC in the U.K and FORBAIRT in Ireland. I wish to acknowledge the contributions of my Whipple collaborators, in particular Julie Mc Enery and Jim Buckley. Acknowledgements also to Ann Breslin, Michael Cawley and Trevor Weekes, all of whom helped with proof-reading of the manuscript.

References

Catanese, M. et al. Proc. workshop on" Towards a major Atmospheric Cerenkov detector" Editor M.Cresti, Padova ( Italy ) p.335 (1996). Cawley, M.F. et al. Proc. 19th Int. Conf. Cosmic Rays (La Jolla) OG1,264 (1985). Cawley, M.F. et al. Exp. Astr. 1,173-193 (1990) Celotti, A., Maraschi, L. and Treves, A. Ap.J. 377, 403 (1991) Coppi, P.S., Kartje, J.F. and K 6nigil, A. Proc. Compton Symposium, Eds. M.Friedlander, N.Gehrels and D.J.Macomb (AIP New York), 559 (1993). Danaher, S., Fegan, D.J. and Hagan, J. Astroparticle Physics 1,357 (1993).

Dermer, C.D. and Schlickeiser, R. Ap.J. 416, 458 (1993). Duncan, G.C. and Hughes, P.A. Ap.J., 436, L119-L122 (1994). Elvis, M. Lockman, F.J. and Wilkes, B.J. Ap.J. 97, 777 (1989).

Akerlof, C.W. et al. Ap.J., 377, L97 (1991).

Eulderink, F. and Mellama, G. Astron. Astrophys., 284, 654-662 (1994).

Begelman, M.C., Blandford, R.D. and Rees, M.J. Reviews of Modern Physics, 56, 2, Part 1,255 (1984).

Fegan, D.J. Proc. workshop on" Towards a major Alanospheric Cerenkov detector ", Editors, P.Fleury and G.Vacanti, Palaiseau (France) p.3 (1992).

Bell, A.R. and Lucek, G.C. Mon.Not. R. Astron. Soc. 277, 1327 (1995)

Fegan, D.J. et al. Proc. workshop on "Towards a major Atmospheric Cerenkov detector III ", Editor, T.Kifune, Tokyo (Japan) p.149 (1994).

Biller, S.D. Astroparticle Physics 3, 385 (1995) Fegan, D.J. Space Science Reviews 75, 137 (1996). Blandford, R.D., Netzer.H., and Woltjer, L. Active Galactic Nuclei, Springer-Verlag, New York (1990).

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

George, I.M., Warwick, R.S. and Bromage, G.E. Mon. Not. R. Astro. Soc. 232, 793-808 (1988). Ghiseilini, G., Maraschi, L. and Dondi, L. Astron. Astrophys in press, (1996). Gomez, J.L. et al. Submitted to Nature Sept.(1996). Kartje, T. et al. (in prepararion 1996).

Mufson, S.L., Huuer, D.J., Kondo, Y., Urry, C.M. and Wisniewski, W.Z. Astrophys. J. 354, 116-123 (1990). Mushotzky,R.F., Boldt, E.A.,Holt, S.S. and Serlemitsos, P.J. Astrophys.J. 232, L17-L19 (1979). Owen, F.N., Porcas,R.W., Mufson, S.L. and Moffett, T.J. Astr. J. 83,685-696 (1978).

Kerrick, A.D. et al. Ap. J. 452, 588, (1995a). Petry, D. et al. Astron. and Astr. (in press 1996). Kerrick, A.D. et al. Ap.J. 438, L59 (1995b). K6nigl, A. Ap.J. 243, 700 (1981). Lessard, R.W. and Buckley, J.H. m preparation (1997).

Punch, M. et al. Proc.22nd ICRC (Dublin) 1,464 (1991). Punch, M. et al. Nature 358, 477 (1992). Quinn, J. et al. Ap.J. 456, L83 (1996).

Lin, Y.C. et al. Ap.J. 401, L61 (1992). Lucek, S.G. and Bell, A.R. Mon.Not. R. Aston. Soc. 281,245 (1996).

Reynolds, P.T., Whelan, D., Fegan, D.J. and Hoey, M. J.Phys. G: Nucl. Part. Phys. 19, 1217 (1993).

Macomb, D.J. et al. Ap.J. 449, L99 (1995).

Reynolds, P.T. and Fegan, D.J. Astroparticle Physics 3, 137 (1995).

Macomb, D.J. et al. Ap.J. 459, L l l l (1996).

Rodriguez, I.F. et al. Ap.J. 401, L15 (1994).

Mannheim, K. Astron. Astrophys. 269, 67 (1993).

Sambruna, R., Maraschi, L. and Urry, C.M. Ap.J. 463, A.~4 (1996).

Mannheim, K. submitted to Rev. Mod. Astr. (1996). Marscher, A.P. and Gear, W.K. Ap.J. 298, 114 (1985). Mart/, J.M-a., MOiler, E and Ibgtfiez, J.M -~. Astron. Astrophys., 281, L9-L12 (1995). Maza, J., Martin, P.G. and Angel, J.R.P. Astrophys J. 224, 368-374 (1978). Mirabel, I.F. et al. Nature 358, 215 (1992). Mirabel, I.F. and Rodriguez, L.F. Annals of New York Academy of Science 759, 21 (1995).

57

Schubnell, M. et al. Amer. Astr. Soc. High Energy Astrophysics Div. Conf. San Diego, CA (1996a). Schubnell, M. et al. Ap.J. 460, 644 (1996b). Sikora, M., Begelman, M.C. and Rees, M.J. Ap.J., 320, L81 (1987). Stecker, F.W., De Jager, O.C. and Salamon, M.H. Ap.J., 390, L49 (1992). Stecker, F.W., De Jager, O.C. and Salamon, M.H. submitted Ap.J. Letters ( 1996 ). Takahashi, T. et al. IAU Circ. 5993 ( 1994 ).

Moriarty, P.M. et al. (in preparation, 1997). Takahashi, T. et al. IAU Circ. 6167 (1995).

58

D.J Fegan/Nuclear Physics B (Proc. Suppl.) 60B (1998) 37-58

Ulrich, M.H., Kinman, T.D., Lynds, C.R., Rieke, G.H. and Ekers, R.D. Ap.J., 198, 261 (1975). Urry, C.M and Padovani, P. PASP 107, 803 (1995). van Putten M.H.P.M. Ap.J., 408, L21-L23 (1993). Xie, G.Z., Lu, R.W., Zhou, Y., Hao, P.J., Zhang, Y., Li, X.Y., Liu, X.D.and Wu, J.X. Astron. Astrophys. Suppl., 72, 163-169 (1988). Zhang, F.J. and B ~i~th, L.B. Astr. Astrophys. 236, 47-52 (1990).