ADASS XIIIASP Conference Series, Vol. XXX, 2004
F. Ochsenbein, M. Allen and D. Egret eds.

Searching for a cosmic string through the graviational lenseffect: Japanese Virtual Observatory science use case
Y. Shirasaki, Y. Mizumoto, M. Ohishi, N. Yasuda, M. Tanaka, S.
Honda, H. Yahagi, M. Nagashima, G. Kosugi, N. Kashikawa
National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan
E. Matsuzaki, F. Kakimoto, S. Ogio
Tokyo Institute of Technology, Meguro, Tokyo, Japan
Abstract. This paper describes a method to search for a cosmic stringusing its unique gravitational effect and its implementation to the Japanese
Virtual Observatory (JVO). Grand unified theory predicts that superheavy cosmic strings with linear mass density of 10 22 g/cm were produced
at a phase transition in the early universe. The lensing effect by a longstraight object can be characterized by undistorted double images which
are almost co-aligned in a direction of the string network and distributed
in a very large scale. Because of its large scale nature, wide field deep
survey is crucial for its discovery, and also the automatic detection system is required to survey the large amount of deep sky data. We haveconstructed databases of Subaru Suprime-Cam catalog/image for selectedareas and applied our search method on the JVO prototype.

1.

Introduction

The standard theory of particle physics predicts a symmetry breaking in theearly universe and, as a result, the production of topological defects (Vilenkinand Shellard 2000). If the defects were generated at the GUT energy, the cosmic
string can be observed as an origin of gravitation lensing (Vilenkin 1984) andits detection can be an observational confirmation of the standard theory. Therecent observations of cosmic microwave background radiation rule out pure
topological defects model as the origin of large scale structure of the universe,
however, they still do not rule out the existence of the defects (Pogosian et al.
2003). So it is of great importance to constrain the existence experimentally.
Several gravitational lens candidates by a cosmic string have been reported(e.g. Sahzin et al. 2003), however, none of them have not been confirmed yet.
Huterer and Vachaspati (2003) pointed out that a deeper observation of thevicinity of the candidate event reveals several additional lensing events. They
predicts that, for a string located nearby (z < 0.5), future wide-field surveys
such as SNAP and LSST find 5 objects lensed by a string in 1 arcmin 2 region,
compared to 0.1 lensed by conventional sources.
The Subaru Suprime-Cam has also a great advantage in observing the wide
field of view (30×30 arcmin2 ) with high sensitivity (R<26 400s exposure), so it is1

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Shirasaki et al.

Figure 1.
Definition of the weight parameter Wi,j . A pair of the solid circles
represents an object lensed by a cosmic
string. The expected directions of a cosmic string for each pair are shown withsolid lines. θi,j is an angle between the two
expected string directions for object i andj. ϕi,j is an angle between the string direction for i and the line from i to j. If
ϕi,j < θi,j , Wi,j = e−θi,j /θ0 . Otherwise,
Wi,j = e−(π−θi,j )/θ0 .
suitable for this research. At present, three and half years of Suprime-Cam dataare publicly available, so it is interesting to search for a cosmic string in thearchived data. In addition, the Japanese Virtual Observatory (JVO) project
has been started and will provide automatic access to the Subaru SuprimeCam database and also provide unified environment for the DB access and dataanalysis (Mizumoto et al. 2003, Ohishi et al. 2004). We are now developing
tools to search for a cosmic string on the JVO system. The current status ofsuch activity is presented here.
2.

Search Strategy

Features of a string lens event are characterized as non-distorted equal-brightness
double images which are aligned in the direction of the string network. So thesearch strategy is simply summarized as follows: 0) Prepare an object catalog
for given images using a source detection software such as the SExtractor. 1)
Select closely located pair objects of similar brightness, color, and morphology.
Use the following parameters for selection of the pair objects: separation angle,
maximum difference of brightness for each band, maximum difference of color,and reduced chi-square for a difference of the pair images. 2) Calculate a degreeof positional and directional
P P coherence C obs for a set of pair objects. The coherence is defined as C = i j6=i Wi,j where Wi,j is calculated as shown in Fig. 1.
3) Calculate chance probability for C obs . This can be estimated by simulating1000 sets of pair objects whose separation directions are randomized to the observational ones and by obtaining the distribution of the degrees of coherence
for them.
3.

Simulation and Application to the Observational Data

We examined ability of the proposed method by simulating lens effect by acosmic string. The assumed string model is described as follows: 1) Three kindsof configuration as shown in Fig. 2 (Left). 2) Mass density is assumed to be 10 22
g/cm which corresponds to the maximum separation angle of 5 00 . 3) The string
direction is perpendicular to the line of sight. A model of background galaxies

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Searching for a cosmic string on the JVO
jig-zag string
curved string (R = 50arcmin)straight string

2.0
jig-zag string
curved string (R = 50 arcmin)straigt string
Redshift Upper Limit

20Y (arcmin)

1001.00.5-10-20-20

1.5-100X (arcmin)

10200.020212223242526

Limiting Magnitude

Figure 2.
Left: String configuration tested for the string detection
method presented here. Right: Maximum redshift of the cosmic string
detected by this method as a function of limiting magnitude (R band)of observations.
is provided by the Numerical Galaxy Catalog (νGC) based on a N-body/semianalytic model with a Λ-CDM cosmology, Ω λ = 0.7, Ωm = 0.3, σ8 = 1.
In Fig. 2 (Right), maximum redshift, where cosmic string can be proved by
the proposed method in 99% confidence limit, are calculated according to thesensitivity of the observation. We applied this method to the data of SubaruXMM Deep Survey (SXDS), Subaru Deep Field (SDF), AGASA 10 20 eV Field,
and 2 deg2 Field. No candidates of the string lens events were detected.
4.

Cosmic String Search on the JVO

Since the detection rate of the GL candidates by a cosmic string is expected tobe very small, a survey of large amount of data is crucial and development ofthe automatic detection system is required. The Japanese Virtual Observatory(JVO) is now being constructed at National Astronomical Observatory of Japanand it will provide an interface to the Subaru Suprime-Cam database, whichenable us to do an automatic search on the whole Suprime-Cam image data.
We developed several tools to implement a GL search on the JVO prototype.
The following tools were developed: (1) JVO Query Language Editor specializedfor GL search. (2) GL candidate viewer. (3) GL candidate search engine. InFig. 3, schematic diagram of cosmic string search system is shown. A searchrequest is written in JVO Query Language (JVOQL, Mizumoto et al. 2003) andthe results are returned in VOTable and FITS format. JVOQL can be generated
by a JVOQL editor (Fig 4), which uses a web browser as an input method. On
the JVOQL editor, one can browse meta data of registered DBs and can select aDB, a table and columns you want. The requested table is retrieved from a DB
server and pair objects are selected on the JVO system server, and the result is4

Shirasaki et al.

Figure 3.
A schematic
diagram of the cosmic
string search system.

Figure 4.
JVOQL Editor (left),
VOTable viewer (right top), GL
candidate viewer (right bottom)

saved as a VOTable. Image data are also retrieved for the pair objects as a FITS
format and the FITS images are converted to JPEG images. User can access tothe VOTable, FITS image and JPEG images through the VOTable viewer shownin Fig 4. A color-color diagram, a SED plot and a three-color composite image
are also generated in accordance with a user’s request. Although the search
procedure described in section 2 is still not fully introduced in the current JVO
prototype, complete implementation will be done in the second version of theJVO prototype.
Acknowledgments. This research was supported by Grant-in-aid ”Information Science” carried out by the MEXT (14019092 and 15017289).
References
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Huterer, D., & Vachaspat, T. 2003, Phys. Rev. D68 041310
Mizumoto, Y. et al. 2003, in APS Conf. Ser., Vol. 295, Astronomical Data
Analysis Software and Systems XII, ed. H. E. Payne, R. I. Jedrzejewski,& R. N. Hook (San Francisco: ASP), 96
Ohishi, M. et al. this volume, [P3-11]