R.Nesci1, A. Maselli2, F. Montagni3, S.Sclavi1

1) Universita’ La Sapienza, Roma; 2) INAF IASF-Palermo; 3) Osservatorio Greve in Chianti

S5 1803+784 revisited

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What is S5 1803+784?

S5 1803+784 is a BL Lac object, a special class of Active Galactic Nuclei (AGN).

BL Lacs take their names form the prototype source, discovered at Sonnenberg in 1929 and then catalogued as the variable star BL Lacertae.

They are charcterized by:

1. featureless optical continuum

2. large and fast variability

3. Appreciable polarization

4. Flat radio spectrum

Most BL Lacs are X-ray sources, and are a substantial part of the extragalactic sources detected in Gamma-rays.

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Current unified model

The current model to explain the emission of BL Lacs is based on a supermassive black-hole, hosted in the galaxy centre which accrete matter from the surroundings.

Part of the matter in the accretion disk is funnelled in narrow relativistic jet, oriented at small angle with the observer line of sight: the existence of such jets has been proved by VLBI radio observations

Most of the observed emission is believed to be produced in this jet, by two different processes:

1. Sinchrotron radiation of relativistic electrons in a strong magnetic field, typically peaking at NIR frequencies;

2. Inverse Compton radiation from these electrons on the ambient photons, typically peaking in the hard X-rays.

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Spectral energy distribution (SED) classification

• Depending on the energy of the electrons and intensity of the magnetic field, the peak of the synchrotron emission, in the Log(nu)-Log(nu*Fnu) plane, may range between far IR and X-rays.

• Accordingly, BL Lacs are classified as Low-frequency (LBL), intermediate frequency (IBL) and high frequency (HBL) peaked sources.

• During a flare, the SED shape is deformed and generally shifted towards higher frequencies.

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Unified scheme

• A schematic view seen edge-on is in this picture

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Time variability

• The light curves of several BL Lacs have been studied since their discovery, also using historical plate archives for long term variability studies.

• Their behaviour has proven to be generally quite erratic, save the case of OJ 287 (Sillampaa et al 1998) which has a periodicity of about 11 years for the major flares.

• In a number of cases time scales of a few years have been found, even though not strict periodicities (e.g. S5 0716+714, Nesci et al. 2005; AO 0235+174, Raiteri et al. 2008; GB6 J1058+5628, Nesci 2010).

• Long term trends have also been found in a few sources, superposed to the short term ones, extending for tens of years and possibly being just part of longer recurrency time-scale (e.g. OQ 530, Massaro et al. 2004; ON 231, Massaro et al. 2001; S5 0716+714, Nesci et al. 2005).

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Possible models

• The long term variability may be due to intrinsic changes in the source or to geometrical effects: in this case the best explanation could be a precession of the relativistic jet, producing a variation of the Doppler boosting and therefore an achromatic variation of the apparent source luminosity.

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The source in short•S5 1803+78 is a bright radio source discovered in 1981 (Kuhr et al.) . •Due to its circumpolar position it can be followed for a large part of the year. •It has a large optical polarization and a redshift z=0.680, based on a single emission line assumed to be MgII (2900 A).•It is well monitored in the radio band because it is a source of the ICRS reference frame and is used also as a geodetic reference source.•It was observed by several X-ray satellites, but was not pointed by EGRET.•Its first optical systematic monitoring was made by Nesci et al. (2002) in the years 1996-2001.•The overall SED is of the LBL type

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Optical follow-up

• Most of the well monitored LBL sources (e.g. BL Lacertae itself), show a correlation between optical luminosity and optical spectral slope, being “bluer when brighter”.

• On the contrary, S5 1803+78 showed a very limited variation in the optical spectral slope, despite it varied by more than 2 mag in flux.

• We decided therefore to continue its monitoring, to check if:• A) the color variation remained small in time;• B) the source showed any recurrent time scale in flux variations;• C) the source showed monotonic long term trend.

The monitoring was performed with the telescopes of Asiago (183 cm), Loiano (152cm), Vallinfreda (50cm) and Greve in Chianti (30cm), mainly in the R band, but with several observations also in the B, V, and I bands.

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The light curve from 1996 to 2010

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expanded

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Main characteristics

1. The source underwent a number of flares:2. We derived by eye estimates of the starting and ending point

for the raising and falling branches of each major well sampled flare;

3. The luminosity variation rates range between 0.03 and 0.07 mag/day.

4. There is no strong evidence that raising and falling rates are statistically different.

5. Inspection of the light curve (by eye and by FFT analysis) shows no strict periodicity for the flares.

6. A possible time scale of 1300 days between major flares may be present.

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Table of major flares parametersRaising

JD

Start

R Start

JD End

R End

Length days

Delta

Mag

Rate mag/day

0669 15.94 0711 14.42 42 1.5 0.036

1108 15.70 1125 14.39 17 1.3 0.077

1929 15.22 1338 14.16 46 1.06 0.02

2400 1543 2421 14.63 21 08 0.04

3144 16.10 3161 15.33 17 0.8 0.047

Falling

0711 14.40 0731 15.80 20 1.4 0.07

1344 14.04 1480 16.04 140 2.0 0.014

1613 15.33 1655 16.82 42 1.5 0.036

2442 14.40 2466 15.90 24 1.5 0.06

2979 15.34 2994 16.83 15 1.5 0.10

3558 14.33 3583 15.49 25 1.2 0.05

5212 14.90 5262 16.34 50 1.4 0.03

5212 14.90 5228 15.56 16 0.6 0.04

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Optical color index

1. The (V-I) color index showed small variations around the average value of <V-I>=1.1 during all our monitoring, a typical value for LBL sources, corresponding to a spectral index -1.6.

2. The most accurate measures, made with the larger Asiago and Loiano telescopes during the strong flare at JD 3550, showed a bluer color during the raising phase and a redder one in the falling part. A similar behaviour was detected in a few other BL Lacs (S5 0716+714, 3C 66A, Ciprini et al. 2004).

3. The UV spectral slope observed by Swift-UVOT was consistent with the optical one from our BVRI data, indicating a common origin (tail of the Synchrotron emission) for the optical and UV light.

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Color variations during major flares: case 1

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Case 2

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Case 3

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The X-ray spectrum

1. The X-ray spectrum was observed by SWIFT in four occasions between 2007 and 2009,

2. It always showed a flux level comparable to that observed by BeppoSAX in 1998, with small variations.

3. Also the X-ray spectral slope (photon index) was stable, around +1.6, suggesting an Inverse Compton origin.

4. During these X-ray observations the R magnitude was not very different, ranging between 15.4 to 16.3: no clear correlation is evident between the X-ray and optical bands.

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X-ray spectral slope

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FermiGST observations

1. S5 1803+78 was not pointed by EGRET.

2. It was detected by Fermi-GST, which performs an all-sky monitoring, and is present in the 11-month catalogue (1FGL, Abdo et al. 2010a).

3. The Gamma-rays light curve, sampled weekly, shows an average level around 2x10-8 phot/cm2/s with oscillations within a factor 2.

4. Our optical light curve has only a few points around the end of this 11 months time interval, and shows a short increase of 0.3 mag two weeks before a short Gamma-ray bump.

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The Gamma ray flare

1. A strong flare, at 9x10-7 phot/cm2/s (E>100 MeV) was reported by Donato et al. (2010), 40 times brighter than the average value.

2. We could observe the source in the optical 4 days after the Gamma burst at R=14.9 with color index (V-I)=1.2.

3. It was already in the decreasing phase at a rate of 0.04 mag/day, a typical value for this source.

4. Extrapolating backwards this trend to the epoch of the Gamma-ray flare gives R=14.7 for the peak value, rather lower than the maximum recorded flares of this source (R=14.0-14.2).

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Spectral Energy Distribution (SED)

We used literature data, as well as our own observations and data analysis, to build the SED of the source. The hard X-ray flux in the 15-30 keV band has been derived from the 54-months BAT all-sky map with the BATIMAGER software (Segreto et al. 2010). Fermi-GST data have been retrieved from the ASI Science Data Center (ASDC) website.

For clarity, not all available data are reported in the figure.

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The SED of S5 1803+784

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SED description

1. The SED is typical of an LBL source, with two large bell-shaped parts:

2. the synchrotron component peaking between 1013 and 1014 Hz, is well fitted by a log-parabola (Maselli et al. 2008);

3. the Inverse Compton branch is well defined by the Swift-XRT, BAT and Fermi-GST instruments: a log-parabolic fit gives a peak at 3.8x1021 Hz, similar to that of other LBL objects observed by Fermi-GST.

4. The Gamma-ray photon spectral index is 2.33, within the range of values for LBL objects (2.21+-0.16, see Abdo et al. 2010b).

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Conclusions

• The optical light curve shows several large (2 mag) flares with a possible time scale of about 1300 days.

• Flux variations are slow, with typical rate of 0.04 mag/day, without marked difference between raising and falling branches.

• No monotonic long term trend is apparent on a 14-years time span.

• The optical spectral slope shows little (10%) variations even in large flares. Accuracy of 0.01 mag are necessary to study these variations.

• In one strong flare we had enough accuracy to detect a “bluer when rising” behaviour.

• The overall SED is well described by two log-parabola, with peak positions and slopes typical of LBL sources.

• Up to now, correlation between Gamma-ray and optical flares is based on one case only: if true we statistically expect another Gamma-ray flare within the lifetime of FermiGST.

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References

• Abdo et al. 2010a, arXiv:1002.0150v2; ApJ in press.• Abdo et al. 2010b, ApJ 710, 1271.• Donato D. et al. 2010, Atel #2386.• Fiorucci et al. 2004, A&A 419, 25.• Maselli A et al. 2008, Blazar Variability Across EM Spectrum,

at http://pos.sissa.it, p.77 .• Massaro et al. 2001, A&A 374, 435• Massaro et al. 2004, A&A 423, 935• Nesci R., et al. 2002, AJ 124, 53.• Nesci R. et al. 2005, AJ 130, 1466• Nesci R, 2010, AJ 139, 2425• Raiteri et al. 2008, A&A 485, L17• Segreto et al. 2010, A&A 510, 47• Sillampaa A et al. 1988 ApJ 325, 628