The millimetre variability of M81 -- Multi-epoch dual frequency mm-observations of the nucl
a r X i v :a s t r o -p h /0611844v 1 28 N o v 2006
Astronomy &Astrophysics manuscript no.6383c
ESO 2008February 5,2008
The millimetre variability of M81*
Multi-epoch dual frequency mm-observations of the nucleus of M81
R.Sch¨o del 1,M.Krips 2,S.Marko ?3,R.Neri 4,and A.Eckart 1
1
I.Physikalisches Institut,Universit¨a t zu K¨o ln,Z¨u lpicher Str.77,D-50937K¨o ln,Germany
e-mail:rainer@ph1.uni-koeln.de,eckart@ph1.uni-koeln.de
2Harvard-Smithsonian Center for Astrophysics,SMA project,645North A’ohoku Place,Hilo,HI 96720e-mail:mkrips@https://www.wendangku.net/doc/0f19358847.html,
3Sterrenkundig Instituut “Anton Pannekoek ”,Universiteit van Amsterdam,Kruislaan 403,1098SJ Amsterdam,The Netherlands e-mail:sera@science.uva.nl
4
Institut de Radio Astronomie Millim′e trique,300rue de la Piscine,Domaine Universitaire,38406Saint Martin d’H`e res,France e-mail:neri@iram.fr
Received September ;accepted
ABSTRACT
Aims.There are still many open questions as to the physical mechanisms at work in Low Luminosity AGN that accrete in the extreme sub-Eddington regime.Simultaneous multi-wavelength studies have been very successful in constraining the properties of Sgr A*,the extremely sub-Eddington black hole at the centre of our Milky Way.M81*,the nucleus of the nearby spiral galaxy M81,is an ideal source to extend the insights obtained on Sgr A*toward higher luminosity AGN.Here we present observations at 3and 1mm that were obtained within the framework of a coordinated,multi-wavelength campaign on M81*.
Methods.The continuum emission from M81*was observed during three epochs with the IRAM Plateau de Bure Interferometer simultaneously at wavelengths of 3and 1mm.
Results.We present the ?rst ?ux measurements of M81*at wavelengths around 1mm.We ?nd that M81*is a continuously variable source with the higher variability observed at the shorter wavelength.Also,the variability at 3and 1mm appears to be correlated.Like Sgr A*,M81*appears to display the strongest ?ux density and variability in the mm-to-submm regime.There remains still some ambiguity concerning the exact location of the turnover frequency from optically thick to optically thin emission.The observed variability time scales point to an upper size limit of the emitting region of the order 25Schwarzschild radii.
Conclusions.The data show that M81*is indeed a system with very similar physical properties to Sgr A*and an ideal bridge toward high luminosity AGN.The data obtained clearly demonstrate the usefulness and,above all,the necessity of simultaneous multi-wavelength observations of LLAGN.
Key words.LLAGN –mm-observations –galactic nucleus
1.Introduction
At high accretion rates onto black holes,the infall of gas and dissipation of energy via mostly thermal processes in a thin disk are fairly well understood in general terms.In con-trast,there is still great debate about which physical mech-anisms are dominant in objects which are accreting at well below the Eddington rate.The most extremely sub-Eddington source currently accessible to observations that allows still rea-sonable statistics for modelling the accretion /emission mech-anisms is the central supermassive black hole of our Galaxy,Sagittarius A*(Sgr A*).Its low luminosity has perplexed theo-rists for decades,and stimulated the re-emergence of radiatively ine ?cient accretion ?ow models (RIAFS,see Melia &Falcke 2001;Quataert 2003),including the advection dominated ac-cretion ?ow model (ADAF,e.g.Narayan &Yi 1994)and its derivatives.These models are characterised by low e ?ciency in converting thermal energy into electromagnetic radiation,but di ?er in terms of the inner boundary conditions such as the presence of strong winds (e.g.Blandford &Begelman 1999)and /or convection (Quataert &Gruzinov 1999),each
of
2R.Sch¨o del et al.:The millimetre variability of M81* as contenders.Recently,Eckart et al.(2004,2006)were suc-
cessful in obtaining the?rst simultaneous measurements of the
emission from Sgr A*at X-ray/NIR wavelengths(with quasi-
simultaneous sub-mm observations).These and ongoing coordi-
nated multi-wavelengths campaigns deliver the decisive obser-
vations for constraining/eliminating weak accretion models fur-
ther.
The nearby spiral galaxy M81(NGC3031)is an Sb spiral
galaxy similar to the Milky Way.It is located at a distance of
3.63±0.34Mpc(Freedman et al.1994).Devereux et al.(2003)
used spectroscopic measurements of the Hα+[N II]emission,
probably emitted from a rotating gas disk inclined at an angle of
14?±2?,to infer a mass of7.0+2
?1×107M⊙for the central black
hole in M81.
The nucleus of M81,termed M81*,shows typical signs of AGN activity.It has a power-law,variable X-ray continuum (Ishisaki et al.1996;Page et al.2004).The X-ray?ux from M81 is highly variable,at scales from days to years(La Parola et al. 2004).The nucleus of M81displays double peaked,broad Hαemission lines(Bower et al.1996).However,the overall lumi-nosity and AGN characteristics of M81*are rather weak and the galaxy is classi?ed as a LINER(low-ionisation nuclear emission-line region,e.g.Ho et al.1996).With a luminosity of the order of1037erg s?1in the radio and1040erg s?1in the optical/X-ray domains(see,e.g.,compilations by Ho et al.1996; Ho1999),its luminosity is<10?5times the Eddington luminos-ity in any wavelength regime.M81*is therefore counted among the low-luminosity AGN(LLAGN).It shows the typical spectral energy distribution(SED)of this class of sources,that is char-acterised by the absence of the so-called big blue bump,the ul-traviolet excess found commonly in the higher power AGN(e.g. Ho1999).
At cm-wavelengths,M81*shows large-amplitude variations (factors up to two at2cm)with timescales of a few months and weaker changes of the?ux density on timescales≤1day (Ho et al.1999).Multi-epoch VLBI observations of M81*at ～0.01pc resolution at20epochs over4.5yr reveal a stationary core with a variable(on timescales of～1yr)one-sided jet of length1mas(3600AU)towards the northeast(Bietenholz et al. 2000).As for the polarisation properties of M81*,circular po-larisation was detected at4.8,8.4(Brunthaler et al.2001),and 15GHz(Brunthaler et al.2006),while linear polarisation ap-pears to be absent at these frequencies.This is an intriguing sim-ilarity to Sgr A*,where circular polarisation also dominates over linear polarisation at these wavelengths(Bower et al.1999a,b,c; Aitken et al.2000;Bower et al.2003,2005).
Sakamoto et al.(2001)present observations of the central kiloparsec of M81at a wavelength of3mm in the CO J=1?0 line and continuum at100pc resolution.They detect molecular gas in a pseudoring or spiral arm at about500pc,but no giant molecular cloud within about300pc of the nucleus.They?nd signi?cant intraday variation of the continuum emission from M81*,suggesting an emitting region of～100AU.
Reuter&Lesch(1996)obtained a spectrum of M81*from the radio to the mm-regime.They?nd an inverted spectrum up to100GHz.Its?ux density can be described well by the law Sν∝ν1/3exp?(ν/νc)with a turnover frequency ofνc= 200GHz.They point out the remarkable similarity between the spectrum of M81*and Sgr A*and conclude that the same physi-cal mechanisms might operate in both galactic nuclei.Therefore Reuter&Lesch(1996)suggest that M81*may be a by～104up-scaled version of Sgr A*.
Thus M81*is a unique source for comparison with Sgr A* and more powerful AGN,and constitutes the next logical step after the successful multi-wavelength observations of Sgr A*. With this aim,a coordinated,multi-wavelength campaign took place in the?rst half of2005,involving instruments from the X-ray to the radio domain:the Chandra X-ray observatory,the Lick telescope(NIR),the SMA,the Plateau de Bure Interferometer (PdBI),the VLA,and the GMRT.A compilation of all observa-tions and the interpretation of the multi-wavelength data is pre-sented by Marko?et al.(in preparation).A detailed description of all instruments and the related data reduction would overload the multi-wavelength paper.Therefore,some papers are dedi-cated to the observations with speci?c instruments.The X-ray emission lines measured in the Chandra observations are dis-cussed by Young et al.(in preparation).In this paper we focus on the three epochs of mm-observations of M81*that were ob-tained with the PdBI during this campaign.
2.Observations and data reduction
M81*was observed with the PdBI on24February,14-15July, and19-20July2005.Six antennae were used for the February observations in the B con?guration of the PdBI,which provides typical beam sizes around1.5′′at～100GHz and around0.8′′at ～200GHz.The July observations were done with?ve antennae in the more compact D con?guration,with typical beam sizes around5′′at～100GHz and2.5′′at～200GHz.All observations are listed in Table1.
The observations aimed to detect continuum emission from M81*.However,due to good conditions the receivers were tuned to the12CO J=1?0and J=2?1transitions at115.3and 230.5GHz in the February observations.Thus,it was possible to search for compact CO-emission within～20′′of the nucleus, while the continuum could be extracted from the line-free chan-nels(The results of the CO-line imaging will be discussed in a forthcoming paper).For greater phase stability in the July obser-vations,an epoch during which the atmosphere at the site con-tains more water vapour,the receivers were tuned to frequencies of80.5and86.2GHz at3mm and241.4and218.2GHz at1mm.
The sources3C273(February),3C454.3(July14-15)and 1741-038(July19-20)were used for bandpass calibration. Phase calibration was performed with the sources1044+719and 0836+710.Primary?ux calibrators to determine the e?ciencies of the antennae were the sources1044+719for February23-24(1.6Jy at3mm/1.1Jy at1mm),1044+719(1.8Jy at3mm) and2200+420(8.7Jy at1mm)for July14/15,and MWC349 (1.0Jy at3mm)and3C454.3(33.0Jy at1mm)for July20.The phase calibrators1044+719and0836+710were used to?t the time-dependent?uctuations of the amplitude for all baselines. Various tests were performed for estimating the uncertainty of the?ux calibration,https://www.wendangku.net/doc/0f19358847.html,ing di?erent primary?ux calibrators and comparing the resulting?uxes of all observed sources.As an additional test,from a comparison of the calibrated?uxes of the sources0836+710,1044+719,and MWC349between the three epochs we estimated the uncertainty of the absolute?ux calibra-tion.To provide a quantitative measurement of the data quality, Table2lists the ranges of the rms values of the phase and am-plitude that were obtained during calibration of these values for the di?erent baselines for each observing epoch.The table also provides values for the estimated uncertainty of the absolute?ux calibration at3mm and1mm.As can be seen,the quality of the February data is highest.The data from20July are clearly better than the data from14/15July.
Individual scans of20min duration were extracted from the calibrated data.Subsequently,a point source was?tted to the re-sulting UV tables in order to determine the?ux of M81*and of
R.Sch¨o del et al.:The millimetre variability of M81*3 Table1.Observations of M81*with the PdBI during2005.ν3mm is the exact frequency used around3mm,ν1mm the one at a wavelength of1mm.N Ant is the number of antennae,“Con?g”refers to the antenna con?guration used.
Start[UT]End[UT]ν3mm[GHz]ν1mm[GHz]N Ant Con?g
Table2.Data quality:The table lists the ranges of the rms values of the phase and amplitude between all baselines for each observing epoch and wavelength.Also,the estimated systematic error of the absolute?ux calibration is indicated.
3mm1mm1mm
phase rms[deg]absolute absolute
10-20≤10%≤15% 14/15-JUL-20058-1140-6025-30
12-28≤15%≤20%
4R.Sch¨o del et al.:The millimetre variability of
M81*
Fig.1.Light curves of M81*and of the calibrators 1044+719and 0836+710from the PdBI observations at 3and 1mm.The x-axes show UT in hours.The ?uxes were all scaled to an average value of 1,with the curves of the calibrators shifted for better
comparison.The vertical lines in the top panels indicate sections for which the average ?ux and the standard deviation of the individual measurements have been calculated.The corresponding values are indicated.a sinusoidal pattern due to polarisation were observed at 3mm,one would expect to observe a similar pattern at 1mm,shifted by several hours.We do not detect the expected sinusoidal pattern in the light curves nor are the light patterns shifted by several hours in time relative to each other.b)Recent observations of M81*with the BIMA array by Brunthaler et al.(2006)indicate
absence of or very low upper limits of a few percent on linear and circular polarisation of M81*at 86and 230GHz.
R.Sch¨o del et al.:The millimetre variability of M81*
5 Fig.2.Light curves of M81*and of the calibrators1044+719and0836+710from the PdBI observations at3and1mm on February 24.The light curves were corrected for possible systematic variations by forcing the calibrator sources to have constant?ux(on
average).
Table3.Average?ux densities of M81*,S3mm and S1mm,measured atν3mm andν1mm during the three epochs of the campaign. The
mean and standard deviation(not the error of the mean)were calculated from the unweighted individual measurements shown in Fig.3.The last column lists the average spectral indices and their standard deviation(not the error of the mean)calculated from all individual measurements as shown in the right panels of Fig.4.
Start[UT]End[UT]ν3mm[GHz]S3mmν1mm[GHz]S1mmα
Fig.3.Flux density of M81*on24February,14/15July,and20July2005.The black boxes mark the?ux density at3mm and the green(gray)crosses mark the?ux density at1mm.The exact frequencies are indicated in the titles of the individual panels.The error bars indicate relative,not absolute,uncertainties.
4.Discussion
Theory and observations indicate that sub-Eddington black holes
are jet-dominated(Falcke&Marko?2000;Falcke et al.2004;
Yuan et al.2002;Fender et al.2003).A compact,variable jet has
in fact been detected in VLBI observations of M81*(Bietenholz
et al.,2000;see also Marko?et al.,in preparation).Falcke
(1996)demonstrate that a compact jet can explain the observed
spectrum of M81*very well.In the jet model,the turnover fre-
quency from a?at(or slightly inverted)radio spectrum to an
optically thin power-law occurs at a frequencyνt.In a simpli?ed
model this frequency depends on the jet power,Q j,and mass of
the black hole,M BH,asνt∝Q2/3
j
M?1
BH
(Falcke et al.2004).In
LLAGN the turnover occurs generally at(sub)mm wavelengths,
while it is located in the near-infrared/optical regime for XRBs
in the low/hard state(see Corbel&Fender2002;Marko?et al.
2003;Falcke et al.2004).
The simultaneous observations of M81*at3and1mm pre-
sented in this work are consistent with a turnover of the syn-
chrotron emission from a jet in M81*into the optically thin
part between3mm and1mm(with the exception of the?rst
～4hours of the24February light curve that would indicate a
higher turnover frequency).This is in good agreement with the
data presented by Reuter&Lesch(1996)that were,however,not
acquired simultaneously.
6R.Sch¨o del et al.:The millimetre variability of
M81*
Fig.4.Spectral index of M81*on24February,14/15July,and20July2005.The spectral index is de?ned by Sν∝να,where Sνis the?ux density at a given frequencyν.The absolute uncertainties(see Tab.2)of the?ux densities at3and1mm have been taken into account for this plot.Since we use upper limits on the absolute uncertainties,the plotted error bars are conservative.
The evidence for the location of the turnover frequency be-tween3and1mm is ambiguous,however.The multi-frequency radio data from the coordinated campaign indicate that bumps are present in the radio spectrum(Marko?et al.,in prep.).These bumps are located at di?erent frequencies at di?erent epochs. Therefore,in spite of the presented mm-data,it may well be pos-sible that there is a submm-bump present in M81*.Observations with the SMA at345Ghz that were obtained during the coordi-nated campaign on M81*indicate that the?ux density of M81* increases toward the submm regime,in agreement with theo-retical predictions on the existence of a submm-bump(Marko?et al.,in preparation).Unfortunately,there was only one epoch (24February2005),where measurements with the PdBI and the SMA were simultaneous.
For?ne-tuning the models of the emission of M81*it is im-portant to know the exact turnover frequency and whether and how it varies with activity of the source.Therefore there is an urgent need for more observational data for comparison with theory in order to understand sub-Eddington accretion and emis-sion.It is essential that the observations across the wavelength regimes are simultaneous.
Due to the gap of several months there is probably no corre-lation between the February and July observations because the light curves at2cm presented by Ho et al.(1999)indicate that radio outbursts of M81*have generally timescales<5months. Also,Sakamoto et al.(2001)have found that M81*shows?ux variations of factors≤2at a wavelength of3mm on time scales <10days.However,the decaying light curve from July19may be related to the same event as the light curve on July14.The spectral index during the two epochs is negative and of similar absolute value(see Tab.3and Fig.4).This behaviour is con-sistent with a decaying light curve from a low-peaking?are in the model by Valtaoja et al.(1992)that describes variability phe-nomena in AGN by a growth/decay of shocks in a jet(see also Marscher&Gear1985).
An estimate of the size scales of the relevant processes can be obtained from the time scales of the observed variability.The drop-o?between07:00UT and12:00UT in the highest quality data set from24February2006shows that a 5σvariability of the?ux at3mm occurs on a time scale of5hours(see also the intraday variability at3mm found by Sakamoto et al.2001).
A similarly rapid change of the?ux can be seen at1mm.Since no signal can travel faster than at the speed of light this cor-responds to an upper limit on the size of the source of merely ～25Schwarzschild radii,when a mass of7.0×107M⊙is as-sumed for the black hole in M81*.5.Summary
We present three epochs of simultaneous1and3mm contin-uum observations of the LLAGN M81*that were obtained in the framework of a coordinated,multi-wavelength campaign (Marko?et al.,in preparation).
The observations of M81*with the PdBI at mm-wavelengths con?rm that M81*is a continuously varying radio contin-uum source as has been found previously,mostly at larger wavelengths(see Ho et al.1999;Bietenholz et al.2000; Sakamoto et al.2001).The measurements present the?rst un-ambiguous detection of M81*at1mm and moreover show that the source is continuously variable at this wavelength as well. The amplitude of the variability is observed to be generally larger at1mm than at3mm by a factor of roughly1.5.This agrees well with the trend found by Ho et al.(1999)that the am-plitude of the variability increases with frequency in LLAGN.A similar behaviour is found in the far weaker source Sgr A*(e.g. Herrnstein et al.2004;Miyazaki et al.2004).
The shortest variability time scales of our observations give upper limits on the size of the emitting region of ～25Schwarzschild radii,assuming a black hole mass of7.0×107M⊙.
The decaying light curves observed on14/15and on20July 2005may have been related to the same radio outburst.They are consistent with a generalized shock-in jet model(Valtaoja et al. 1992).
The simultaneous measurements of M81*at3and1mm are consistent with a turnover of the?ux in the mm-to-submm regime as predicted by models for a jet dominated source.Some ambiguity remains concerning the exact peak frequency and the related interpretation of variability events.
The observations con?rm previous?ndings that there are many similarities between M81*and Sgr A*,the source re-lated to the supermassive black hole at the center of the Milky Way.This underlines the similarity between LLAGN despite of several orders of magnitudes of di?erence between their lumi-nosities.M81*can serve as a bridge from the extremely sub-Eddington Sgr A*toward higher luminosity LLAGN.The radio emission from M81*is apparently dominated by a compact jet. The same may be the case for Sgr A*,although such a jet has not yet been discovered due to the strong interstellar scattering toward this source.This work poses a clear case for follow-up observations of the highly sub-Eddington nucleus M81*and for the necessity of simultaneous multi-wavelength observations in general in order to constrain the physical mechanisms in black
R.Sch¨o del et al.:The millimetre variability of M81*7 holes that accrete at rates several orders of magnitude below the
Eddington limit.
Acknowledgements.Part of this work was supported by the Deutsche
Forschungsgemeinschaft,DFG project number494.We thank Lena Lindt and
Jan Martin Winters for their help with the data reduction.Special thanks to
M.Nowak fo his e?orts in setting up the coordinated campaign on M81.
References
Aitken,D.K.,Greaves,J.,Chrysostomou,A.,et al.2000,ApJ,534,L173
Bagano?,F.K.,Bautz,M.W.,Brandt,W.N.,et al.2001,Nature,413,45
Bietenholz,M.F.,Bartel,N.,&Rupen,M.P.2000,ApJ,532,895
Blandford,R.D.&Begelman,M.C.1999,MNRAS,303,L1
Blandford,R.D.&Konigl,A.1979,ApJ,232,34
Bower,G.A.,Wilson,A.S.,Heckman,T.M.,&Richstone,D.O.1996,AJ,111,
1901
Bower,G.C.,Backer,D.C.,Zhao,J.,Goss,M.,&Falcke,H.1999a,ApJ,521,
582
Bower,G.C.,Backer,D.C.,Zhao,J.-H.,Goss,M.,&Falcke,H.1999b,ApJ,
521,582
Bower,G.C.,Falcke,H.,&Backer,D.C.1999c,ApJ,523,L29
Bower,G.C.,Falcke,H.,Wright,M.C.,&Backer,D.C.2005,ApJ,618,L29
Bower,G.C.,Wright,M.C.H.,Falcke,H.,&Backer,D.C.2003,ApJ,588,
331
Brunthaler,A.,Bower,G.C.,&Falcke,H.2006,A&A,451,845
Brunthaler,A.,Bower,G.C.,Falcke,H.,&Mellon,R.R.2001,ApJ,560,L123
Corbel,S.&Fender,R.P.2002,ApJ,573,L35
Devereux,N.,Ford,H.,Tsvetanov,Z.,&Jacoby,G.2003,AJ,125,1226
Eckart,A.,Bagano?,F.K.,Morris,M.,et al.2004,A&A,427,1
Eckart,A.,Bagano?,F.K.,Sch¨o del,R.,et al.2006,A&A,450,535
Falcke,H.1996,ApJ,464,L67+
Falcke,H.&Biermann,P.L.1995,A&A,293,665
Falcke,H.,K¨o rding,E.,&Marko?,S.2004,A&A,414,895
Falcke,H.&Marko?,S.2000,A&A,362,113
Fender,R.P.,Gallo,E.,&Jonker,P.G.2003,MNRAS,343,L99
Freedman,W.L.,Hughes,S.M.,Madore,B.F.,et al.1994,ApJ,427,628
Genzel,R.,Sch¨o del,R.,Ott,T.,et al.2003,Nature,425,934
Ghez,A.M.,Wright,S.A.,Matthews,K.,et al.2004,ApJ,601,L159
Herrnstein,R.M.,Zhao,J.-H.,Bower,G.C.,&Goss,W.M.2004,AJ,127,
3399
Ho,L.C.1999,ApJ,516,672
Ho,L.C.,Filippenko,A.V.,&Sargent,W.L.W.1996,ApJ,462,183
Ho,L.C.,van Dyk,S.D.,Pooley,G.G.,Sramek,R.A.,&Weiler,K.W.1999,
AJ,118,843
Ishisaki,Y.,Makishima,K.,Iyomoto,N.,et al.1996,PASJ,48,237
La Parola,V.,Fabbiano,G.,Elvis,M.,et al.2004,ApJ,601,831
Marko?,S.,Falcke,H.,&Fender,R.2001a,A&A,372,L25
Marko?,S.,Falcke,H.,Yuan,F.,&Biermann,P.L.2001b,A&A,379,L13
Marko?,S.,Nowak,M.,Corbel,S.,Fender,R.,&Falcke,H.2003,A&A,397,
645
Marscher,A.P.&Gear,W.K.1985,ApJ,298,114
Melia,F.&Falcke,H.2001,ARA&A,39,309
Miyazaki,A.,Tsutsumi,T.,&Tsuboi,M.2004,ApJ,611,L97
Narayan,R.&Yi,I.1994,ApJ,428,L13
Page,M.J.,Soria,R.,Zane,S.,Wu,K.,&Starling,R.L.C.2004,A&A,422,
77
Quataert,E.2003,Astronomische Nachrichten Supplement,324,435
Quataert,E.&Gruzinov,A.1999,ApJ,520,248
Reuter,H.-P.&Lesch,H.1996,A&A,310,L5
Sakamoto,K.,Fukuda,H.,Wada,K.,&Habe,A.2001,AJ,122,1319
Valtaoja,E.,Terasranta,H.,Urpo,S.,et al.1992,A&A,254,71
Yuan,F.,Marko?,S.,&Falcke,H.2002,A&A,383,854