Break in the VHE spectrum of PG 1553+113: new upper limit on its redshift?

PG 1553+113 is a known BL Lac object, newly detected in the GeV-TeV energy range by H.E.S.S and MAGIC. The redshift of this source is unknown and a lower limit of $z > 0.09$ was recently estimated. The very high energy (VHE) spectrum of PG 1553+113 is attenuated due to the absorption by the low energy photon field of the extragalactic background light (EBL). Here we correct the combined H.E.S.S and MAGIC spectrum of PG 1553+113 for this absorption assuming a minimum density of the evolving EBL. We use an argument that the intrinsic photon index cannot be harder than $\Gamma = 1.5$ and derive an upper limit on the redshift of $z < 0.69$. Moreover, we find that a redshift above $z = 0.42$ implies a possible break of the intrinsic spectrum at about 200 GeV. Assuming that such a break is absent, we derive a much stronger upper limit of $z < 0.42$. Alternatively, this break might be attributed to an additional emission component in the jet of PG 1553+113. This would be the first evidence for a second component is detected in the VHE spectrum of a blazar.Comment: revised version submitted to Ap

The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lac Object 1E 1207.9+3945 in its brightest state

We studied the temporal and spectral evolution of the synchrotron emission from the high energy peaked BL Lac object 1E 1207.9+3945. Two recent observations have been performed by the XMM-Newton and Swift satellites; we carried out X-ray spectral analysis for both of them, and photometry in optical-ultraviolet filters for the Swift one. Combining the results thus obtained with archival data we built the long-term X-ray light curve, spanning a time interval of 26 years, and the Spectral Energy Distribution (SED) of this source. The light curve shows a large flux increasing, about a factor of six, in a time interval of a few years. After reaching its maximum in coincidence with the XMM-Newton pointing in December 2000 the flux decreased in later years, as revealed by Swift. The very good statistics available in the 0.5-10 keV XMM-Newton X-ray spectrum points out a highly significant deviation from a single power law. A log-parabolic model with a best fit curvature parameter of 0.25 and a peak energy at ~1 keV describes well the spectral shape of the synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides a milder curvature (b~0.1) and a peak at higher energies (~15 keV), suggesting a different state of source activity. In both cases UVOT data support the scenario of a single synchrotron emission component extending from the optical/UV to the X-ray band. New X-ray observations are important to monitor the temporal and spectral evolution of the source; new generation gamma-ray telescopes like AGILE and GLAST could for the first time detect its inverse Compton emission.Comment: 7 pages, 6 figures, accepted for publication in A&

Six years of BeppoSAX observations of blazars: a spectral catalog

We present a spectral catalog for blazars based on the BeppoSAX archive. The sample includes 44 High-energy peaked BL Lacs (HBLs), 14 Low-energy peaked BL Lacs (LBLs), and 28 Flat Spectrum Radio Quasars (FSRQs). A total of 168 LECS, MECS, and PDS spectra were analyzed, corresponding to observations taken in the period 1996--2002. The 0.1--50 keV continuum of LBLs and FSRQs is generally fitted by a single power law with Galactic column density. A minority of the observations of LBLs (25%) and FSRQs (15%) is best fitted by more complex models like the broken power law or the continuously curved parabola. These latter models provide also the best description for half of the HBL spectra. Complex models are more frequently required for sources with fluxes F_{2-10 keV} > 10^-11 cm-2 s-1, corresponding to spectra with higher signal-to-noise ratio. As a result, considering sources with flux above this threshold, the percentage of spectra requiring those models increases for all the classes. We note that there is a net separation of X-ray spectral properties between HBLs on one side, and LBLs and FSRQs on the other, the distinction between LBLs and FSRQs is more blurry. This is most likely related to ambiguities in the optical classification of the two classes.Comment: 6 pages, 4 figures, 7 tables. Accepted for publication in A&

Swift detection of all previously undetected blazars in a micro-wave flux-limited sample of WMAP foreground sources

Almost the totality of the bright foreground sources in the WMAP CMB maps are blazars, a class of sources that show usually also X-ray emission. However, 23 objects in a flux-limited sample of 140 blazars of the WMAP catalog (first year) were never reported before as X-ray sources. We present here the results of 41 Swift observations which led to the detection of all these 23 blazars in the 0.3-10 keV band. We conclude that all micro-wave selected blazars are X-ray emitters and that the distribution of the micro-wave to X-ray spectral slope $\alpha_{mu x}$ of LBL blazars is very narrow, confirming that the X-ray flux of most blazars is a very good estimator of their micro-wave emission. The X-ray spectral shape of all the objects that were observed long enough to allow spectral analysis is flat and consistent with inverse Compton emission within the commonly accepted view where the radiation from blazars is emitted in a Sychrotron-Inverse-Compton scenario. We predict that all blazars and most radio galaxies above the sensitivity limit of the WMAP and of the Planck CMB missions are X-ray sources detectable by the present generation of X-ray satellites. An hypothetical all-sky soft X-ray survey with sensitivity of approximately $10^{-15}$ erg/s would be crucial to locate and remove over 100,000 blazars from CMB temperature and polarization maps and therefore accurately clean the primordial CMB signal from the largest population of extragalactic foreground contaminants.Comment: 13 pages, 4 figures, 5 tables, A&A in pres

Swift observations of IBL and LBL objects

BL Lacs are an enigmatic class of active galactic nuclei (AGNs), characterized by the non-thermal continuum typically attributed to synchrotron and inverse Compton emission. Depending on the frequency location of the maxima of these components, they are subdivided into three subclasses LBLs, IBLs, and HBLs. We present the results of a set of observations of eight BL Lac objects of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift satellite between January 2005 and November 2006. We are mainly interested in measuring the spectral parameters, and particularly the steepness between the UV and the X-ray band, useful for determining the classification of these sources. We compare the behavior of these sources with previous XMM-Newton, BeppoSAX obser- vations and with historical data in the X-ray and in the optical band. We are also interested in classifying the sources in our sample on the basis of the observations and comparing them with their classification presented in literature. We performed X-ray spectral analysis of observed BL Lac objects using a simple powerlaw and in a few cases the log-parabolic model. We also combined the UV emission with the low energy X-ray data to We used observational data to classify sources in our sample and derived parameters of their spectral energy distribution. We found that for the IBLs X-rays low states show features of the high energy component, usually interpreted as due to inverse Compton emission. Sources in our sample exhibit a range of temporal UV and X-ray behaviors, some objects having clear and neat correlated UV and X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO 0235+164) UV and X-ray correlation. Finally, we also note that our estimates of spectral curvature are in the range of that measured for the HBLs.Comment: 10 pages, 7 figures, published in A&

Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN

Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we study correlations between X-ray spectral features. The sample consists of 350 time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to the central black hole that is reprocessed and absorbed by material around the black hole. To test the robustness of our results, we performed Monte Carlo simulations of the spectral sample. We find a complex relationship between the iron line equivalent width (EW) and the underlying power law index (Gamma). The data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2, but finds a weak anti-correlation for steeper photon indices. We propose that this relationship is driven by dilution of a disk spectrum (which includes the narrow iron line) by a beamed jet component and, hence, could be used as a diagnostic of jet-dominance. In addition, our sample shows a strong correlation between the reflection fraction (R) and Gamma, but we find that it is likely the result of modeling degeneracies. We also see the X-ray Baldwin effect (an anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample as a whole, but not for the individual galaxies and galaxy types.Comment: Accepted for publication in ApJ, 14 page

SWIFT observations of TeV BL Lac objects

Context: We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. Aims: We are mainly interested in measuring the spectral parameters, and particularly the intrinsic curvature in the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac TeV objects using either a log-parabolic or a simple power-law model . Results: We found that many of the objects in our sample do show significant spectral curvature, whereas those having the peak of the spectral energies distribution at energies lower than ~0.1 keV show power law spectra. In these cases, however, the statistics are generally low thus preventing a good estimate of the curvature. Simultaneous UVOT observations are important to verify how X-ray spectra can be extrapolated at lower frequencies and to search for multiple emission components. Conclusions: The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.Comment: 10 pages, 7 figures. Accepted for publication in the Astronomy and Astrophysic