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Issue
A&A
Volume 543, July 2012
Article Number A111
Number of page(s) 5
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201118289
Published online 06 July 2012

© ESO, 2012

1. Introduction

The number of known AGN whose emission extends up to the very high energy (VHE; E > 100 GeV) γ-ray band has more than doubled in the last couple of years. Nowadays, the TeV sources catalogue counts 44 AGN1, located both in the northern and in the southern hemisphere. This achievement has been made possible thanks to the good performances of the last generation of imaging atmospheric Cherenkov telescopes (IACT), namely MAGIC, HESS and VERITAS. A key ingredient for the detection of new sources has been the cooperation of these telescopes with satellite experiments, especially those operating at optical, X-ray and soft γ-ray frequencies.

The great majority of AGN detected at VHE belongs to the class of blazars. In these sources, the emission is produced in relativistic jets that point towards the observer.

PKS 0447 − 439 is a blazar located at RA(J2000) = 04h49m24.s88\hbox{$\rm{RA}(J2000)\,=\,04^{\rm{h}} 49^{\rm{m}} 24\fs88$}, Dec(J2000) = −43°50′09.″7\hbox{$\rm{Dec}(J2000)\,=\,-43\degr 50\arcmin 09\farcs 7$}. It was discovered in 1981 at radio wavelenghts by the Molonglo Telescope (Large et al. 1981) and detected by the PMN radio Survey in 1993 (Gregory et al. 1994).

The source was subsequently detected in the UV with EUVE (Lampton et al. 1997) and X-rays with ROSAT (White et al. 1994). Some confusion arose in the determination of the optical counterpart because at first a near-UV bright spectrum with prominent emission lines in the optical was related to PKS0447 − 439 (Craig et al. 1997). The source was therefore classified as a Type I Seyfert at z = 0.107. Instead Perlman et al. (1998) reported a featureless optical continuum typical of a BL Lac object.

We investigated this classification discrepancy, also inspecting some archival Swift/UVOT exposures of the field. We understood that the association of the Seyfert spectrum with this source is probably due to some databasing mistake, because the PKS 0447 − 439 coordinates reported in Craig et al. (1997) are incorrect, and point to a region that is source-free both in their optical finding chart and in our UVOT images.

The redshift of PKS 0447 − 439 was measured by Perlman et al. (1998), who reported a value of z ~ 0.205 based on few weak absorption features in the optical spectrum that were interpreted as the CaH,K doublet. Another study performed by Landt & Bignall (2008) provided only a lower limit of 0.176, based on the non-detection of the host galaxy, which is assumed to be a giant elliptical of inferred luminosity (Piranomonte et al. 2007). Recently, a different lower limit z > 1.246 was obtained (Landt 2012) based on the analysis of absorption lines in the optical spectrum. This new value is well above the measurement reported by Perlman et al. (1998).

The Large Area Telescope (LAT) instrument on board Fermi identified the source as one of the brightest of the southern hemisphere in the 100 MeV–300 GeV energy range (Abdo et al. 2009). Dedicated studies on the weekly light curve above 300   MeV revealed that the source is variable in this energy range (Abdo et al. 2010a). Inspired by the Fermi/LAT observations, the HESS IACT observatory performed a dense observation campaign between November 2009 and January 2010. This led to the detection of a VHE signal, statistically significant above the 13σ level and positionally consistent with PKS 0447 − 439, as preliminarily reported in Zech et al. (2011).

In this Paper, we derive an independent estimate of the distance to PKS 0447 − 439 using combined GeV and TeV spectral information, following the method we proposed in Prandini et al. (2010). Our goal is to provide a new measurement of the source’s redshift, which is still uncertain, as discussed above. In the next section we briefly outline the method and present the result, which basically confirms the estimate of z ~ 0.2 given by Perlman et al. (1998). Then, we build a quasi-simultaneous SED, which is fitted with a homogeneous, leptonic SSC model. This allows the determination of the main physical parameters governing the jet physics.

2. Inferring the distance of TeV emitting blazars

The VHE spectrum of blazars suffers from the absorption arising from the interaction with the extragalactic background light (EBL) (Hauser & Dwek 2001). This absorption, caused by electron-positron pair production (Nikishov 1962), induces a partial/total deformation of the VHE part of the spectrum that is strongly redshift-dependent. In general, for nearby sources that are located at redshift below 0.1, it affects the spectral points above some TeV. At higher redshifts, between 0.1 to 0.5, it affects the spectrum already at some hundred of GeV, while above 0.5 it becomes effective already above some GeV. Therefore, given an intrinsic VHE spectrum, the observed one depends on the distance of the emitter. In other words, the optical depth associated to the absorption process depends on the distance covered by the energetic photon and on its energy. Unfortunately, owing to strong foreground emissions that are difficult to suppress, there are no solid measurements of EBL. Upper and lower limits are provided by indirect techniques such as galaxy counts, which provide solid lower limits. In addition to these measurements, many models of the EBL energy density and evolution have been proposed in the last years (Stecker et al. 2006; Franceschini et al. 2008; Kneiske & Dole 2010; Domínguez et al. 2011). In this work we adopt the model presented in Franceschini et al. (2008).

To estimate the distance to PKS 0447 − 439, we applied the method that we proposed in Prandini et al. (2010) and updated in Prandini et al. (2011). This method is based on the comparison between the high energy (HE; 0.1 < E < 100    GeV) γ-ray spectrum measured by Fermi/LAT and that measured at VHE by the last generation of imaging atmospheric Cherenkov telescopes (MAGIC, HESS and VERITAS).

We refer the reader to the original paper (Prandini et al. 2010) for the full description of the method. Briefly, to infer the distance of an HE and VHE gamma-ray emitting blazar, we calculate the redshift, z∗, at which the power law slope fitting the VHE spectrum corrected for the EBL absorption equals the slope measured by Fermi/LAT at lower energies. As empirically demonstrated in the paper using a set of known redshift sources, this value is related to the true redshift of the source by a simple linear relation. Therefore, once one has obtained z ∗ , it is possible to give an estimate on the source distance, zrec, by using the inverse formula: zrec=(A−z∗)B,\begin{equation} \label{eq:1} z_{\rm rec} = \frac{(A - z^*)}{B}, \end{equation}(1)with A and B given in Prandini et al. (2011). The law was applied to infer the distance of the unknown redshift blazar PKS 1424 + 240. The value obtained has been confirmed in a study on the host galaxy performed by Meisner (2010).

2.1. The reconstructed z of PKS 0447−439

The blazar PKS 0447−439 was observed by the HESS IACT observatory between November 2009 to January 2010, for a total of 13.5 h of data. A clear signal of 13.8σ of significance was detected at energies above 250 GeV (Zech et al. 2011). The preliminary spectrum extends from 300 GeV up to more than 1 TeV, and is compatible with a steep simple power law of index Γ = 4.36  ±  0.49, where the error reported is statistical only. The significant spectral points are drawn in Fig. 1, open markers.

The Fermi/LAT spectral slope in the energy range 0.1 − 100 GeV is ΓLAT = 1.95  ±  0.03, as reported in the 1FGL catalogue (Abdo et al. 2010b). We adopted this slope, and not the one extracted from only simultaneous data, to be consistent with the analysis performed for the determination of the z∗    −    ztrue law.

thumbnail Fig. 1

Observed (light grey, open circles) and deabsorbed VHE spectra of PKS 0447 − 439, as reported in Zech et al. (2011). Deabsorption is applied assuming the Franceschini et al. (2008) EBL model, and a redshift of the source z∗ = 0.36 (black, filled circles) and zrec = 0.2 (dark grey, filled squares).

The redshift that we obtain when requiring that the deabsorbed PKS 0447 − 439 spectrum has a slope equal to ΓLAT is z ∗  = 0.357  ±  0.065, where the error takes into account both the errors on the TeV slope and on the Fermi/LAT slope. The corresponding spectrum is drawn in Fig. 1, filled black circles. A simple power law fits the data very well, with a probability higher than 99%, and χ2/n.d.f. = 0.1/3.

thumbnail Fig. 2

Diagram adapted from Prandini et al. (2011) representing the linear relation between the redshifts of known distance sources and their z ∗  values. The value of z ∗  obtained for PKS 0447 − 439 in this work is superimposed in black.

Figure 2 shows the z ∗     −    ztrue plot in linear scale, adapted from Prandini et al. (2011). The grey markers represent the z ∗  values of the sixteen sources with known redshifts used to infer the linear relation (dashed-dotted line). The continuous line is the z ∗     =    ztrue line. All points lie above or on the line, meaning that the values z ∗  are higher than the real redshifts. This indicates z ∗  as a good upper limit on the source distance. In our case, we have z ∗  of 0.357 ± 0.065 and the corresponding upper limit is 0.49 at 2 sigma level. Applying Eq. (1) to our data, we obtain for PKS 0447 − 439 the distance zrec = 0.20  ±  0.05, where the error is statistical only. The differential energy spectrum obtained assuming a distance z of 0.20 is drawn in Fig. 1, filled squares. It lies between the observed spectrum (not corrected for EBL absorption) and the spectrum obtained assuming a distance z ∗ . A simple power law of index Γ = 3.2  ±  0.5 fits the data very well (probability  > 99%).

thumbnail Fig. 3

SED of PKS 0447 − 439 during the epoch of HESS observation (November 2009 − January 2010). Red points represent Swift/UVOT, Swift/XRT, Fermi/LAT and de-absorbed (assuming z    =    0.2) HESS data. Cyan data are the observed HESS data. Green symbols report historical data obtained through the ASI/ASDC tools. Green LAT points are from the 1LAC catalogue. The black lines are the result of the one-zone emission leptonic model discussed in the text for two different values of the minimum Lorentz factor of the emitting electrons, γmin    =    1 (solid) and γmin    =    3 × 103 (dashed). See text for details.

The estimated distance obtained perfectly agrees with the value z = 0.205 reported by Perlman et al. (1998), and also confirms the lower limit of z > 0.176 based on photometric estimates. Therefore, our study strongly supports the value of z = 0.205 for PKS 0447 − 439.

Table 1

Input model parameters for the models reported in Fig. 3.

3. Spectral energy distribution

The single-epoch multiwavelength SED of PKS 0447−439 is plotted in Fig. 3, and includes, in addition to the HESS data presented in previous section, Swift/UVOT, Swift/XRT, and Fermi/LAT data acquired during the epoch of the HESS detection, and other historical data.

Swift observed PKS 0447 − 439 for about one week at the end of the HESS campaign (Zech et al. 2011). Data from the UVOT (Roming et al. 2005) observation taken on December 25 (obs. ID 00038100009) were analysed by means of the uvotimsum and uvotsource tasks with a source region of 5′′, while the background was extracted from a concentric source-free annular region with inner and outer radii of 10′′ and 15′′, respectively. The extracted magnitudes were corrected for Galactic extinction using the values of Schlegel et al. (1998) and applying the formulae by Pei (1992) for the UV filters, and eventually were converted into fluxes following Poole et al. (2008).

XRT (Burrows et al. 2005) νFν points corresponding to the observation of Dec. 25 (MJD 55190, obs. ID 00038100010) were obtained through the ASI/ASDC on-line analysis tool2 (Stratta et al. 2010). The X-ray fluxes were obtained assuming a power law spectrum (best-fit photon index Γ =  −2.46) and fixing the absorption to the Galactic value NHGal=1.24×1020\hbox{$N_{\rm H}^{\rm Gal}=1.24 \times 10^{20}$} cm-2 (after Kalberla et al. 2005).

We retrieved the publicly available data taken by the LAT γ-ray telescope on board the Fermi satellite (Atwood et al. 2009) in scanning mode from the NASA database3. We selected the good quality (“DIFFUSE” class) events observed within 10° from the source position, taken between MJD 55170 and MJD 55190, and with measured energy in the 0.2−100 GeV interval. We excluded events observed at zenith distances greater than 105° to avoid contamination from the Earth albedo. We performed the analysis by means of the standard science tools, v. 9.18.6, including Galactic and isotropic extragalactic backgrounds and the P6 V3 DIFFUSE instrumental response function. We applied an unbinned likelihood algorithm (gtlike) to the data, modelling the source spectrum with a power law model, with the integral flux in the 0.2 − 100 GeV energy band and photon index left as free parameters. We clearly detected the source with a test statistics (Mattox et al. 1996) TS    =    154, and determined F0.2−100   GeV    =    6.93    ±    1.26 × 10-8 ph cm-2 s-1 and Γ    =    1.93    ±    0.14.

We also performed the same analysis on the dataset split into logarithmically equal bins of energy, accepting spectral points with TS  > 25 and more than three events attributed to the source in the model. The results are plotted in Fig. 3.

PKS 0447 − 439 belongs to the group of highly peaked BL Lac objects. Within the framework of leptonic models, the emission of this type of sources is generally modelled with the one-zone SSC model (e.g. Tavecchio et al. 1998, 2010; Abdo et al. 2011). The idea that the emission mainly originates in a single, uniform, region is supported by the correlated variability observed at different frequencies, especially X-rays and gamma-rays (e.g. Fossati et al. 1998).

The good spectral coverage allows one to firmly constrain the low-energy bump, which is related to synchrotron radiation produced by accelerated electrons in the jet, whose peak lies around 1016 Hz (see Fig. 3). LAT and HESS data track the so-called SSC component quite well, which shows a broad peak in the GeV region. As detailed in Tavecchio et al. (1998), in the framework of the one-zone SSC model knowing of the peak frequencies and luminosities, together with an estimate of the variability timescale, allows one to determine the physical parameters of the emitting region. An indication of the variability timescale can be derived from the multi-frequency observations, especially those at X-ray energies, which reveal variability on timescale of  ≈ days. We therefore assume an upper limit of tvar < 1 day for the variability timescale.

We modelled the SED with the one-zone leptonic model described in Maraschi & Tavecchio (2003). The emission region is spherical with radius R, in motion with bulk Lorentz factor Γ at an angle θ with respect to the line of sight. Special relativistic effects are fully accounted for by the relativistic Doppler factor, δ    =     [Γ(1 − βcosθ)] -1. The energy distribution of the emitting electrons is assumed to be well described by a smoothly connected broken power law function, with minimum, maximum and break Lorentz factor γmin, γmax and γb, respectively. The SSC emission is calculated assuming the full Klein-Nishina cross section (Jones 1968). We recall that within the framework of the one-zone SSC model the synchrotron self-absorption causes the source to be opaque below frequencies of about 1011 − 1012 Hz. The emission below these frequencies is therefore produced by more distant, transparent regions of the jet.

Two possible SEDs are reported in Fig. 3, while the corresponding input parameters are listed in Table 1. In the table we also report the derived powers carried by the different components, relativistic electrons, magnetic field and protons (assuming a composition of one cold proton per relativistic electron) and the total radiative power of the jet, Pr ≃ Lobs/δ2 (in which we assumed δ ~ Γ).

The jet power is strongly dependent on the total density of particles in the jet that, in turn, is dominated by the number of relativistic electrons at γmin. While this parameter can be relatively well determined for FSRQs, because the low-energy end of the electron energy distribution is directly accessible through the inverse Compton emission that falls in the X-ray band (e.g. Ghisellini et al. 2010), for BL Lac objects emitting through SSC the constraints are quite loose. As an example of the impact of different values of γmin on the derived SED, we report in Fig. 3 two curves, corresponding to γmin    =    1 (solid line) and γmin    =    3 × 103 (dashed). The two curves clearly show that the value of γmin mainly affects the low-energy branch of the synchrotron and SSC bumps. Clearly, any value between 1 and 103 is allowed by the available data, which determines a large uncertainty on the inferred powers (see Table 1).

The magnetic field energy density appears to be several orders of magnitude below the equipartition value with the electron energy density and, consequently, the corresponding Poynting flux is negligible compared to the power carried by protons and electrons. This is a feature that PKS 0447 − 439 shares with several other TeV BL Lacs (e.g. Tavecchio et al. 2010; Ghisellini et al. 2010).

4. Conclusions

We have presented the results of our study on the distance to PKS 0447 − 439. This work was triggered by the preliminary measurement of the differential energy spectrum emitted by the source at VHE gamma-rays performed by the HESS Collaboration (Zech et al. 2011). The redshift we found with our method, based on both HE and VHE spectra, perfectly agrees with the measurement performed by Perlman et al. (1998), which reports a redshift of 0.205. But it contradicts with the recent assertion of Landt (2012), who claimed a redshift higher than 1.246.

Assuming this redshift, we have built a broadband SED including quasi-simultaneous ultraviolet, X-ray and gamma-ray data acquired during the epoch of the HESS detection. The SED can be well fitted within the framework of a homogeneous, leptonic SSC model. The available data leave a large uncertainty on one of the parameters of the model, γmin, the minimum Lorentz factor of the emitting electrons. This uncertainty propagates to the inferred powers. In any case, the physical parameters that we found suggest a strongly matter-dominated jet, in agreement with the other sources observed at these extreme energies.

The method used in this paper has great potential. A large part of GeV emitting blazars, indeed, about 60% of the BL Lac objects present in the second catalogue of AGN detected by Fermi/LAT (Ackermann et al. 2011), has unknown redshift. If detected in the VHE band, the redshift of these powerful objects could be investigated through this technique. Therefore, we encourage the observation of promising TeV emitters among the unknown/uncertain redshift Fermi blazars.

Acknowledgments

This research has made use of public Swift and Fermi data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASAs Goddard Space Flight Center through the Science Support Center (SSC). Part of this work is also based on archival data and on-line services provided by the ASI Science Data Center (ASDC).

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All Tables

Table 1

Input model parameters for the models reported in Fig. 3.

In the text

All Figures

thumbnail Fig. 1

Observed (light grey, open circles) and deabsorbed VHE spectra of PKS 0447 − 439, as reported in Zech et al. (2011). Deabsorption is applied assuming the Franceschini et al. (2008) EBL model, and a redshift of the source z∗ = 0.36 (black, filled circles) and zrec = 0.2 (dark grey, filled squares).
In the text
thumbnail Fig. 2

Diagram adapted from Prandini et al. (2011) representing the linear relation between the redshifts of known distance sources and their z ∗  values. The value of z ∗  obtained for PKS 0447 − 439 in this work is superimposed in black.
In the text
thumbnail Fig. 3

SED of PKS 0447 − 439 during the epoch of HESS observation (November 2009 − January 2010). Red points represent Swift/UVOT, Swift/XRT, Fermi/LAT and de-absorbed (assuming z    =    0.2) HESS data. Cyan data are the observed HESS data. Green symbols report historical data obtained through the ASI/ASDC tools. Green LAT points are from the 1LAC catalogue. The black lines are the result of the one-zone emission leptonic model discussed in the text for two different values of the minimum Lorentz factor of the emitting electrons, γmin    =    1 (solid) and γmin    =    3 × 103 (dashed). See text for details.
In the text

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