X-ray data

Even though most of the X-ray spectra presented here have been already fitted with more complex models and their results published, we decided not to use the spectral parameters reported in the literature and to redo the fitting using a model as simple as possible, still considering the goodness of the fit. This choice is motivated by the necessity of having a database of fluxes and spectral parameters as uniform as possible, both in the model used and in the fitting range considered. A simple power law absorbed by a Galactic column density of N_H = 1.8×10²⁰cm⁻²reveals to be a good (χ²_red ∼ 1) model for most of the X-ray spectra analysed here. In the small percentage of cases for which the χ²_red is found to be too high (≥ 1.3), we have a closer look at the spectra and try to understand the reasons of the discrepancy between the model and the data. In most of these cases, excluding a few noisy bins or adding an Fe line significantly improves the fit (details about these spectra are given in the following sections). In order to fit independently the soft-excess, the medium X-rays and the hard X-rays as well as to have similar fitting ranges for all instruments, the spectra are separately fitted in bands as close as possible to the 0.5–2, 2–10 and>10 keV range. The exact energy ranges that are used are reported in Table 8.1 and more details for the single cases are given in the following sections.

Thanks to the presence of some simultaneous or quasi-simultaneous (within one day) observations with different satellites, we are able to inter-calibrate the different instruments, assuming XMM/PN as the reference one, and to renormalise the light curves accordingly. For further details and the inter-calibration factors see Soldi et al. (2008, Sect. 2.4 and Table 2).

1After the completion of this work, Poole et al. (2008) published a paper on the calibration of the Swift/UVOT instru-ment, where they propose an empirical polynomial correction of the coincidence losses for isolated point sources.

The new data 111 BeppoSAX data

During its 6-years lifetime, BeppoSAX observed 3C 273 in 9 occasions from July 19, 1996 to June 14, 2001. The spectra of the main narrow field instruments on board, i.e. LECS, MECS and PDS, have been retrieved from the ASI Science Data Centre (ASDC) archive². The absorbed power-law fit in the three separated ranges well represents the X-ray emission as measured by BeppoSAX.

One of the LECS fits gives a χ²_red of about 1.4. Excluding the bins above 1.9 keV improved significantly the fit (χ²_red = 1.1). In two cases the MECS fit in the 2–10 keV range results in aχ²_red higher than 1.3. Adding a Gaussian to the model improves the fit results, confirming the 1996 and 2001 detections of the iron Kαline reported by Grandi et al. (1997) and Grandi & Palumbo (2004).

In another case an already acceptableχ²_red of 1.06 is further improved when a Gaussian is included in the model (1998 detection of the Fe line reported by Grandi & Palumbo 2004). For three of the PDS spectra theχ²_red is higher than 1.5 when fitted in the 15–200 keV range. In two of these cases (January 15, 1997 and June 2000 spectra), the spectra are particularly noisy above 100 keV and therefore the 100–200 keV range is excluded from the fitting, resulting in acceptableχ²_red (0.8 and 1.2). In the spectrum of January 17, 1997 an excess is observed around 55 keV, which might be due to contamination by the tantalum Kαline (at 57 keV) from the collimator (Vink et al. 2001). When the 51–56 keV range is excluded, a significant improvement of theχ²_red is obtained (from 1.9 to 0.7, with an F-test probability of 10⁻³). In order to provide the best fit results, but still maintaining the uniformity in the modelling of the spectra, we include in the database the improved fitting parameters obtained when excluding noisy bins, but still keep the single power law results for the spectra that show evidence of a line.

RXTE data

During its first 10 years of observations, RXTE monitored 3C 273 intensively, performing 957 obser-vations spanning the time period from February 9, 1996 to December 11, 2005. We downloaded PCA and HEXTE spectra from the RXTE archive³ and performed a combined fit of the data from the two instruments, because, when fitted alone, the low signal-to-noise of the HEXTE spectra introduces a too large scatter in the resulting parameters. PCA data below 3 keV are excluded due to the effect of the gain drift on the channel-energy relation. In fact, even a small drift of only 1% is able to produce a significant change in the maximum energy of channel 2 of the PCA. As this energy shifted from 2.02 to 2.96 keV over the mission lifetime, channel 2 would be included or excluded from a fit starting below 3 keV only depending on the gain period. Since the inclusion of channel 2 results in very high χ²_redand in order to reject it from all the analysed spectra, a low-energy limit of 3 keV is chosen for the fitting range (Fig. 8.4).

Recently, the RXTE team discovered a number of problems in the South Atlantic Anomaly (SAA) history files, as for example missing or repeated data⁴. Another, but independent bug, was found in the pcabackest background estimation program when using the model pca_bkgd_cmfaintl7_eMv2005112 8.mdl. This latest bug does not influence our results, as the archival data were processed with pre-vious versions of the background model for faint sources, as 3C 273 can be considered for RXTE observations (e.g. Courvoisier et al. 2003a). The dates of data gaps have been checked and two

2http://www.asdc.asi.it/

3http://heasarc.gsfc.nasa.gov/docs/xte/recipes/stdprod_guide.html

4http://www.universe.nasa.gov/xrays/programs/rxte/pca/doc/bkg/bkg-2007-saa/

Figure 8.4:χ²_redas obtained when an absorbed power-law model is applied toRXTE/PCA+HEXTE spectra in the 2.5–100 keV (upper panel) and in the 3–100 keV range (lower panel) for 3C 273. The inset display shows on a larger y scale theχ²_redfrom the 2.5–100 keV fit in the 1999–2001 period, when channel 2 had a maximum energy higher than 2.5 keV, resulting in the largeχ²_red observed. The dashed line indicates aχ²_redlevel of 1.3, corresponding to a probabilityP(χ²≥127.4)=2.5%.

RXTE observations in our database have been flagged as uncertain data as they fall into these periods.

Missing data for the SSA actually can affect the results reported here, even though it is not possible to estimate how many observations are concerned without redoing the analysis of the whole sample.

Nevertheless, the RXTE team estimates that for faint sources the combination of the two problems (SSA data missing and pcabackest bug) will cause errors on the PCA background estimation that are at maximum about 9% below 5 keV and 2% above, but usually the errors will be half of these values in the affected cases. Therefore, waiting for a reprocessing of the RXTE archive with the revised SAA history files, we consider that the spectra and light curves used here have a sufficient precision for our purposes.

An absorbed power law model represents well the large majority of the RXTE spectra. In 19 cases out of the almost 1000 spectra, a χ²_red ≥ 1.3 was obtained. A more detailed analysis of these spectra reveals that in 11 cases adding a Gaussian line at 6.4 keV (in the AGN rest frame) improves significantly the fit. In 2 of these 11 spectra it is necessary also to exclude the bins below 5 keV to obtain an acceptable fit. In 3 cases, the highχ²_red is due to the low signal-to-noise of the HEXTE data from one of the two detector clusters and in another case to the data above 70 keV. In the last 4 cases, the PCA spectra appear “wavy” and a power-law model turns out to be not an accurate representation.

In cases like the “wavy” PCA spectra, the power-law slope has to be considered only a description of the global trend of the spectrum rather than a precise representation of it.

The new data 113 XMM-Newton data

Several studies on the XMM data of 3C 273 have been already published. We collected 16 XMM/PN spectra from the works of Dr Martin Stuhlinger (see M. Stuhlinger Ph.D. Thesis, 2004, for analysis details) and Chernyakova et al. (2007), covering the period from June 14, 2000 to July 10, 2005.

Several other observations have been performed since then, but they were not yet publicly available at the time of this work. 4 calibration observations in the period June 2000, 13–16 have not been included as 3C 273 was from 1.5’ to 7’ off-axis. The January 9, 2002 PN data, taken in timing mode, were also excluded. Due to the higher signal-to-noise ratio of the PN spectra compared to the MOS ones, the former are preferred to the latter for the analysis. Separated fits in the 0.5–2 and 2–10 keV range are performed.

In 6 out of the 16 spectra analysed, the 0.5–2 keV fit results inχ²_red>1.3. This is not surprising, as Page et al. (2004) have already shown for these same XMM data how complex the model of the soft-excess has to be and that a multiple-blackbody component or a Comptonisation model are a better representation of this spectral feature in 3C 273. Nevertheless, we include in the database the results of the fit with a single power-law model for the same reasons as for BeppoSAX spectra.

INTEGRAL data

The region around 3C 273 represents the deepest extragalactic field observed by INTEGRAL up to date, with about 4 Ms of data collected. About 1 Ms of them (January 2003–July 2005) were available at the time of this work and are included in the database. In order to obtain a good signal-to-noise, the data from different observations are summed up in 5 periods, all of them containing IBIS/ISGRI data. Due to the narrower field of view of the JEM-X monitors, a smaller amount of INTEGRAL data below 20 keV is available, but all the 5 periods are covered. On the other hand, SPI data are available only during 3 of the 5 periods, because in one case the instrument was undergoing an annealing phase and in the other the data were taken in staring mode that makes unreliable the deconvolution of SPI data, due to the difficulty of the background determination. All the details about the INTEGRAL data analysis can be found in Chernyakova et al. (2007). For each time period, a combined fit including all the available INTEGRAL spectra is performed in the range 5–200 keV. In all cases, the simple absorbed power-law model is a good representation of the data.

Swift/XRT data

Since its launch on November 20, 2004, Swift has already performed several pointed observations of 3C 273, collecting data in the X-ray domain with the XRT monitor and optical-UV data with the UVOT camera. Among the 18 XRT observations performed up to the end of 2006, we selected those for which the instrument was operating in Photon Counting (PC) readout mode, because they have full imaging and spectral resolution. Furthermore, the observations shorter than 3 ks have been disregarded, due to the low signal-to-noise. We end up with 5 data sets, covering the period between November 15 and December 19, 2005. The data analysis was performed with the XRTDAS software v2.5a distributed within the HEAsoft 6.1.1 package. The latest calibration files available at the time of the analysis (CALDB files 2007-03-30) have been used.

All the XRT images extracted show a clear evidence of photon pile-up. This effect can be observed in bright sources as more than one photon can arrive in the same or neighboring pixels before the pixel is read out, making it impossible to distinguish between this group of photons and a single photon

Figure 8.5: Swift/XRT image of 3C 273 during the observation on November 15, 2005. The color-scale have been chosen in order to emphasize the central pixels showing a drop in the count rate as an indirect consequence of the pile-up effect (see the text for explanation).

at higher energies. As a consequence, both the spectral response and the point spread function are affected, resulting in artificially harder spectra and in a “hole” in the centre of the source image (Fig. 8.5), because the software rejects the large size events (as those induced by cosmic rays). In order to correct for this effect, the XRT spectra were extracted in an annular region around the source, with 6 and 20 pixels inner and outer radius, respectively, as suggested by Vaughan et al. (2006).

On May 27, 2005, the XRT detector was hit and damaged by a micro-meteorite and, since then, CCD columns DETX = 294 and 320 have been turned off (Abbey et al. 2006, Grupe et al. 2006). During the December 19, 2005 observation, the source is located on the dead-pixel columns, resulting in a

Figure 8.6:Swift/XRT image of 3C 273 during the observation on December 19, 2005. During that observation the dead columns of pixels crossed the position of the source reducing the actual collecting area (see the black stripe crossing the source on its right side). This effect has been corrected for with thexrtexpomaptool before extracting a spectrum.

The new format 115 decrease of the collecting area. To take this effect into account, we corrected the exposure map using the xrtexpomap tool. When comparing the spectrum obtained with the corrected exposure map and the uncorrected one, no differences are found in the fitting parameters.

The 5 XRT spectra obtained are then separately fitted in the 0.5–2 and 2–8 keV range, showing that the absorbed power law gives good results in all cases analysed.