A&A 626, A60 (2019) Astronomy https://doi.org/10.1051/0004-6361/201834363 & c ESO 2019 Astrophysics
F-GAMMA: Multi-frequency radio monitoring of Fermi blazars The 2.64 to 43 GHz Effelsberg light curves from 2007–2015?
E. Angelakis1, L. Fuhrmann1,2, I. Myserlis1, J. A. Zensus1, I. Nestoras1,3, V. Karamanavis1,2, N. Marchili4, T. P. Krichbaum1, A. Kraus1, and J. P. Rachen5,6
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [email protected]; [email protected] 2 Fraunhofer Institute for High Frequency Physics and Radar Techniques, Fraunhoferstraße 20, 53343 Wachtberg, Germany 3 Deimos Space UK, Fermi Avenue R103, Harwell, Oxford OX11 0QR, UK 4 Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Rome, Italy 5 Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands 6 Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
Received 2 October 2018 / Accepted 7 December 2018
ABSTRACT
Context. The advent of the Fermi gamma-ray space telescope with its superb sensitivity, energy range, and unprecedented capability to monitor the entire 4π sky within less than 2–3 h, introduced a new standard in time domain gamma-ray astronomy. Among several breakthroughs, Fermi has – for the first time – made it possible to investigate, with high cadence, the variability of the broadband spectral energy distribution (SED), especially for active galactic nuclei (AGN). This is necessary for understanding the emission and variability mechanisms in such systems. To explore this new avenue of extragalactic physics the Fermi-GST AGN Multi-frequency Monitoring Alliance (F-GAMMA) programme undertook the task of conducting nearly monthly, broadband radio monitoring of selected blazars, which is the dominant population of the extragalactic gamma-ray sky, from January 2007 to January 2015. In this work we release all the multi-frequency light curves from 2.64 to 43 GHz and first order derivative data products after all necessary post-measurement corrections and quality checks. Aims. Along with the demanding task to provide the radio part of the broadband SED in monthly intervals, the F-GAMMA programme was also driven by a series of well-defined fundamental questions immediately relevant to blazar physics. On the basis of the monthly sampled radio SEDs, the F-GAMMA aimed at quantifying and understanding the possible multiband correlation and multi-frequency radio variability, spectral evolution and the associated emission, absorption and variability mechanisms. The location of the gamma- ray production site and the correspondence of structural evolution to radio variability have been among the fundamental aims of the programme. Finally, the programme sought to explore the characteristics and dynamics of the multi-frequency radio linear and circular polarisation. Methods. The F-GAMMA ran two main and tightly coordinated observing programmes. The Effelsberg 100 m telescope programme monitoring 2.64, 4.85, 8.35, 10.45, 14.6, 23.05, 32, and 43 GHz, and the IRAM 30 m telescope programme observing at 86.2, 142.3, and 228.9 GHz. The nominal cadence was one month for a total of roughly 60 blazars and targets of opportunity. In a less regular manner the F-GAMMA programme also ran an occasional monitoring with the APEX 12 m telescope at 345 GHz. We only present the Effelsberg dataset in this paper. The higher frequencies data are released elsewhere. Results. The current release includes 155 sources that have been observed at least once by the F-GAMMA programme. That is, the initial sample, the revised sample after the first Fermi release, targets of opportunity, and sources observed in collaboration with a monitoring programme following up on Planck satellite observations. For all these sources we release all the quality-checked Effelsberg multi-frequency light curves. The suite of post-measurement corrections and flagging and a thorough system diagnostic study and error analysis is discussed as an assessment of the data reliability. We also release data products such as flux density moments and spectral indices. The effective cadence after the quality flagging is around one radio SED every 1.3 months. The coherence of each radio SED is around 40 min. Conclusions. The released dataset includes more than 3 × 104 measurements for some 155 sources over a broad range of frequencies from 2.64 GHz to 43 GHz obtained between 2007 and 2015. The median fractional error at the lowest frequencies (2.64–10.45 GHz) is below 2%. At the highest frequencies (14.6–43 GHz) with limiting factor of the atmospheric conditions, the errors range from 3% to 9%, respectively. Key words. astronomical databases: miscellaneous – galaxies: active – galaxies: jets – radio continuum: galaxies – quasars: general – BL Lacertae objects: general
? Full Tables 7, 9 and 10 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http:// cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/626/A60
Article published by EDP Sciences A60, page 1 of 35 A&A 626, A60 (2019)
1. Introduction events that probe the emission and variability mechanisms; and finally (d) the availability of multi-frequency linear and circular The current work constitutes the release of the first part of polarisation light curves (Myserlis et al. 2018) which give access the F-GAMMA (Fuhrmann et al. 2016) dataset, which includes to the microphysics of the emitting plasma (Myserlis et al. 2016; the centimetre and subcentimetre light curves obtained with the Angelakis et al. 2017). Element (c) is particularly important as it 100 m Effelsberg radio telescope between 2007 and 2015. The allowed us to collect a large sample of events from an otherwise F-GAMMA programme collected a vast amount of monthly inherently biased sample as we discuss later. monitoring data for more than 100, almost exclusively, Fermi The analysis that has already been carried out for a lim- blazars over an unprecedentedly broad radio spectrum down to ited part of the dataset has led to a series of noteworthy submillimetre wavelengths. The current release alone contains findings. An examination of the F-GAMMA dataset with ref- more than 3 × 104 measurements that have survived quality fil- erence to the first Fermi releases (Abdo et al. 2009, 2010b) tering, and naturally raises the question of what is the motivation showed that the detectability in the giga electron volt (GeV) for such a massive observational effort on blazars? energy range was an increasing function of the variability in the Blazars are active galactic nuclei (AGN) with their collimated, radio regime (Fuhrmann et al. 2016) as found by other studies relativistic plasma outflow (jet) in close alignment to the line (e.g. Richards et al. 2011, with observations at 15 GHz). After of sight of the observer (a few degrees, Blandford et al. 1978; accounting for the numerous biases affecting a flux-flux cor- Blandford & Königl 1979). The relativistic beaming induced by relation analysis we found that radio and gamma-ray emis- this configuration causes the jet emission to outshine all other sions are correlated with a significance that increases with components thus making it an ideal probe of the physical con- radio frequency (Fuhrmann et al. 2016). This finding was inter- ditions and processes in the exotic environments of relativis- preted as an indication that the gamma-ray emission is produced tic jets. These sources exhibit an exceptionally broad spectral very close to (or in the same region as) the millimetre-band emis- energy distribution (SED), which often spans 20 orders of mag- sion region. The radio spectral evolution was used to model and nitude in frequency or even more (e.g. Giommi et al. 2012), mak- quantify the broadband variability mechanism (Angelakis et al. ing blazars the dominant high-energy population. In the Fermi- 2012). On the basis of the cross-correlation of radio and LAT third source catalogue (3FGL; Acero et al. 2015) blazars gamma-ray light curves of selected cases, we managed to comprise 60% of the detected sources and more than 90% of constrain the gamma-ray emission site (Fuhrmann et al. 2014; the associated sources. The high-energy-peaked component of Karamanavis et al. 2016). We further developed a method that their characteristic double-peaked SED (e.g. Abdo et al. 2010a) parametrises each outburst separately allowing us to account for is argued to be the signature of a photon field that is inverse different variability mechanisms operating in the same source Compton up-scattered by plasma producing its low-energy-peak at different times. This method was then used to quantify the synchrotron component. Blazars exhibit intense variability at all presence of a relativistic jet in narrow-line Seyfert 1 galaxies energies (e.g. Padovani et al. 2017) over timescales from min- (Angelakis et al. 2015) and constrain the multi-frequency vari- utes (e.g. Aleksic´ et al. 2011, inferred doubling time at TeV ability Doppler factors (Liodakis et al. 2017). Finally, assuming energies of around 10 minutes) to several months or more the radio variability to be caused primarily by traveling shocks (e.g. Fuhrmann et al. 2016). They typically appear significantly (Angelakis et al. 2012) we constructed a realistic radio jet emis- polarised (Strittmatter et al. 1972), especially at higher energies sion model able to reproduce the linearly and circularly polarised (e.g. Angelakis et al. 2016), and their polarisation also undergoes radiation (Myserlis et al. 2018) and quantify the physical condi- intensive variability not only in terms of fraction but also polar- tions and their evolution in those systems (Myserlis et al. 2016; isation plane orientation (e.g. Marscher et al. 2008; Blinov et al. Angelakis et al. 2017). 2015). The blazar phenomenology and the richness of the relevant In the following we present the F-GAMMA dataset from jet physics becomes even more apparent with their role as complex 2.64 to 43 GHz. The higher frequencies datasets will be released particle accelerators and, as of recently, even confirmed neutrino in subsequent publications. We begin with a detailed description emitters (IceCube Collaboration 2018; Padovani et al. 2018) urg- of the sample that is included in the data release (Sect.2) and ing the re-evaluation of our understanding of their dominant emis- then we give a detailed description of the observations (Sect.3) sion mechanisms. and the post-measurement data treatment (Sect.4). The raw data The F-GAMMA programme was initiated with the scope are presented in Sect.5 in the form of multi-frequency light to provide necessary multi-frequency radio monitoring comple- curves. Finally, in Sect.6 we present spectral indices as a higher mentary to the Fermi-LAT (Atwood et al. 2009) monitoring of level data product. The content of this paper is strictly confined the gamma-ray sky, and to study certain aspects of relevant radio to the needs of a data release. Radio astrophysical studies and physics. Among the notable advantages of radio monitoring of interpretation of the data will be presented in subsequent publi- blazars is that the radiation in these bands originates almost cations entirely at the plasma jet and the contamination with other sources of radio emission is insignificant, if any. The unique- 2. Source sample ness of the F-GAMMA programme in particular is attributed to four elements: (a) its multi-frequency character, which allows As we discuss in Sect. 2 of Fuhrmann et al.(2016), the us to follow the evolution and dynamics of the radio SED F-GAMMA programme was optimised to complement the Fermi and link it with underline physical processes; (b) the relatively blazar monitoring. Specifically, we developed the programme to high cadence observations, which are optimal (in most cases) quantify and understand the broadband blazar variability, localise for a satisfactory sampling of the spectral evolution, and most the gamma-ray emission site, and study the evolution of condi- importantly, which resolves the inevitable degeneracies that stem tions in the emitting elements. The initial F-GAMMA sample from merging several evolving elements into one comparatively included 62 sources previously detected by the Energetic Gamma large telescope beam; (c) its long duration, which allows us to Ray Experiment Telescope (EGRET) with flat-spectrum radio acquire a firm understanding of the source behaviour at different quasars (FSRQs) comprising roughly 52% and BL Lacs 37% of timescales and to collect a large number of spectral evolution the total.
A60, page 2 of 35 E. Angelakis et al.: F-GAMMA: Multi-frequency radio monitoring of Fermi blazars
With the release of the First Fermi-LAT Source Catalogue 2.00 (Abdo et al. 2010b), the sample was revised around mid-2009 to include exclusively LAT-detected sources. The updated sample 1.75 3FGL blazars: frsq, bll, bcu comprised a total of 65 Fermi sources, 25 of which are already F-GAMMA monitored in the first sample. The new sample was chosen on the basis of 1.50 the observability of the sources from Effelsberg and IRAM obser- 1.25 vatories, their variability in radio and gamma rays, whether they were monitored by other programmes and several cases of special 1.00 interest such as narrow-line Seyfert 1 galaxies. The sources were observed with different cadences and priorities depending on all 0.75 these parameters (cf. Sect.3). A considerable amount of observ- Probability Density ing effort was put on targets of opportunity. 0.50 As a consequence, the F-GAMMA sample includes: (a) sources that have been observed only for the first 2.5 years 0.25 until the sample revision, (b) sources that have been observed 0.00 over the entire duration of the programme (before and after the 13 12 11 10 9 8 − Log(− νF (100MeV− 100GeV− )/ erg cm−2 s 1) − sample revision), (c) sources that were included after the sam- ν − − − ple revision, and (d) targets of opportunity that were observed 2.5 occasionally. In this work we release the data for every source with at least one measurement. The exception to this rule con- 3FGL blazars: frsq, bll, bcu stitutes a sample of TeV sources (Wakely & Horan 2008) and 2.0 F-GAMMA monitored narrow-line Seyfert 1 galaxies (Angelakis et al. 2015) that will be published in separate papers. Table A.1 lists all the sources in our data release and includes five sections: (a) the main mon- 1.5 itored sample that was finalised with the mid-2009 revision, (b) sources monitored until that revision (labelled “old”); (c) sources observed within the F-GAMMA-Planck satellite MoU; 1.0
(d) other sources observed mostly as targets of opportunity; and Probability Density (e) calibrators. The sample in section (c) was the result of the 0.5 partial overlap of the F-GAMMA monitoring and the regular sky-scans of ESA’s Planck satellite1. The source sample and science goals are discussed by Rachen et al.(2016). Part of the 0.0 dataset is also presented elsewhere (Planck Collaboration XIV 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2011; Planck Collaboration XV 2011; Giommi et al. 2012). Log(3FGL variability index) Given the primary aim of the programme to understand mech- Fig. 1. Distribution of energy flux (upper panel) and variability index anisms and not population statistics, the F-GAMMA sample has (lower panel) in the band 100 MeV–100 GeV. The grey area corre- been inevitably biased towards the brightest and most variable sponds to all the sources in the 3FGL catalogue designated as “fsrq”, blazars; the former guarantee the highest quality datasets and the “bll’,’ and “bcu” while the black solid line to the F-GAMMA monitored latter frequent outbursting events. Figure1 provides a qualitative sample. impression of how our monitored sample is representative of the entire blazar population. There we show the distributions of the (Table A.1, Col. 8), were observed on a monthly basis in every energy flux in the range 100 MeV–100 GeV (upper panel) and the session. Another 30 slower variable sources were grouped in two variability index (lower panel), respectively, for all the sources sets of 15 sources each, labelled groups “s1” and “s2’, and were in the 3FGL catalogue (Acero et al. 2015) designated as “fsrq”, observed every other session. The two groups comprised a total “bll’,’ and “bcu”. These plots show that the F-GAMMA moni- of 65 sources that were being monitored. tored sample populates the upper end of high-energy distribution The observations were conducted with the secondary focus as well as that of the variability index. In Fig.2 we compare the heterodyne receivers of the 100 m Effelsberg telescope (Table1). redshift distribution of the F-GAMMA monitored sources to that The systems at 4.85, 10.45, and 32 GHz are equipped with mul- of all BZCAT catalogue sources (Massaro et al. 2015a). A two- tiple feeds allowing differential measurements which partially sample KS test showed that the distribution F-GAMMA moni- remove effects of disturbing atmospheric emission or absorption tored sample over redshift is not qualitatively different from that perturbations. Practically, only linear tropospheric disturbances of all the BZCATsources (p-value: 0.009). From this we conclude are treated (only partial overlap of the atmosphere columns that despite the biases in its selection the F-GAMMA sample is “seen” by the feeds). representative of the blazar population at least in terms of source Because the sources are point-like or only slightly extended cosmological distribution. for the 100 m telescope beam, the observations were conducted with “cross-scans” i.e. by recording the antenna response while repeatedly slewing over the source position in two orthogo- 3. Observations nal directions. One slew in one direction has been termed a After the sample revision in mid-2009 the F-GAMMA adopted “sub-scan”. In our case the scanning was carried out over the an optimised observing scheme for the more efficient usage azimuthal (AZI) and the elevation (ELV) directions. This tech- of time. The 35 fastest varying sources, labelled group “f” nique offers immediate detection of extended source structure or confusion from field sources and pointing offset corrections. The 1 https://www.esa.int/Our_Activities/Space_Science/ observing cycle typically included two scans: one for telescope Planck pointing and one as the actual measurement.
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1.0 The opacity τatm at the source position is a function of its ELV, and is given by 1 0.8 τatm = τ (ELV) = τz · AM = τz · , (3) sin(ELV)
where τz is the atmosperic opacity at the zenith and AM the air 0.6 mass. Correcting therefore for the atmospheric opacity at the
CDF source position requires the knowledge of the opacity at zenith, 0.4 τz. The zenith opacity is computed from the observed depen- dence of Tsys on ELV (or equivalently on the airmass AM) and this is done for each individual session. First, a lower envelope 0.2 All BZCAT sources (straight line) is fitted to the scatter plot of Tsys against AM. The F-GAMMA monitored sources fitted line is then extrapolated to estimate the system tempera- ture at zero airmass. This point is subsequently connected with 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 the system temperature of the actual measurement to compute redshift the opacity for that scan. In Fig.4 we plot the computed zenith Fig. 2. Redshift distribution for the F-GAMMA monitored sources com- opacity at three characteristic frequencies. pared to all the BZCAT sources (Massaro et al. 2015a). The mean opacity at zenith for the receivers used is tabulated in Table2. As can be seen there, the opacity becomes particu- larly important towards higher frequencies. Because the opacity 4. Data reduction and system diagnostics correction is that with the largest impact, in Fig.5 we present For the reconstruction of the true source flux density every its effect at the three typical bands. In those plots we show the observation was subjected to a series of post-measurement cor- fractional increase of the pointing corrected antenna temperature rections. The median fractional effects of these corrections are Tpoi when opacity correction is applied. reported in Table2. Below we discuss them in order of execution. 4.3. Elevation-dependent gain correction This post-measurement operation accounts for losses caused by 4.1. Pointing offset correction small-scale gravity-induced departures of the geometry of the primary reflector from that of an ideal paraboloid. The power At this first stage the reduction pipeline accounts for the power loss can be well approximated by a second order polynomial loss caused by possible differences between the commanded and function of the source ELV. For an observed antenna tempera- actual source position. The pointing offset is deduced from the ture T, the corrected value, is difference between the expected source position and the maximi- −1 sation of the telescope response. If we approximate the telescope Tgc = T · G , (4) main beam pattern with a Gaussian, and the antenna tempera- where G is the “gain curve” value at the observing frequency and ture observed by scanning over a direction “i” is Ti, then that at the source ELV. It is given by corrected for pointing offset is G (ELV) = A + A · ELV + A · ELV2. (5) 0 1 2 ∆p !2 j In Fig.6 we show the functions used for our observations. Ti,poi = Ti · exp 4 · ln 2 · , (1) θ The parameters A0, A1, and A2 are tabulated in Table1. As shown there, the ELV of maximum gain for the lowest frequencies where i, j is the scanning direction indices with i: ELV, AZI and tends to be at lower elevations. The enhancement of the beam j: AZI, ELV, ∆p j the pointing offset in j direction, and θ the full side lobes at these frequencies imposes an overestimation of the width at half maximum (FWHM) at the observing frequency. We opacity at those elevations. Hence, for low elevations the source note that the offset in the j direction is used for the correction of flux densities tend to be over-corrected. As a result, because the the measurement in the i direction. gain curves are computed from opacity-corrected data, they tend In Fig.3 we show the pointing o ffsets at three characteris- to overestimate the gain at those lower elevations. The fractional tic frequencies: 4.85 GHz (low), 14.60 GHz (intermediate band), effect of this operation is constrained to mainly less than one and 32.00 GHz (high band). The black solid line corresponds to percent (Table2). the AZI and the grey area to the ELV scan. Table3 summarises the corresponding pointing parameters. As shown in Table2 we conclude that this effect is of the order of a few percent at maxi- 4.4. Absolute calibration mum for all the receivers. The corrected antenna temperatures are finally converted into Jy by comparison with the observed antenna temperatures of stan- 4.2. Atmospheric opacity correction dard targets termed primary calibrators. This operation also cor- rects for variations in the antenna and receiver gains between This operation is correcting for the attenuation induced by different observing epochs. The calibrators used here along with the signal transmission through the terrestrial atmosphere. The the assumed flux densities are shown in Table4. In case of mul- opacity-corrected antenna temperature Topc is computed from tiple calibrator observations a mean correction factor was used. the observed antenna temperature T, as For NGC 7027, our calibration procedure accounts for the tem- poral evolution of its spectrum (Zijlstra et al. 2008) and a par- T = T · eτatm , (2) opc tial power loss is caused by its extended structure relative to the 00 00 where τatm is the atmospheric opacity at the source ELV. beam size above 10.45 GHz (7 × 10 , Ott et al. 1994).
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Table 1. Calibration parameters of the used receivers.
ν ∆ν Tsys θ Feeds Polarisation Aperture A0 A1 A2 Epoch ELVmax (GHz) (GHz) (K) (00) (%) (10−4) (10−5) (Deg) 2.64 0.08 17 260 1 LCP, RCP 58 1.00000 0.0000 −0.0000 Feb. 2007 . . . 4.85 0.5 27 146 2 LCP, RCP 53 0.99500 5.2022 −1.2787 Feb. 2008 20.3 8.35 1.1 22 82 1 LCP, RCP 45 0.99500 4.3434 −1.0562 Feb. 2007 20.6 10.45 0.3 52 68 4 LCP, RCP 47 0.99000 8.2490 −1.7433 Feb. 2007 23.7 14.60 2.0 50 50 1 LCP, RCP 43 0.97099 18.327 −2.8674 Feb. 2007 32.0 23.05 2.0 77 36 1 LCP, RCP 30 0.91119 47.557 −6.2902 Feb. 2007 37.8 32.00 4.0 64 25 7 LCP 32 0.91612 49.463 −7.1292 Feb. 2007 34.7 43.00 (a) 2.8 120 20 1 LCP, RCP 19 0.88060 58.673 −7.1243 Feb. 2007 41.2
Notes. Entry in each column is as follows: 1: central frequency; 2: receiver bandwidth; 3: system temperature; 4: FWHM; 5: number of available feeds; 6: available polarisation channels; 7: telescope aperture efficiency at the corresponding frequency; 8, 9, and 10: parameters A0, A1, and (a) A2 defining the gain curve, respectively; 11: epoch of the gain curve observation; and 12: ELV where the gain is maximised. We used 43 GHz occasionally with set-ups centred at frequencies slightly different than 43 GHz within a range of a couple hundred megahertz however. For simplicity we always assume 43 GHz as the central frequency.
Table 2. Median percentage effect of each post-measurement correction These quality checks were eventually followed by the ulti- applied to the data for each observing frequency. mate test, which included two steps. First, we checked the shape of the radio SED in which every finally reduced frequency was compared against all other observing frequencies with the ν Pointing Opacity Gain curve τz σ (GHz) (%) (%) (%) requirement that the line resemble a physically sensible shape. Second, we tested whether the finally reduced radio SEDs were 2.64 0.5 3.5 0.0 0.020 (a) 0.003 following physically sensible evolution (mostly smooth). 4.85 0.4 3.7 0.6 0.020 (a) 0.003 (a) 8.35 0.5 3.7 0.5 0.020 0.003 4.6. Error budget 10.45 1.2 4.1 0.6 0.022 0.006 14.60 1.3 3.8 0.3 0.023 0.016 The end product of the data reduction pipeline is flux densities 23.05 1.6 12.2 0.7 0.077 0.076 and their associated uncertainties, which are computed following 32.00 3.1 11.8 0.6 0.058 0.021 a modified formal error propagation recipe assuming Gaussian 43.00 5.1 26.0 1.1 0.090 0.027 behaviour. For the computation of the uncertainty ei of a flux density measurement S i the information of the entire light curve Notes. Columns: (1) observing frequency; (2), (3), and (4) median per- is used as follows: centage effect of pointing, opacity, and gain correction, respectively; p 2 2 (5): average opacity at zenith; and (6): standard deviation around the ei = σ0 + (m · S ) , (6) (a) mean. The opacity at the low end of the bandpass may be overesti- σ m mated as a result of the enhanced beam side-lobes. The tabulated values where 0 is the flux density independent term, flux dependent should then be seen as upper limits. term proportionality coefficient, and S source mean flux density. The term σ0, is defined as
4.5. Data editing and final quality check σ0 = max(σ, e), (7) The previously discussed reduction pipeline provides the end- where σ is the standard deviation of the flux densities over the to-end framework for recovering the real source flux densities mean flux density for the corresponding observing session; and from the observables. In practise, each individual sub-scan of e the error in the mean flux density, assuming Gaussian statistics each pointing (scan) was examined and quality checked manu- and after propagating the error of each correction described in ally by a human. The quality check protocol included various Sect.4. diagnostic tests at various stages of the data reduction pipeline. The term m is a measure of the flux-density-dependent part of the error. It is computed from the scatter of the flux density of Sub-scans. Each sub-scan was inspected for (a) FWHM sig- each one of the calibrators which is assumed to be invariant, at nificantly different from that expected; this could indicate source least over timescales comparable to the length of the data trains structure, field source confusion, or variable atmospheric con- discussed in this work2. The proportionality coefficient m hence- ditions; (b) excessively large pointing offset, which could lead forth can be seen as a measure of the “repeatability” of the sys- to irreversible power loss. (c) extraordinarily high atmospheric tem and has incorporated cumulatively all the sources of errors. absorption or emission, which could cause destructive increase In Table5 we present the mean values of σ and m used for of noise; (d) large divergence from the mean amplitude of all 0 each receiver. A physical interpretation of both σ0 and m can be sub-scans in the scan; (e) excess system temperature; and (f) found in Angelakis et al.(2009). In Fig.7 we show the distribu- possible radio frequency interference. The irreversibly corrupted tion of the fractional error in three characteristic bands while in sub-scans were vetoed from further analysis. Table6 we list the median fractional errors for all our receivers Sensitivity. Highest quality observations of calibrators are calculated from all the measurements released here. clearly necessary at this stage. With human supervision this step 2 The normality of the flux density distribution of each calibrator, was executed repeatedly until sensible estimates of the Jy-to-K which is expected from the assumption of intrinsically invariant flux factors were computed. This step required special care as atten- density with the addition of random noise, has been confirmed with uators could be activated even in the same observing session. exhaustive tests.
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0.12 Table 3. Parameters of the Gaussian function fitted in the distributions of pointing offsets for three characteristic frequencies. 4.85 GHz 0.10 AZI ELV Frequency AZI Scan ELV Scan 0.08 N ∆p σ N ∆p σ (00)(00)(00)(00) 0.06 4.85 9140 1.0 6.0 9140 −1.1 8.9 14.60 9824 −1.1 5.8 9740 −0.8 5.4 0.04 32.00 7761 −0.4 3.8 7498 −0.6 4.0 Probability Desntiy Probability Notes. The entry in each column is as follows: 1: observing frequency; 0.02 2 and 5: number of data points; 3 and 6 the median pointing offsets; and 4 and 8 the scatter of the pointing offset distribution. The latter is rather 0.00 increased as compared to single frequency observations as a result of − − − the instrumental effects caused by the usage of different receivers. Offset (arcsec)
The normality test was run on each individual sub-scan (Sect.3) of one representative (in terms of tropospheric condi- tions) observing session. That amounts a total of several hun- dred datasets. In Fig.8 we first show the quantile–quantile (Q–Q) probability plot at the three characteristic receivers for a visual inspection of the normality. Each dataset is first shifted to its average (hence it centres at zero) and is having its standard deviation normalised to unity. These transforma- tions allow us to compare the Q–Q plots of all the different datasets and make the interpretation of the Q–Q plots easier.