10.3.2 Calibration procedure at Bure

10.3.2.0.1 Some basic points

Because of the physics of quasars, the spectral index may be variable with time as the source intensity. Simultaneous measurements at 2 frequencies are thus needed to estimate it accurately, IRAM instruments (30-m and PdBI) use the frequencies of 86.7 GHz and 228 GHz. At the 30-m, flux density measurements are done during the pointing sessions while they are performed in special sessions at Bure, usually after baseline measurements.

The results of the flux sessions are regularly reduced and published in an internal report (usually each 4 months). These reports are currently available on the web, in the local IRAM page (see).

10.3.2.0.2 How we proceed at Bure

In practice, it is impossible (and not necessary) to follow all the quasars used as amplitude calibrator at the IRAM interferometer. Monitoring of the RF bandpass calibrators which are strong quasars with flux density >2 Jy (no more than 4-8 sources) is enough. In the meantime, planets are observed as primary calibrators. These sessions require to calibrate the atmosphere (T_sys) on each source and to check regularly the focus.

At the Bure interferometer, the flux density measurements on quasars are done by pointings in interferometric mode. Pointings on planets are actually done in total power mode because they are resolved by interferometry and strong enough. Total power intensity is not affected by the possible decorrelation due to atmospheric phase noise. However, it is then necessary to accurately determine the efficiencies of the individual antennas (conversion factor in Jy/K) in interferometric mode ( $\ensuremath{\mathcal{J}} _I$) and in single-dish mode ( $\ensuremath{\mathcal{J}} _S$).

10.3.2.0.3 Determining the antenna efficiencies (Jy/K)

For each flux session, $\ensuremath{\mathcal{J}} _S$ is measured on planets by comparison with the models (see GILDAS programs ASTRO or FLUX).

For a given antenna, the interferometric efficiency $\ensuremath{\mathcal{J}} _I$ is always $\geq \ensuremath{\mathcal{J}} _S$. Pointings measurements in interferometric mode are not limited by the atmospheric decorrelation because the timescale of the atmospheric decorrelation is usually significantly larger than the time duration of the basic pointing integration time (< a few sec). On the contrary, all instrumental phase noise on very short timescale can introduce a significant decorrelation and degrades $\ensuremath{\mathcal{J}} _I$. This is what may happen from time to time at a peculiar frequency due to a bad optimization of the receiver tuning.

For example, in the initial 1.3mm observations, strong decorrelation was introduced by the harmonic mixer of the local oscillator system which degraded $\ensuremath{\mathcal{J}} _I$ by a factor of 2-4 depending of the antennas. This problem has been solved recently. Now at 3mm, it is reasonable to neglect the instrumental noises and take $\ensuremath{\mathcal{J}} _I = \ensuremath{\mathcal{J}} _S$. At 1.3mm, the new harmonic mixers have been installed only recently and statistics on the site are rare but laboratory measurements show that the loss in efficiency should be small. The Table 10.2 gives the antenna efficiencies $\ensuremath{\mathcal{J}} _S$, as measured in flux sessions or by holography.

 

Table 10.2: Conversion factor from K to Jy for the 15-m antennas of Plateau de Bure

Antenna	3mm efficiency	1.3mm efficiency
number	(Jy/K)	(Jy/K)
1	22	37
2	21	27
3	21	36
4	21	29
5	22	34

 

These values are the current efficiencies (since November 1997); older values are given in flux reports. They assume that the focus is optimum and do not include any instrumental phase noise. $\ensuremath{\mathcal{J}} _I$ agrees usually within 10 % at 3mm and 15 % at 1.3mm with $\ensuremath{\mathcal{J}} _S$, note that $\ensuremath{\mathcal{J}} _I$ must be $\geq \ensuremath{\mathcal{J}} _S$.

Being able to cancel out most of the instrumental phase noise even at 1.3mm makes the IRAM interferometer a very reliable instrument. It is reasonable to think that, in the near future, the flux calibration will be systematically performed at Bure at the beginning of each project by reference to the antenna efficiencies. This is indeed already the case: after pointing and focusing, we systematically measure the flux of calibrators when starting a new project (data labeled FLUX in files). Up to now, for typical weather conditions, most (more than 90 %) of the flux measured at 3mm are correct within 10 % and more than 60 % at 1.3mm are within 15 %.

10.3.2.0.4 CRL618 and MWC349 as secondary flux calibrators

Finally, for each project, a complementary flux check is systematically done using the continuum sources CRL618 or MWC349 (pointing + cross-correlations). However these sources must be used with some caution. CRL618 is partially resolved in A and B configurations at 3mm and in A,B,C at 1.3mm. Moreover it has strong spectral lines which may dominate the average continuum flux; this must be checked before using it for flux estimates. MWC349 is unresolved and remains a reliable reference in all antenna configurations. The only strong lines for MWC349 are the Hydrogen recombination lines. The adopted flux densities are:

For CRL618 (see flux reports 13 and 15):

• CRL618 F(87 GHz) = 1.55 Jy (+/-0.15)
  At 87 GHz, the flux density of CRL618 (free-free emission from the HII region) has increased since 1990 (where it was $\sim$ 1.1 Jy instead of 1.55 Jy).
• CRL618 F(231.9 GHz) = 2.0 Jy (+/-0.3),
  from [Martin-Pintado et al 1988]. Since the flux density has increased at 87 GHz, this value needs to be observationally confirmed. Beware of the line contamination which can be high in CRL618.

For MWC349:

• Spectrum of MWC349 F $(\nu ) = 1.69(\nu/227~{\rm GHz})^{0.6}$
• MWC349 F(87 GHz) = 0.95 Jy
• MWC349 F(227 GHz) = 1.69 Jy

These values agree within 1 $\sigma$ with the measurement performed at 87 GHz [Altenoff et al 1994], (0.87 $\pm 0.09$ Jy).