E. Bottacini

Photometric redshifts for gamma-ray burst afterglows from GROND and Swift/UVOT

Aims. We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and Swift/UVOT, photo-zs are derived for five GRBs for which spectroscopic redshifts are not available. Methods. We use UV/optical/NIR data and synthetic photometry based on afterglow observations and theory to derive the photometric redshifts of GRBs and their accuracy. Taking into account the afterglow synchrotron emission properties, we investigate the application of photometry to derive redshifts in a theoretical range between z ∼ 1t oz ∼ 12. Results. The measurement of photo-zs for GRB afterglows provides a quick, robust and reliable determination of the distance scale to the burst, particularly in those cases where spectroscopic observations in the optical/NIR range cannot be obtained. Given a sufficiently bright and mildly reddened afterglow, the relative photo-z accuracy η =Δ z/(1 + z) is better than 10% between z = 1. 5a nd z ∼ 7 and better than 5% between z = 2a ndz = 6. We detail the approach on 5 sources without spectroscopic redshifts observed with

INTEGRAL observations of the blazar Mrk 421 in outburst : Results of a multi-wavelength campaign

Context. If one wants to understand the physics of blazars, better simultaneous observations are important at all wavelengths, so it was fortunate that a ToO observation of the TeV-emitting blazar Mrk 421 with INTEGRAL could be triggered in June 2006 by an increase in the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments, with the exception of the spectrometer SPI, for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM- X. Multiwavelength observations were immediately triggered, and the source was observed at radio, optical, and X-ray wavelengths up to TeV energies. Aims. The data obtained during these observations were analysed with respect to time variability, time lags, correlated vari ability, and spectral evolution and then compiled in aFspectrum. Methods. The observations of the different instruments/telescopes were analysed with the usual correlation and time-analysis methods. The spectral analysis of the X-ray data was performed with XSPEC. Results. Four strong flares at X-rays were observed that were not seen a t other wavelengths (partially because of missing data). From the fastest rise in the X-rays, an upper limit could be derived to the extension of the emission region. A time lag between high-energy and low-energy X-rays was observed, which allowed an estimation of the magnetic-field strength. The spectr al analysis of the X-rays revealed a slight spectral hardening of the low-energy (3-∼43 keV) spectral index. The hardness-ratio analysis of the Swift-XRT (0.2 - 10 keV) data indicated a small correlation with the intensity; i. e., a hard-to-soft evolution was obser ved. At the energies of IBIS/ISGRI (20 - 150 keV), such correlations are less obvious. A multiwavelength spectrum was composed and the X-ray and bolometric luminosities calculated. Conclusions. The observed flaring activity of Mrk 421 is mainly visible at X -rays. It is found that the spectral change with intensity is small. But at least one flare showed a completely di fferent spectral behaviour than the other flares, so one can con clude that each blob of relativistic-moving electrons has its own individu al physical environment that leads to different emission characteristics. From a fit of a leptonic emission model to the data, one finds that the observed variability may be due to a varying effi ciency of particle acceleration.

The multifrequency campaign on 3C 279 in January 2006

We present the results of a multifrequency campaign from radio to hard X-ray energies on the blazar 3C 279 during an optical high-state in January 2006. We give the observational results (multifrequency light curves and spectra) and compile an SED. This complements an SED from an optical low-state in June 2003. Surprisingly the two SEDs differ only in their high-energy synchrotron emission (near-IR - UV), while the low-energy inverse-Compton emission (X- to Gamma-rays) remained unchanged. By interpreting with a steady-state leptonic emission model, the variability among the SED can be reproduced by a change solely of the low-energy cutoff of the relativistic electron distribution. In an internal shock model for blazar emission, such a change could e.g. achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet.

XMM-Newton and INTEGRAL observations of the black hole candidate XTE J1817-330

Aims. The galactic black hole candidate XTE J1817-330 was discovered in outburst by RXTE in January 2006. We present here the results of an XMM-Newton target of opportunity observation, performed on 13 March 2006 (44 days after the maximum), and an INTEGRAL observation performed on 15–18 February 2006 (18 days after the maximum). Methods. The EPIC-pn camera on-board XMM-Newton was used in the fast read-out burst mode to avoid photon pile-up, while the RGS was used in spectroscopy high count rate mode. We fit both the XMM-Newton and the INTEGRAL spectra with a twocomponent model consisting of a thermal accretion disk (represented by diskbb or diskpn models) and a comptonizing hot corona (represented by a power-law or the compTT model). Results. The soft X-ray spectrum is dominated by an accretion disk component, with a maximum temperature decreasing from 0.96±0.04 keV at the time of the INTEGRAL observation to 0.70±0.01 keV on 13 March. The optical monitors on board INTEGRAL and XMM-Newton showed the source with magnitudes V:11.3–11.4, U:15.0–15.1 and UVW1:14.7–14.8. The soft X-ray spectrum, together with the optical and UV data, show a low hydrogen column density towards the source, and several absorption lines, most likely of interstellar origin, are detected in the RGS spectrum: OIKα ,O IKβ, OII, OIII and OVII, which trace both cold and hot components of the interstellar medium. The soft X-ray spectrum indicates the presence of a black hole, with an estimate for the upper limit of the mass of 6.0 +4.0 −2.5 M� .

PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe

Context. The high-redshift (z = 3.1) blazar PKS 0537-286, belonging to the flat spectrum radio quasar blazar subclass, is one of the nmost luminous active galactic nuclei (AGN) in the Universe. Blazars are very suitable candidates for multiwavelength observations. nIndeed, the relativistic beaming effect at work within the jet enhances their luminosity. This in turn allows the properties of the nextragalactic jets, the powering central engine, and the surrounding environment to be derived. nAims. Our aim is to present the results of a multifrequency campaign from the near-IR to hard X-ray energies on PKS 0537-286 and ngive insight into the physical environment where the radiation processes take place. nMethods. We observed the source at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with nground-based optical telescopes. We also analyzed two archival spectra taken with XMM-Newton in 1999 and 2005. A comparative nanalysis of the results is performed. nResults. The X-ray continuum of the blazar, as sampled by XMM, is described by a power law of index Γ = 1.2, modified by nvariable absorption at the soft X-rays, as found in other high-redshift QSOs. Modest X-ray continuum variability is found in the Swift nobservations. The combined Swift/BAT and Swift/XRT spectrum is very hard (Γ = 1.3). This, together with the non simultaneous nEGRET detection and the more recent non detection by Fermi-LAT, constrains the peak of the high-energy component robustly. nThe optical/UV data, heavily affected by intervening Ly α absorption, indicate the presence of a bright thermal accretion disk that ndecreased in luminosity between 2006 and 2008. We infer from this a reduction of the BLR radius. When taking this into account, nthe 2006 and 2008 SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering where nthe accretion-disk luminosity decreases between the 2 epochs by a factor 2, while the bulk Lorentz factor remains unchanged and the nmagnetic field changed only marginally.

INTEGRAL and Multiwavelength Observations of the Blazar Mrk 421 during an Active Phase

A ToO observation of the TeV‐emitting blazar Mrk 421 with INTEGRAL was triggered in June 2006 by an increase of the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments with the exception of the spectrometer SPI for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM‐X. Multiwavelength observations were immediately triggered and the source was observed at radio, optical and X‐ray wavelengths up to TeV energies. The data obtained during these observations are analysed with respect to spectral evolution and correlated variability. Preliminary results of the analysis are presented in this poster.

XMM-Newton observations of XTE J1817-330 and XTE J1856+053

The black hole candidate XTE J1817‐330 was discovered in outburst on 26 January 2006 with RXTE/ASM. One year later, another X‐ray transient discovered in 1996, XTE J1856+053, was detected by RXTE during a new outburst on 28 February 2007. We report on the spectra obtained by XMM‐Newton of these two black hole candidates.