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Astroparticle Physics

Astroparticle Physics 11 (1999) 193-195

www.elsevier.nl/locate/astropart

Short time scale variability in Mkn 501 Thomas J. Maccarone, Paolo S. Coppi

Deparbnent of Astronomy, Box 208101. Yale Universiiy, New Haven, CT 06511, USA

Abstract

In the TeV blazars Mkn421 and MknSOl, the same electrons are thought to be responsible for the synchrotron X-rays and TeV Comptonized emission. As a prelude to simultaneous X-ray/TeV analysis, we present results from variability studies of Mkn 421 and Mkn 501. We find that each source has a variability of 5% r.m.s. amplitude on time scales shorter than 20 minutes. We find some evidence that the hard X-ray flux may be more variable than the soft X-ray flux in Mkn 421. For Mkn 501 we also present comparisons with nearly simultaneous TeV observations taken by the HEGRA collaboration. @

1999 Elsevier Science B.V. All rights reserved.

Keywords; BL Lac objects: individual (Mkn 501, Mkn421); X-rays: observations

1. Introduction

The spectra of BL Lac objects show two peaks, one in the infrared to X-ray regime, and the other at

X-ray to y-ray energies. It is thought that the same

electrons are responsible for producing both peaks, the lower one through synchrotron radiation and the

higher energy peak through Comptonization. Study- ing the short time scale variability of both the syn-

chrotron and Compton peaks is necessary to under- stand whether the seed photons for the Compton ra- diation are internal (synchrotron self-Comptonization or SSC models) or external (external radiation Comp- tonization, or ERC models) to the jet.

2. Observations

We looked at archival XTE data from the sources Mkn 421, from April 2-12, 1997, and April 30-May 6, 1997, as well as Mkn 501, from July 11-16, 1997. We then compared the results for Mkn 501 with the

already published TeV observations of the HEGRA

telescope. Spectral fits for the July 1997 Markarian 501 data which differ from our fits only in that we used the more recent response matrices for XTE have been

previously published (Lamer and Wagner 1998).

3. Variability analysis

We applied X-squared and Kolomogorov-Smirnov tests to the lightcurves of the two objects. We also compute structure functions for each object. Note that the structure function for the hard X-rays in Mkn 421 rises more quickly than that for the soft X-rays (Fig. 1)) and that the chi-squared test reveals variabil- ity at the 3~ level on time scales of about 1300 sec- onds in both the 4.7 to 9.8 keV and 9.8 to 20.5 keV

bands, while it takes about 2000 seconds for variabil- ity to be revealed in the 2.5 to 4.7 keV band. Count- ing statistics prevent searches on finer energy scales or at higher energies. The maximum r.m.s. variations are about 3% in the lowest band, about 4% in the

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194 7X Maccarone, PS. Coppi/Astroparticle Physics II (1999) 193-195

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Fig. 1. The structure function for Mkn 421 in the energy bands 254.7 keV (circles) and 4.7-9.8 keV (triangles). The structure function

is defined as V*(T) = ([ s( t + T) - s(t) I*), so it is in units of absolute counts squared, which is why the variability in the low band,

which has more source counts, eventually overtakes the high energy variability.

Fig. 2. The above plot shows the X-ray and TeV luminosities plotted against one another, In both cases the lines shown represent the

best fits to the data. In the X-rays, the lines drawn are the 1~ error bounds of the fits. The X-ray observation start times are (MJD):

50640.97 (solid, thick), 50641.97 (dashed,thick), 50642.97 (dashed,thin), 50643.15 (dot-dashed, thin). The TeV observation start times

are: 50641.03 (solid,thick), 50642.05 (dashed,thick), 50643.07 (dashed,thin).

middle band and about 15% in the highest band on the 2000 second time scale. Similar results are found

for Mkn 501, albeit at lower amplitude. No variabil- ity is found from 2.5 to 4.0 keV using the chi-squared test, while time scales of 3400 seconds and 2000 sec- onds are found for ~IY variability in the 4.0 to 7.0 and 15.0 to 30.0 keV bands, respectively. The two lower bands show r.m.s. variability amplitudes of less than 2%, while the highest shows roughly 7% variability on 2000 seconds time scale. In no source does the KS test show variability within a single XTE dwell (a

dwell is defined as any period of time where the point- ing instruments of XTE continually point at the same

location on the sky). Cross-correlation function anal- ysis reveals a hard lag of questionable significance in both sources, and in contradiction to previous results (Takahashi et al. 1996). This, if true, would contra- dict the simplest picture of synchrotron radiation as the source of the X-ray emission. Hardness-intensity comparisons reveal no conclusive results.

T. J. Maccarone, PS. Coppi/Astroparticle Physics 1 I (I 999) 193-I 95 19.5

4. Nearly simultaneous TeV observations

We compare the results in the X-rays with the TeV observations from the HEGRA collaboration, which

were generally taken within a few hours of the XTE dwells ( Aharonian et al. 1998). A simple SSC model can fit the data for Mkn 50 1 in both the X-ray and TeV

bands, but significant fine-tuning is required to fit the same model to the particular X-ray and TeV observa- tions which are closest in time. The one TeV obser-

vation that is separated by less than 5 hours from two different X-ray observations is sandwiched between the highest and lowest X-ray flux states in the sample. Additionally, over the three day span of the observa- tions, the TeV luminosity varies by a factor of two,

while the X-ray luminosity varies only by about 30%.

The confusing nature of these results underscores the

importance of getting truly simultaneous X-ray/TeV observations as opposed to the nearly simultaneous

results that have been obtained so far. These observa-

tions are shown in Fig. 2.

5. Conclusions

We examine the variability of the two brightest ex-

tragalactic TeV sources. We find that on time scales of less than a few hours, the sources are more strongly variable in the hard X-ray bands than in the soft bands. Finally, we find that both the X-rays and TeV gamma rays vary in intensity rapidly enough that we cannot fit the same models to multiwavelength observations

separated by as little as a few hours.

Acknowledgements

This research has made use of the NASA Astro-

physics Data Archive.

References

Aharonian F. et al., astro_ph/9808296. preprint.

Lamer G., Wagner S.J., 1998, A&A 331L. 13.

Takahashi T. et al. 1996. ApJL 470. L89.