Blazar microvariability at hard Blazar microvariability at hard X-raysX-rays

Luigi FoschiniINAF/IASF-Bologna

on behalf of a larger collaboration

Simbol-X: The hard X-ray Universe in focus – Bologna, May 14-16, 2007

According to the common paradigm, blazars are composed of a supermassive black hole with relativistic jets extending from poles. Since the angle between the jet axis and the observer direction is very small, relativistic effects are important (see the talks by Tavecchio and Giommi).

However, the step from this general picture to the details of a scientific model is much larger than one can ever think. A lot of issues are still opened.

With particular reference to the topic of today’s talk (variability), we do not know:

• how jets are generated, accelerated and collimated, • what is their content of matter (e+, e-, p+, …), • what is their energy budget, • what is the origin of flares, • what is the role of shocks, • and many many more questions…

Introduction

We still not have a complete observational frame (or we have a very limited data base), particularly at high-energy, mainly due to limitations of the present technology. When we try to measure variability at short timescales, we collide against the instruments limits.

Gamma-rays (>511 keV): CGRO/EGRET changed our view of the γ-ray sky (and particularly of blazars!), but still limited to study variability. Most blazars observed during outbursts; short timescales only with exceptional flares (e.g. ~3 hours with PKS 1622-297 in 1995 or 3C 279 in 1996). Great improvements expected with GLAST.

What do we have at hands? - I

3C 279 in 1996 (Hartman et al. 2001)

PKS 1622-297 in 1995

(Mattox et al. 1997)

Soft X-rays (0.1-10 keV): Chandra & XMM-Newton have opened new windows in X-ray astronomy. Possibility to study X-ray emission from jets (Chandra, e.g. Sambruna et al. 2004) or to study short term variability (XMM-Newton, e.g. S5 0716+71, Foschini et al. 2006a), also for blazars during quiescent phases (e.g. 3C 454.3, Raiteri et al. in preparation).

What do we have at hands? - II

S5 0716+71, Foschini et al. (2006a)INTEGRAL/ISGRI (256 ks): only a 4.5 σ detection in the 30-60 keV energy band (Pian et al. 2005).

Hard X-rays (10-511 keV): Great advancements with INTEGRAL and Swift, but still not sufficient for blazars, which can be observed only during outbursts (S5 0716+71, Pian et al. 2005; 3C 454.3, Pian et al. 2006, Giommi et al. 2006); otherwise they are below the instruments detection limits.

What do we have at hands? - III

Swift/XRT (10 ks) Swift/BAT

Swift/BAT transient monitor results provided by the Swift/BAT team

The outburst of 3C 454.3 in 2005

INTEGRAL/ISGRI

Time bin 1 day

Time bin 1 hour

Another example of the difference between the possibilities offered in the variability studies by the present instruments in the energy band below and above 10 keV. Please note that we are presenting observations of a handful of bright blazars. The general situation is much worse!

What do we have at hands? - IV

Swift/BAT transient monitor results provided by the Swift/BAT team

Time bin 1 day

The outburst of Mkn 421 in 2006

Time bin 1 hour

Swift/BAT

Swift/XRT

Other examples with INTEGRAL can be

viewed in the poster by Bianchin et al. and in

the talk by Sambruna.

The paradigmatic case of NRAO 530 - IThe paradigmatic case of NRAO 530 - I

A short hard X-ray flare of NRAO 530 (z=0.902) occurred on 17 February 2004 and detected serendipitously by IBIS/ISGRI on board INTEGRAL during the Galactic Centre Deep Exposure (GCDE).

Peak flux ≈ 210-

10 erg cm-2 s-1 in the 20-40 keV

energy band in a time scale of less

than 1 hr.5 σ detection!

Background: stable

GX 17+2 (that has the ISGRI

SPSF overlapping with NRAO530):

stable

Foschini et al. (2006c)

The paradigmatic case of NRAO 530 - IIThe paradigmatic case of NRAO 530 - II

Since the blazar is apparently located in the Galactic plane (l,b) = (12.03, +10.81), there could be the doubt of a contaminating Galactic source inside the 3’ error circle of IBIS/ISGRI.

However, 4 snapshots with Swift/XRT revealed that NRAO 530 is the only source present.

NRAO 530 is known to display strong and erratic variability

The paradigmatic case of NRAO 530 - IIIThe paradigmatic case of NRAO 530 - III

up to Δmag ≈ 3 at optical wavelengths (Webb et al. 1988)

up to a factor 6 in flux at E > 100 MeV during the giant radio and γ-ray flare in 1995 (Bower et al. 1997)

1977

1995

Another short hard X-ray flare detected this time with Swift/BAT (3σ, low confidence)

April 6th, 2007Work in progress to improve the detection

Search for simultaneous or nearly simultaneous data at other wavelengths: only one radio observation at 2 cm (MOJAVE Project, Lister & Homan, 2005) performed on 11 February 2004 revealed a moderate increase of the polarization. After about 1 year, there was a clear expansion of the radio jet.

The paradigmatic case of NRAO 530 - IVThe paradigmatic case of NRAO 530 - IV

Animations are courtesy of the MOJAVE Project (http://www.physics.purdue.edu/astro/MOJAVE/index.html)

The paradigmatic case of NRAO 530 - VThe paradigmatic case of NRAO 530 - V

Possible explanations of the short hard X-ray flare (first event of this type recorded to date in a FSRQ): • unsteadyness of the jet flow, that might be due to a single non stationary shock (e.g. Hughes et al. 1985);• collision of two relativistic plasma shells (internal shock, Spada et al. 2001);• anything else?

This would require large Doppler factors (δ 100), while Bower & Backer (1998) measured δ = 15 with VLBI.

Recently, Homan et al. (2006) proposed a new method to measure δ and applied it to the sources of the MOJAVE Project. They found values much larger than previously thought and, particularly, γ-ray loud blazars have jets faster than γ-ray quiet agn.

NRAO 530 is one of the fastest sources, with apparent jet speed 30c !! ( δ ~ 30-40, MOJAVE Team, private communication).

How Simbol-X can contribute to these studies?How Simbol-X can contribute to these studies?Simulations performed with the XSPEC package available for the Workshop

We considered the Swift/XRT spectrum published by Kadler et al. (astro-ph/0704.1465):NH=(5±1)·1021 cm-2; Γ=1.5±0.3; Flux 0.3-10 keV = 2.2·10-12 erg cm-2 s-

1; Exp. = 10 ksSimulation 1: the same spectrum observed by Swift/XRT.SX with 1 hour of exposure:Errors: NH= 20%; Γ = 7%

Simulation 2: the same flux observed by

INTEGRAL.SX with 1 hour of exposure:Errors: NH = 2%; Γ = 0.5%

It will be possible to follow the spectral evolution on hourly timescales!

Final RemarksFinal Remarks

High-energy variability studies are of paramount importance in the blazar research field, but they require very good statistics.

Presently, this type of studies is hampered by instruments limits, particularly at short timescales.

Simbol-X can offer a unique opportunity to investigate – for the first time – the hard X-ray spectral variability at short timescales (< 1 day; hourly timescales).

A little wish: an optical monitor (something like OM onboard XMM-Newton or UVOT onboard Swift) is presently not foreseen for SX, but it would be of great advantage for this type of studies to have optical/UV data simultaneous to X-rays.

A little dream: if Simbol-X will be launched with GLAST still operative…