Variability of AGN on Diverse Time Scales

Alok C. Gupta

Yunnan Astronomical Observatory, Kunming, China

alok@ynao.ac.cn

Projects

• Optical / Near Infra-red Intra-day and Short Term Variability of Blazars (Gupta et al. 2004; 2007 a, b)

• Optical / Radio Long Term Variability of Radio-Loud AGN (Fan, ..., Gupta, et al. 2006, 2007)

• Quasi - Simultaneous two Color Optical Intra-day Variability of Radio-Quiet QSOs (Gupta & Joshi 2005; Gupta & Yuan 2007)

Outline• Introduction

• Motivation of the Projects

• Observations and Data Reductions

• Data from Published Archive

• Results

• Main Conclusions

• Ongoing and Future Projects

Introduction

Active Galactic Nuclei (AGN)

refers to the existence of energetic phenomenon in the nuclei, or central region of galaxies. Total energy emitted by the compact central source at visible wavelength is comparable to the energy emitted by all the stars in the Galaxy (i.e. ≈ 1011 L⊙).

AGN Classifications

• Radio-Quiet AGN (housed in spiral galaxy)

• Radio-Loud AGN (housed in elliptical galaxy)

If F5GHz / FB ≥ 10 (radio-loud AGN)

only about 10 – 15 % AGN are radio-loud.

• Super massive Black Hole at center surrounded by accretion disk

• Obscuring torus of gas and dust

• Clouds of line emitting gas

Different AGNs from Different Angles

• If jets are important. Blazars along jet axis, RLQSOs at modest angles and Radio Galaxies at larger angles.

• No jets: Luminous Quasars seen close to perpendicular to the disk

• Weaker: Type 1 or Type 2 Seyferts modest angle with disk or near disk plane.

Blazar is a subclass of radio-loud AGNs BL Lacs + FSRQs + OVVs + HPQs BLAZARS (named by Ed Spiegel in 1978 in a conference on BL Lac objects at Pittsburg). BL Lac objects have feature less spectrum. Blazars display variable polarization and flux variability in the complete EM spectrum. Blazars radiation at all wavelengths is predominantly non-thermal. Blazars make angle of < 200 between the line of sight and the emitted jet from the source (Urry & Padovani 1995). Blazars Classification LBL ↔ RBL (Red ↔ Low Energy ↔ Radio selected) HBL ↔ XBL (Blue ↔ High Energy ↔ X-ray selected)

Spectral Energy Distribution (SED)

Blazars SED have two peaks. The first component peaks anywhere from IR to Optical in LBL and at UV/X-rays in HBL. The second component extends upto Gamma-rays, peaking at GeV in LBL and at TeV in HBL.

The electromagnetic emission is dominated by synchrotron emission from relativistic electrons in the jet at low-energy and at high energy probably by inverse Compton (IC) scattering of

seed photons of the jet's electrons.

Blazars Variability

• Irregular

• Non periodic

• All time scales which can be broadly divided into 3 classes: (i) intra-day (ii) short term (iii) long term

• Intra-day VariabilityIDV (few minutes to

within a day) or intra-night or micro-variability

• Short term (days to weeks)

• Long Term (months to several years)

What are the sources of Intrinsic Variability?

• Shock - in - jet (IDV and short term)

• Instabilities or hot spots on the accretion disk(variability in Low-state and periodic variability)

• Binary black hole model (long term periodic variability)

Variability Models

• Shock-In-Jet Models: (i) relativistic shock

propagating down a jet and interacting with

irregularities in the flow (Qian 1991, Marscher et al.

1992); (ii) Relativistic shocks whose direction to the

line of sight is variable (Gopal-Krishna & Wiita 1992)

• Accretion Disk Based Models: Numerous

flares of hot spots on the surface of the accretion disk

belived to surround the central engine (Witta et al.

1991, 1992a,b; Chakrabarti & Wiita 1993; Mangalam &

Wiita 1993).

Shock- in – Jet Model

(From A. P. Marscher's website)

Motivation: Blazars & Radio-Loud AGNs

• Search for IDV in a large homogeneous sample of different classes of luminous AGNs (1. Blazars, 2. Radio-loud AGN (non blazars), 3. Radio-quiet QSOs) (statistics of IDV variability behaviour will give an important clue on the central engine of different classes of AGNs).

• Shortest time scale of IDV will give: size of emitting region, upper limit of mass of the black hole

• t = R/c, R = 5 Rs where R

s = 2GM/c2 ➔ M = c3t/10G

• Find the optical IDV and short term variability of blazars in low state (support for accretion disk based models).

• Search for periodicity in IDV and short term variability light curves (support for a prominent hot spot on accretion disk).

• Search for optical/NIR high states of blazars (useful to initiate WEBT multi-wavelength campaigns).

• Search for long term variability using large long time data base of radio-loud AGNs in Optical and Radio bands (useful to predict the next outburst time).

Radio-Quiet QSOs• Search for two colour quasi-simultaneous IDV in luminous radio-

quiet QSOs (-30.0 < Mv < -24.4) & (15 < V < 16.5). • Detection of IDV in radio-quiet QSOs will be a support to the

Wills (1996) work. He/she has mentioned that radio-quiet QSOs also posses jets, but they propagate through denser gas close to the host galaxies and are thus quickly snuffed out.

• IDV detection in radio-quiet QSOs can be also caused by the disturbances on the accretion disk.

• Quasi-simultaneous 2 optical bands (V & R), IDV. If V-R color variation is detected, it will support accretion disk based models (work in progress).

• Simultaneous CHANDRA (X-ray) and Optical observations of radio-quiet QSOs (planned). This will be helpful to know the real cause of IDV.

Telescopes Used & Archive Data

• 2.34 meter VBT, Kavalur and 1.2 meter Optical/NIR

Telescope, Mount Abu, India

• 1.02 meter, 1.56 meter and 2.16 meter Optical Telescopes

in China.

• 61 cm Optical and 1.8 meter Optical/NIR Telescopes in

South Korea.

• 1.4 meter NIR telescope in South African Astronomical

Observatory (SAAO).

• UMRAO (University of Michigan Radio Astronomy

Observatory). Data Base at 5.0, 8.0 and 14.5 GHz for more

than 33 years.

NIR Observations of Mrk 421 (z = 0.031) Gupta et al. (2004)

• Observations: Mrk 421 was observed in NIR J band during Feb. 25 – Mar. 5, 2003 using NICMOS-3 camera mounted on 1.2 meter Mount Abu, India telescope.

• Motivation: Observations were carried out in NIR J band. Source was also observed in this period by using RXTE (X-rays) and ground based gamma-ray telescopes. Search for IDV and short term variability.

• Data Reduction: Aperture photometry was done for Mrk 421 and standard stars image frames using APPHOT software.

• Photometric Calibration: It was done by using UKIRT standard stars HD 105601 and HD 95884.

Optical Observations of Blazar: Mrk 501 (z = 0.034), Gupta et al. (2007 a)

• Observations: Mrk 501 was observed in BVR optical

passbands during March - May 2000 (12 nights).• Telescopes: (a) 1.2 meter telescope, Mt. Abu, India

(b) 61 cm telescope, Sobaeksan, Korea • Motivation: Search for IDV & short term variability and

variability time scale in the low-state of the source. • Data Reduction: Aperture photometry was done by using

APPHOT task in IRAF software and stand alone software DAOPHOT.

• Photometric Calibration: Local standard star was used.

Structure Function analysis was used to get the time scale.

Optical Observations of 9 Blazars (Gupta et al. 2007b)

• Observations: 9 blazars were observed in V & R passbands

during Oct. 2006 – April 2007 (13 nights).• Telescope: 1.02 meter YNAO telescope, Kunming.

• Motivation: (i) Search for IDV and high state of blazars. Data is

also for WEBT campaigns for 4 blazars (Raiteri, ..., Gupta, et al. 2007). (ii) Blazars in low-state for IDV and short term variability.

• Data Reduction: Partial data analysis were done by using

stand alone software DAOPHOT II and Aperture Photometry technique (in progress).

• Photometric Calibration: Data were calibrated using local

standard stars.

Optical Observations of the Blazar: 4C 29.45 (z = 0.729) (Fan, .., Gupta et al. 2006)

• Observations: 4C 29.45 was observed in VRI optical

passbands during April 1997 – March 2002.• Telescopes: 1.56 meter Shanghai Observatory, China • Motivation: Search for long term variability. We also used

published data available of the source till date. • Data Reduction: Aperture photometry was done by using

APPHOT task in IRAF software.• Photometric Calibration:Local standard stars were used.

• Periodicity Analysis: Jurkevich and Power Spectral

Analysis method. False periods were cleaned by using CLEAN algorithm.

Long Term Radio Variability Properties of

168 radio-loud AGN (Fan, ..., Gupta, et al. 2007)

• Radio data were taken for 168 radio-loud AGN from

UMRAO (University of Michigan Radio Astronomy

Observatory) data base for more than 33 years.

Sources are observed at 5.0, 8.0 and 14.5 GHz.

• Variability Time Scale: Structure Function Analysis It

is a method to detect the time scale and periodicity in

unevenly sampled light curves. It is an interpolation

algorithm calculated twice, first from starting and then from

the end of the time series data.

• Periodicity Analysis: Jurkevich Method & Power

Spectral (Fourier) Method These methods are useful to

search for periodicity in non evenly sampled data.

• Variability Cross-correlation in different EM bands: Descrete Correlation Function Method This method is

useful to search for cross correlation and time lags in the

light curves of different EM bands.

Optical observations of radio-quiet QSOs

(Gupta & Joshi 2005; Stalin, Gupta, et al. 2005; Gupta & Yuan 2007)

• Observations: 10 radio-quiet QSOs were observed in (14 nights).

• Telescopes: 2.34 m VBT; 1.2 m telescope, Mt. Abu, India and 2.16 m telescope, Beijing Observatory, China.

• Motivation: Search for IDV in radio-quiet QSOs.

• Data Reduction: DAOPHOT software was used. Differential light curves were generated with 2 comparison stars from 4 selected stars. Variance method is used to detect the IDV in the (differential instrumental magnitude vs time) light curves.

Results based on Mrk 421

• IDV and flaring activity ~ 0.4 magnitude have been detected on 6 nights out of 8 nights observations.

• Short term variability is also detected in the source.

• Mrk 421 was in the high-state during our observing run. A maximum variation of 0.89 magnitude is seen over the entirety of the observed period.

• Using RXTE archive data, we found X-ray and NIR variability are correlated. X-ray leading NIR by 7 days(work in progress).

Results based on Mrk 501

• In 12 nights of data (March – May 2000), we found IDV of 6-7% in one night in R band. No genuine IDV is detected in B and V bands light curves.

• Using structure function analysis on one night IDV in R band, we found time scales of variability ~ 15 min.

• Short term variations in V and R bands are seen. Source has varied about 15% and 25% in V and R bands respectively. Similar variation is also noticed by (Ghosh et al. 2000) in the low state of the source.

• No B-R color variation is detected but source has shown V-R color variation on one occassion.

• Using the time scale of variability, we got the mass of black hole of Mrk 501 is 1.2 x 108 M

⊙.

IDV of blazars (YNAO observations)• In partial data analysis we have detected outburst phase in

three blazars namely AO 0235+164, S5 0716+714 and 3C 279 (more than 1000 image frames).

• IDV and day to day variations of large amplitude are also noticed in these sources.

• We have detected the brightest state of AO 0235+164 after 1999. So, our detection is pre or post outburst of the source. Our observations also confirm the 8 years periodicity predicted earlier by Raiteri et al. (2006).

• The outburst state detected in S5 0716+714 and 3C 279 may be also the pre or post outburst state. The brightest level of the sources detected by us is comparable to the previous outburst state of these sources.

Light Curve of AO 0235+164

Light Curve of S5 0716+714

Light Curve of S5 0716+714

Results based on 4C 29.45

• Compiled post-1974 and our 1997-2002 optical photometric

data of 4C 29.45, we found max. variations in different

passbands: △U = 4.41mag, △B = 5.55 mag, △V = 4.53

mag, △R = 5.80 mag and △I = 5.34 mag.

• The average color indices U – B = -0.54±0.18 mag, B – V =

0.56±0.21 mag, B – R = 0.93±0.18 mag, B – I = 1.51±0.24

mag, V – R = 0.44±0.15 mag and V – I = 1.03±0.23 mag.

• We found possible period of 3.55 years and 1.58 years in

post – 1974 data.

Results based on 168 radio sources

• Out of 168 Radio-loud AGN, 66 sources have shown

periodicity ranging from 2.2 to 20.8 years.

• We found that the value of variability parameter C for BL

Lac candidates are larger that FSRQs.

• C = (AGN – StarA)

/ (Star A – Star B)

& (AGN – StarB)

/

(Star A – Star B)

• If C > 2.57, variability confidence limit is more than 99%

Results on IDV of radio-quiet QSOs

• IDV was detected in radio-quiet QSOs.

• We compiled the IDV data of different classes of AGNs reported to date. Statistical analysis shows the maximum variation amplitude is ~ 10% for radio-quiet AGN, ~ 100% for blazars and ~ 50% for radio-loud AGN (non blazars).

• Generally 10% radio-quiet AGN, 35-40% radio-loud AGN show IDV. For any blazar, if observed continuously for less than 6 hour and more than 6 hour, the chances of detecting IDV are 60-65% and 80-85% respectively.

• In radio-quiet AGN, variation occur in less than few hours.

Important Conclusions

• Mrk 421 was in high-state during our observations. Total flux variation was about ~80% in continuous 9 nights of obs. in Feb. - March 2003. IDV, flaring activity of 0.4 mag have been detected on 6 nights. Short term variability was also noticed.

• Mrk 501 was in low-state during our observation period of 8 weeks. Total flux variation in V and R passbands was 15% and 25% respectively. A possible time scale of variability of ~ 15 min is detected which gives the mass of BH = 1.2 X 108 M

⊙.

• We have detected outburst phase (pre or post outburst in 3 blazars namely AO 0235+164, S5 0716+714 and 3C 279.

• We found IDV was present all classes of AGN which show the nature of central engine in all these AGN classes are similar if not identical.

• Variation detected in high state of blazars is supported by shock-in-jet model.

• Variation in low state of blazar can be explained by the disturbance on accretion disk.

• Variation detected in radio-quiet QSOs can be explained as relativistic particle jets ejected by the central engine of radio-quiet QSOs. IDV in radio-quiet QSOs can also be supported by the alternative standard model having numerous flares or hot spots on the accretion disk surrounding the central engine which can produce IDV.

• Open Question?: IDV in RQQSOs and blazars in low-state are similar? If yes → IDV in RQQSOs will be only supported by accretion disk based models??

• In post-1974 data, we found the blazar 4C 29.45 has

shown large amplitude long term flux variation: U = 4.41

mag, B = 5.55 mag, V = 4.53 mag, R = 5.80 mag, I =

5.34 mag.

• We found the period of 3.55 years and 1.58 years in the

long term optical light curves of 4C 29.45.

• We found 2.2 to 20.8 years of meaningful period in 66

sources out of 168 radio sources. BL Lac objects show

larger variability parameter value compare to FSRQs.

Ongoing and Future Projects• Long term optical and radio variability of blazars using

published data (with J. H. Fan's group).

• Observations of blazars OJ 287, AO 0235+164 and 3C 279 for WEBT campaigns.

• Observations of blazars in low-state (with J. M. Bai)

• Search for radio-quiet BL Lac objects using GMRT observations

(in collaboration with D. J. Saikia & W. Yuan).

• Simultaneous 2 color IDV of radio-quiet QSOs (based on current observations from 2.16 meter Beijing Observatory Telescope, China) (in collaboration with W. Yuan).

• Simultaneous CHANDRA (X-ray) and Optical IDV of radio-quiet QSOs (collaborators in USA and Poland).

• Near-Infrared IDV and short term variability of blazars in outburst

state (in collaboration with S. Pak & S. Lee)

• BL Lac candidates from Veron-Cetty & Veron's (2006) catalog in

SDSS spectra (Classifications in HBL and LBL and SED) (in

collaboration with T. G. Wang's & W. Yuan's group).

• Search for new BL Lac candidates in SDSS spectra by recently

developed technique by our group to do proper modeling of the

stellar component (Lu et al. 2006) and decompose the SDSS

spectra of AGN into stellar and non stellar nuclear components

(in collaboration with T. G. Wang's & W. Yuan's group).

Important Recent Publications• A. C. Gupta et al., 2004, A & A, 422, 505

• A. C. Gupta & U. C. Joshi, 2005, A & A, 440, 855

• C. S. Stalin, A. C. Gupta, et al. 2005, MNRAS, 356, 607

• J. H. Fan, ..., A. C. Gupta, et al., 2006, PASJ, 58, 797

• J. H. Fan, ..., A. C. Gupta, et al., 2007, A & A, 462, 547

• A. C. Gupta et al., 2007, ASP Conference Series (in press)

• A. C. Gupta et al., 2007a, AJ (submitted revised version)

• C. M. Raiteri, ..., A. C. Gupta, et al., 2007, A & A (submitted)

• A. C. Gupta et al., 2007b, ApJ (To be submitted in August)

• A. C. Gupta & W. Yuan, 2007, AJ (To be submitted in August)

Thanks