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In the news: Gamma-ray superflares from Blazars

November 25, 2010

According to the AAVSO (Special Notice #233), the blazar known as 3C 454.3 has very recently (in the last couple of days) again begun flaring extremely brightly in gamma-rays and in radio:

As was noted by AAVSO observer E. Erdelyi (Carlsbad, CA, USA) in a post to cvnet-discussion, the BL Lac object 3C 454.3 (AUID 000-BDC-612) appears to be undergoing a bright flare; the object was recorded by Erdelyi at V=13.659(0.013) on 2010 November 10.2708 (JD 2455510.7708). The last flare of comparable brightness was observed in 2007, when the object reached V ~ 13.4 around JD 2454335 (late August 2007). Photometry of this interesting event is encouraged. Gamma-ray flaring and bright millimeter-wavelength radio emission from this source were both detected on 2010 November 17 by AGILE (Striani et al., ATel #3034) and SMA (Gurwell & Wehrle., ATel #3036).

I have a soft spot for these types of astronomical objects, as I have just submitted my dissertation on gamma-rays emitted by their smaller galactic cousins, microquasars. So why do blazars flare? And why does this particular one flare so violently?

According to a paper published this summer in the Astrophysical Journal by a collaboration of Italian astronomers (Striani et al 2010), the gamma-ray “superflare” from 3C 454.3 in November/December 2009

repeatedly reached a flux near F?,Vela for about 2 weeks, and then produced a very intense super-flare on December 2-3, 2009, with F > 2F?,Vela. This flare turns out to be even more intense than that detected from PKS 1622-29 (Mattox et al. (1997)), and then qualifies as the most intense gamma-ray flare ever observed from a cosmic source at energies above 100 MeV.

How bright is the flare? Well, the F?,Vela talked about by Striani et al is the flux of the Vela pulsar which is the brightest steady gamma-ray source in the sky. It normally has a flux of 900 × 10-8 ph cm-2 s-1 above 100 MeV (i.e. 0.000009 photons arriving per square centimetre of the detector per second). This may not seem at all bright in real terms, but in gamma-ray astronomy, this is extremely luminous (as can be seen in this map of the gamma-ray sky here).

This enhanced luminosity (called Doppler boosting) is a consequence of special relativity, and is called relativistic beaming. Because blazars (also known as BL Lac objects after the canonical object) are simply AGN where the relativistic jet (i.e. moving at or close to the speed of light) is aligned along our line of sight, and as a result to us the jet appears much brighter than in reality. According to Zhou & Su (2007):

In the relativistic jets of AGNs or GRBs, the observed flux is related to their intrinsic ux by Fobs= δ3+αF, where δ is [the] Doppler factor, Fobs and F are the observed and intrinsic flux respectively , and α is the spectral index (Blandford & Konigl 1979). If δ is greater than 1, then the observed flux will be enhanced, which is called [the] Doppler boosting effect.

But another consequence of special relativity in these circumstances is that of time dilation. In summary, this can be explained by the single phrase moving clocks run slow. In practise, this means that, from the point of view of a stationary or near-stationary outside observer (i.e. us), events that occur in a frame of reference moving at a significant fraction of the speed of light either appear speeded up, in the case of the direction of motion being towards us, or slowed down, in the case of the motion being away from us. For blazars, this means that events that happen in the approaching jet appear speeded up (Blandford & Konigl 1979).

This would suggest that what we see as short-period flares from a blazar jet would actually be longer-period events in the frame of reference of the jet itself, which is indeed the case. But what are the cause of the flares in the first place?

The standard hypothesis, which has been applied to BL Lac itself by Marscher et al (2008) is that existing ‘knots’ or ‘clumps’ of more energetic material in the jet (which are produced by periodic injections of energy into the jet) interact with pre-existing shockwaves or areas of turbulence and as a result locally increased amounts of extreme particle acceleration occur. From Rani et al (2010):

The substantial flux variations we and others have observed in these LBLs can be reasonably explained by models that involve relativistic shocks propagating outward (e.g., Marscher & Gear 1985; Wagner & Witzel 1995; Marscher 1996). The larger flares are expected to be produced by the emergence and motion of a new shock triggered by some strong variation in a physical quantity such as velocity, electron density or magnetic field moving into and through the relativistic jet. Smaller variations may be nicely explained by turbulence behind a shock propagating down the jet (Marscher et al. 1992).

There are other possible factors at play as well: Raiteri et al (2010) have recently suggested that geometric variations in the position of the jet itself could also result in variations in observed flux from the jet in this object.

So does the jet in 3C 454.3 show these ‘knots’? The basic answer is, yes. According to Qian et al (2007):

These VLBI observations show that the source has a core-jet structure, ejecting superluminal knots following major radio outbursts. The complex core was once observed to show a ‘superluminal’ increase in its size. The VLBI jet is very curved and acceleration of the motion of its knots was observed (apparent speed from ~ 5c to ~ 20c). However, in 3C 454.3 some features were observed to be remained stationary, showing their orientation could be directed to us (with viewing angle ~ 0◦). The VLBI measurements (less than ~ 2◦ − 5◦) and a large bulk Lorentz factor (larger than ~20).

But the extreme luminosity of the recent outburst remains somewhat a puzzle. Pacciani et al (2010) modelled the 2009 flare and suggested a particular increase in the injection of energetic electrons into the jet and increased particle accleration could explain the increase in luminosity. It may well be the case that such circumstances have repeated themselves and thus may explain the most recent outburst

References:

R.D. Blandford & A. Konigl 1979 Apj 232 34 (ADS)
A. Marscher et al 2008 Nature 452 966 (ADS)
C.M. Raiteri et al 2010 accepted for publication in A&A (ADS)
L. Pacciani et al 2010 Apjl 716 170 (ADS)
S.J. Qian et al 2007 Chin. J. Astron. Astrophys. 7 364 (ADS)
B. Rani et al 2010 MNRAS 404 1992 (ADS)
E. Striani et al 2010 ApJ 718 455 (ADS)
J. Zhou & Y. Su 2007 Doppler boosting and de-boosting effects in relativistic jets of AGNs and GRBs Proceedings of the International Astronomical Union (2006), 2: 477-478 Cambridge University Press (ADS)

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