Long-term variability of AGN at hard X-rays

Variability at all observed wavelengths is a distinctive property of AGN. Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Swift/BAT offers the unique opportunity to follow, on time scales of days to years and with a regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. We study the amplitude of the variations, and their dependence on sub-class and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of the AGN in the sample are found to exhibit significant variability on months to years time scales, radio loud sources being the most variable. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes of the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are well correlated, suggesting a common origin of the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on month time scale. In addition, sources with harder spectra are found to be more variable than softer ones. These properties are generally consistent with a variable power law continuum, in flux and shape, pivoting at energies >~ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).

In-flight calibration of the INTEGRAL/IBIS mask

Since the release of the INTEGRAL Offline Scientific Analysis (OSA) software version 9.0, the ghost busters module has been introduced in the INTEGRAL/IBIS imaging procedure, leading to an improvement of the sensitivity around bright sources up to a factor of 7. This module excludes in the deconvolution process the IBIS/ISGRI detector pixels corresponding to the projection of a bright source through mask elements affected by some defects. These defects are most likely associated with screws and glue fixing the IBIS mask to its support. Following these major improvements introduced in OSA 9, a second order correction is still required to further remove the residual noise, now at a level of 0.2-1% of the brightest source in the field of view. In order to improve our knowledge of the IBIS mask transparency, a calibration campaign has been carried out during 2010-2012. We present here the analysis of these data, together with archival observations of the Crab and Cyg X-1, that allowed us to build a composite image of the mask defects and to investigate the origin of the residual noise in the IBIS/ISGRI images. Thanks to this study, we were able to point out a simple modification of the ISGRI analysis software that allows to significantly improve the quality of the images in which bright sources are detected at the edge of the field of view. Moreover, a refinement of the area excluded by the ghost busters module is considered, and preliminary results show improvements to be further tested. Finally, this study indicates further directions to be investigated for improving the ISGRI sensitivity, such as taking into account the thickness of the screws in the mask model or studying the possible discrepancy between the modeled and actual mask element bridges.

High-energy emission from NGC 5506, the brightest hard X-ray Narrow Line Seyfert 1 galaxy

We present results on the hard X-ray emission of NGC 5506, the brightest narrow line Seyfert 1 galaxy above 20 keV. All the recent observations by INTEGRAL, Swift and Suzaku have been analysed and spectral analysis during nine separated time periods has been performed. While flux variations by a factor of 2 were detected during the last 7 years, only moderate spectral variations have been observed, with the hint of a hardening of the X-ray spectrum and a decrease of the intrinsic absorption with time. Using Suzaku observations it is possible to constrain the amount of Compton reflection to R = 0.6-1.0, in agreement with previous results on the source. The signature of Comptonisation processes can also be found in the detection of a high-energy cut-off during part of the observations, at Ec = 40-100 keV. When a Comptonisation model is applied to the Suzaku data, the temperature and the optical depth of the Comptonising electron plasma are measured at kT = 60-80 keV and tau = 0.6-1.0, respectively. The properties inferred for NGC 5506 in this study agree with those based on other data sets for the same AGN, and fit the picture of NLS1 having in general lower high-energy cut-offs at hard X-rays than their broad line equivalent.

Compton processes in the bright AGN MCG+8-11-11

We present preliminary results on the hard X-ray emission properties of the Seyfert 1.5 galaxy MCG+8-11-11 as observed by INTEGRAL and SWIFT. All the INTEGRAL IBIS/ISGRI data available up to October 2009 have been analyzed together with two SWIFT/XRT snapshot observations performed in August and October 2009, quasi-simultaneously to INTEGRAL pointed observations of MCG+8-11-11. No correlation is observed between the hard X-ray flux and the spectral slope, while the position of the high-energy cut-off is found to have varied during the INTEGRAL observations. This points to a change in the temperature of the Comptonising medium from a minimum value of kT = 30-50 keV to values larger than 100-150 keV. There is no significant detection of Compton reflection, with a 3 sigma upper limit of R < 0.2, and no line has been detected at 112 keV, as previously claimed from HEAT observations (112 keV flux F < 2.4e-4 ph/cm^2/s). The variability behaviour of MCG+8-11-11 is found to be similar to that shown by IC 4329A, with different temperatures of the electron plasma for similar flux levels of the source, while other bright Seyfert galaxies present different variability patterns at hard X-rays, with spectral changes correlated to flux variations (e.g. NGC 4151).

AGN variability at hard X-rays

We present preliminary results on the variability properties of AGN above 20 keV in order to show the potential of the INTEGRAL IBIS/ISGRI and Swift/BAT instruments for hard X-ray timing analysis of AGN. The 15-50 keV light curves of 36 AGN observed by BAT during 5 years show significantly larger variations when the blazar population is considered (average normalized excess variance = 0.25) with respect to the Seyfert one (average normalized excess variance = 0.09). The hard X-ray luminosity is found to be anti-correlated to the variability amplitude in Seyfert galaxies and correlated to the black hole mass, confirming previous findings obtained with different AGN hard X-ray samples. We also present results on the Seyfert 1 galaxy IC 4329A, as an example of spectral variability study with INTEGRAL/ISGRI data. The position of the high-energy cut-off of this source is found to have varied during the INTEGRAL observations, pointing to a change of temperature of the Comptonising medium. For several bright Seyfert galaxies, a considerable amount of INTEGRAL data have already been accumulated and are publicly available, allowing detailed spectral variability studies at hard X-rays.

The variability of the quasar 3C 273: a radio to gamma-ray view

We have analysed the first 15 months of Fermi/LAT data of the radio loud quasar 3C 273. Intense gamma-ray activity has been detected, showing an average flux of F(> 100 MeV) = 1.4e-6 ph/cm^2/s, with a peak at F(> 100 MeV) = 5.6e-6 ph/cm^2/s detected during a flare in September 2009. Together with the brightening of the source, a possible hardening of the gamma-ray spectrum is observed, pointing to a shift of the inverse Compton peak toward higher energies than the 1-10 MeV range in which 3C 273 inverse Compton emission is typically observed to peak. During the 15 months of observations the photon index is measured to vary between 2.4 and 3.3, with an average value of 2.78 +/- 0.03. When compared to the observations at other wavelengths, the gamma-rays show the largest flux variations and we discuss the possibility that two different components are responsible for the inverse Compton hump emission below and above the MeV peak.

The multiwavelength variability of 3C 273

We present an update of 3C 273s database hosted by the ISDC, completed with data from radio to gamma-ray observations over the last 10 years. We use this large data set to study the multiwavelength properties of this quasar,especially focussing on its variability behaviour. We study the amplitude of the variations and the maximum variability time scales across the broad-band spectrum and correlate the light curves in different bands, specifically with the X-rays, to search for possible connections between the emission at different energies. 3C 273 shows variability at all frequencies, with amplitudes and time scales strongly depending on the energy and being the signatures of the different emission mechanisms. The variability properties of the X-ray band imply the presence of either two separate components (possibly a Seyfert-like and a blazar-like) or at least two parameters with distinct timing properties to account for the X-ray emission below and above ~20 keV. The dominant hard X-ray emission is most probably not due to electrons accelerated by the shock waves in the jet as their variability does not correlate with the flaring millimeter emission, but seems to be associated to long-timescale variations in the optical. This optical component is consistent with being optically thin synchrotron radiation from the base of the jet and the hard X-rays would be produced through inverse Compton processes (SSC and/or EC) by the same electron population. We show evidence that this synchrotron component extends from the optical to the near-infrared domain, where it is blended by emission of heated dust that we find to be located within about 1 light-year from the ultraviolet source.