An observing campaign was devoted to the search for the Sunyaev-Zeldovich
(S-Z) effect towards X-ray ROSAT Clusters in the millimetric spectral domain. A
double channel (1.2 and 2 {\it mm}) photometer was installed at the focus of
the 15m Swedish ESO Submillimeter Telescope (SEST) in Chile in september 1994
and 1995 and observations of the targets S1077, A2744, S295 and RXJ0658-5557
were gathered. Detections were found for A2744 at 1 {\it mm} and in both
channels (at 1.2 and 2 {\it mm}) towards RXJ0658-5557. For the first time there
is evidence for the S-Z enhancement and both the latter and the decrement were
detected on the same source. We discuss astrophysical and systematic effects
which could give origin to these signals.Comment: 6 pg Latex file (style file included) including 1 ps figure, XVIth
  Moriond Astrophysics Meeting "The Anisotropies of the Cosmic Microwave
  Background", Les Arcs, Savoie-France, March 16-23 199

Andreani, P.

Boehringer, H.

Booth, R.

Dall'Oglio, G.

Lemke, R.

Martinis, L.

Nyman, L. -A.

Pizzo, L.

Shaver, P.

Whyborn, N.

English

arXiv

Abstract: An observing campaign was devoted to the search for the Sunyaev-Zeldovich (S-Z) effect towards X-ray ROSAT Clusters in the millimetric spectral domain. A double channel (1.2 and 2 {it mm}) photometer was installed at the focus of the 15m Swedish ESO Submillimeter Telescope (SEST) in Chile in september 1994 and 1995 and observations of the targets S1077, A2744, S295 and RXJ0658-5557 were gathered. Detections were found for A2744 at 1 {it mm} and in both channels (at 1.2 and 2 {it mm}) towards RXJ0658-5557. For the first time there is evidence for the S-Z enhancement and both the latter and the decrement were detected on the same source. We discuss astrophysical and systematic effects which could give origin to these signals.

Andreani, P.,

Dall'Oglio, G.,

Pizzo, L.,

Martinis, L.,

Shaver, P. A.,

Böhringer, H.,

Lemke, R.,

Nyman, L. A. A.,

Booth, R.,

Whyborn, N.,

University of Groningen Digital Archive

The Sunyaev-Zel'dovich Effect at 1 and 2 mm towards ROSAT ClustersThe Sunyaev-Zeldovich Effect at 1 and 2 mm towards ROSAT Clusters

Shaver, P. A.

Böhringer, H.

Nyman, L. A. A.

University of Groningen Research Database

  
 University of Groningen
The Sunyaev-Zel'dovich Effect at 1 and 2 mm towards ROSAT ClustersThe Sunyaev-
Zeldovich Effect at 1 and 2 mm towards ROSAT Clusters
Andreani, P.; Dall'Oglio, G.; Pizzo, L.; Martinis, L.; Shaver, P. A.; Böhringer, H.; Lemke, R.;
Nyman, L. A. A.; Booth, R.; Whyborn, N.
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Andreani, P., Dall'Oglio, G., Pizzo, L., Martinis, L., Shaver, P. A., Böhringer, H., ... Whyborn, N. (1996). The
Sunyaev-Zel'dovich Effect at 1 and 2 mm towards ROSAT ClustersThe Sunyaev-Zeldovich Effect at 1 and
2 mm towards ROSAT Clusters. Default journal.
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THE SUNYAEV-ZELDOVICH EFFECT
AT 1 AND 2 MM TOWARDS ROSAT CLUSTERS
P. Andreani1, G. Dall’Oglio2, L. Pizzo2, L. Martinis3, P. Shaver4, H. Bo¨hringer5, R. Lemke4,
L.-A. Nyman4, R. Booth6, N. Whyborn7
1Dip.Astr.,Univ. Padova, Italy; 2Dip.Fis.,III Univ. Roma, Italy
3ENEA Frascati, Italy; 4ESO Garching, Germany
5MPE Garching, Germany; 6ONSALA Sweden; 6SRON, Groningen
Abstract
An observing campaign was devoted to the search for the Sunyaev-Zeldovich (S-Z) ef-
fect towards X-ray ROSAT Clusters in the millimetric spectral domain. A double channel
(1.2 and 2 mm) photometer was installed at the focus of the 15m Swedish ESO Submil-
limeter Telescope (SEST) in Chile in september 1994 and 1995 and observations of the
targets S1077, A2744, S295 and RXJ0658-5557 were gathered. Detections were found for
A2744 at 1 mm and in both channels (at 1.2 and 2 mm) towards RXJ0658-5557. For the
rst time there is evidence for the S-Z enhancement and both the latter and the decrement
were detected on the same source. We discuss astrophysical and systematic eects which
could give origin to these signals.
1 Introduction
The S-Z eect is one of the major sources of secondary anisotropies of the Cosmic Microwave
Background (CMB), arising from (inverse) Compton scattering of the microwave photons by
hot electrons in clusters of galaxies. This eect generates a peculiar signal with a decrement
at wavelengths longer than 1.4mm and an enhancement at shorter ones relative to the CMB
planckian value.
The original computation by Sunyaev and Zeldovich [1−3] of the the net transfer of energy
from the hot e− to the microwave photons predicts a signal for the relative temperature change:
(
T
T
)therm = y (x
ex + 1
ex − 1
− 4): (1)
where T is the CMB temperature, x = h=kT and y =
R
(kTe=mc2) neTd‘ is the comp-
tonization parameter, ne, Te being the electron density and temperature. Equation 1 is an
approximated solution of the full kinetic equation for the change of the photon distribution
due to scattering. A more accurate solution gives rise to corrections which are not negligible
at high frequencies [4].
If the cluster has a peculiar velocity relative to the frame where the CMB is isotropic an
additional kinematic eect should be measured. The motion of the gas cloud will induce a
Doppler change whose relative amplitude, (T
T
)kin, does not depend on the frequency but only
on the peculiar velocity, vr, and cloud optical depth for Thomson scattering,  ,: (
T
T
)kin = −
vr
c

(where the minus sign refers to a cluster receding from the observer). Since both eects are very
small the net relative temperature change is the sum of the two: (T
T
)SZ = (
T
T
)therm+(
T
T
)kin.
There is considerable interest in the detection of this eect also because of its potential in
determining the distance of clusters, their peculiar velocities and in studying the intracluster
medium [1−3]. Most of the observations carried out so far (e.g. [5]) were taken in the Rayleigh
- Jeans (R-J) part of the spectrum, where the scattering leads to an intensity decrement.
The more recent radio observations agree in nding this decrement at centimeter wavelengths
towards A2218, A665, 0016+16, A773, A401, A478, A2142, A2256 [6−9] and Coma [10] and at
2.2mm towards A2163 ([11] and Lange, these proceedings).
Measurements near the planckian peak and on the Wien side have several advantages: (a)
the intensity enhancement relative to the planckian value is larger than the magnitude of the
R-J decrement; (b) the simultaneous detection of the enhancement (positive) and decrement
(negative) on the same cluster provides an unambiguous signature of their presence and min-
imize systematic errors and spurious signals; (c) sources in the cluster are expected to give a
negligible contribution at high frequency, while radio observations are plagued by the possi-
ble radio emission from sources within the clusters; (d) because of the large bandwidth the
sensitivity of bolometer systems is excellent.
2 The Instrument
A double channel photometer was built and devoted to the search for the enhancement and
decrement of the S-Z eect. The system works, in fact, simultaneously at 1.2 mm and 2 mm
using two bolometers cooled at 0.3 K by means of a 3He refrigerator. The 2mm band includes
the peak brightness of the decrement in the S-Z thermal eect, while the 1.2mm bandwidth is
a compromise between the maximum value of the enhancement in the S-Z and the atmospheric
transmission. The collecting optics, cooled at 0.3 K, dene a eld of view in the sky of 4400 at
both frequencies. The beam separation in the sky was limited by the antenna chopping system
and was set to the maximum chopping amplitude: 13500.
This photometer was built to feed the O.A.S.I. (Osservatorio Antartico Submillimetrico In-
frarosso) telescope installed at the Italian base in Antarctica [12] and was adapted to the focus
of the SEST and its performance was tested during an observing run in September 1994. Details
of the instrument can be found in [13].
Responsivities, beam shapes and widths were measured with planets and the main gures
measured at the focus are listed in table 1. Sensitivities are also given in terms of relative
change of the thermodynamic temperatures in one second integration time.
Table 1. Performances of the photometer at focus
c  FWHM noise Responsivities N.E.T. (
T
T
)therm
(m) (m) (0) (nV/
p
Hz) (V=K) (mK/s) (1s)
1200 360 44 45 3.0 7.5 0.010
2000 580 46 31 1.4 10.7 0.007
3 Observations
The targets were selected because of their high X-ray luminosity in the ROSAT band (0.52.4
keV)  2 − 5 1045 erg/s and because of their high redshift (0.3-0.42). The apparent size of
the core radius, being small, well matches the beam size: for a core radius of 250400 kpc
in standard cosmologies the apparent size is (4060)00. However, a chop throw of 13500 means
that at the reference beam position the ratio between the electron density, ne( = 13500), to its
central value, ne( = 0), is 0.25 - 0.35, i.e. 20 - 30 % of the signal could be lost because of the
limited chop throw.
The present paper deals with observations of A2744 and RXJ0658-5557 carried out in four
dierent nights during september 1-5 1995. A total integration time of 10800s and 12400s were
spent on A2744 and RXJ0658-5557 respectively, and the same integration time was spent on a
blank sky located 15m ahead in right ascension with respect to the source position. During the
observations the sky opacity was very low (1mm < 0:1 with an average value of < 1mm >= 0:07,
2mm < 0:05 with an average value of < 2mm >= 0:03) and the sky emission very stable thus
producing a very low sky-noise.
In order to get rid of the major sources of noise in this kind of experiment, fluctuations in
the atmospheric emission and systematics from the antenna, the observing strategy makes use
of two combined procedures: the common three-beam technique, beam-switching + nodding,
which gets rid of the linear spatial and temporal variations in the atmospheric emission, and
the observations of blank sky regions located at right ascension position of 15m ahead with
respect to the location of the source. This latter implies that for each 10m integration ON the
source (10m integration + overheads give a total tracking time of 15m) a similar integration
is performed on blank sky. The comparison between the two signals is a measurement of the
systematics introduced by the antenna. In fact, the instrument tracks the same sky position
twice with respect to the local environment, once ON the source and the other on the blank
sky (position OFF). The choice of 600s of integration ON and OFF the source is a compromise
between the minimization of the time wasted on overheads and the need of minimizing the
atmospheric variations between one observation and the other.
The data analysis procedure is described in detail elsewhere[14]. Briefly, data on each sky
position (on the source and on the blank sky) consist of dierential measurements of the antenna
temperature, Tant, with integration chunks lasting 3 x 200s. 200s data are averaged and a
variance is assigned for each of this "scan".
Weighted means are computed over the 600s integration when the antenna tracked the source
(hereafter called TON) and when the antenna tracked the blank sky (hereafter called TOFF).
Cluster signals are then estimated from the subtraction: TSZ = TON − TOFF and the
quadratic sum of the two standard deviations are used to estimate errors: 2SZ = 
2
ON + 
2
OFF .
Figure 1 (a,b,c,d) shows the values ON (lled squares) and OFF (empty squares) of the antenna
temperatures of the two clusters and for both channels. The solid lines represent the maximum
likelihood estimates of the TSZ, while the dotted lines correspond to the 70% condence range.
This was found by computing the values of the signal where the likelihood drops by a factor
of 1:71 from its maximum. From the gure one can infer that a 3 detection can be claimed
in both channels for RXJ0658-5557: T1mm = 0:3 mK, T2mm = −0:46 mK, while a signal of
T1mm = 0:18 mK, is present only at 1mm for A2744.
To check whether these detections were spurious additional measurements were carried out to
test the behaviour of (a) the photometer + the electronic chain and (b) the antenna systematics.
The same observing strategy (on the source and on the blank sky) was applied in two cases: (a)
blocking the photometer entrance window and (b) following an empty sky region for integration
times comparable with those used for the observations of the sources.
No signals are detected at a level of 0.1 mK (3) so that we believe that the adopted
observing strategy gets rid of most of the unknown systematics (however one must note that if
during the observation of the sources the antenna, because of a loss of synchronization, did not
track precisely the same paths relative to the local environment, when looking at the source
and at the blank sky some, very small spurious signals can survive).
4 Discussion
If the detected signals are due to clusters several questions are raised. We briefly discuss here
only the case of RXJ0658-5557.
Let us assume that the decrement seen at 2mm is due to the SZ eect. From this value the
expected 1mm signal for the thermal eect can be easily computed by convolving eq.1 with the
beam shape, the optics transmission and the atmospheric spectrum. However, the dierence
with the observed value is larger than a factor of 2 and this can be explained either with a
large peculiar velocity of the cluster, of the order of -1000 km/s, or with a source contaminating
mainly the 1mm channel. To our knowledge there is no determination of the peculiar velocity
from optical data and we are not able to verify the rst hypothesis. The latter can be checked
in several ways.
(1) No sources in the IRAS Faint Source Catalogue or in the 5GHz NRAO survey are present
at the position of the main beam and/or of the reference beams.
(2) If we scale the 60m IRAS flux limit of 240 mJy at 1.2mm by using the average flux of
nearby spirals 15, we nd that a normal spiral would give rise to a signal not larger than 0.02
mK in antenna temperature.
(3) If we assume a contribution of many unresolved sources fluctuating in the beam and take
the estimation made by Franceschini et al. ([16]), the expected signal will be not larger than
0.02 mK.
(4) Irregular emission from the Galactic cirrus can also give origin to a signal at these
wavelengths. If we take the estimation by Gautier et al. ([17]) and extrapolate the 100m flux
at 1.2 mm using the average Galactic spectrum a maximum signal of 0.02 mK is found.
(5) If part of the signal is due to CMB anisotropies at these scales, it will be hard to
disentangle them from the S-Z kinematic eect since this latter has a spectrum identical to
that of the anisotropies (see e.g. [18]).
We conclude that it is very likely that part of the signal at 1.2 mm is due to one or more
sources but at present it is hard to identify it. This point however deserves further investigations
and it will be the goal of future research.
The authors are indebted to the ESO/SEST teams at La Silla and in particular to Peter
de Bruin, Peter Sinclair, Nicolas Haddad and Cathy Horellou. This work has been partially
supported by the P.N.R.A. (Programma Nazionale di Ricerche in Antartide). P.A. warmly
thanks ESO for hospitality during 1995, when part of this work was carried out.
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0 10 20 30 40
-4
-2
0
2
4
scan number
0 10 20 30 40
-4
-2
0
2
4
0 10 20 30
-4
-2
0
2
4
0 10 20 30
-4
-2
0
2
4
Figure 1: Dierential antenna temperatures at 1 and 2 mm for A2744 and RXJ0658-5557.
Filled squares refer to data ON the source, while empty squares to blank sky regions located
15m ahead in R.A. with respect to the source position. The cluster signals are estimated by
subtracting TSZ = TON −TOFF . The maximum likelihood values of these dierences is
shown as a solid line while the dashed lines correspond to 70% condence interval. For the 2
mm signal of A2744 only the 3 upper limit is reported.


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The Sunyaev-Zeldovich Effect at 1 and 2 mm towards ROSAT Clusters

The Sunyaev-Zeldovich Effect at 1 and 2 mm towards ROSAT Clusters

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