MOA Data Flow System

This section introduces the internal details of the MOA data flow system before describing the actual implementation of real time transient notification architecture. This will provide a big picture of how MOA captures imaging data from the observatory through to generating the VOEvent packet. It should be noted that this MOA data flow system is not the subject of this research.

The MOA data flow system, that generates the necessary data for transient notification, involves four processes; capturing images, image subtraction, transient identification, and issuing notifications about new microlensing candidates or other interesting transient events to the microlensing community.

In the first step, MOA uses their powerful 1.8m ground based telescope to capture images from the Bulge during each observation night, and there are a total of 14 fields of Bulge observed. These observation images are called raw images and are archived in external HDDs immediately after capture by the telescope.

After the raw images are archived in external HDDs, the second step of image subtraction begins. This process is very complex and includes two steps. First, a reference image is required. The reference image is selected from the best viewing image, or a combination of the best images of a time series of observation images of a given field, and this reference image is fixed during the entire time series. Second, a difference imaging technique [137] is applied to generate the subtracted images through subtracting the observation’s raw image from its corresponding reference image. Moreover, all

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subtracted images are stored on disk, because they are frequently used by the object identification process.

The transient identification process includes human interaction, with the observers looking at the brightness change of objects on subtracted images one by one to find interesting transient events. Once the observers discover an interesting object, they use a program to calculate the light curve by extracted photometric measurements from the subtracted images using PSF Fitting photometry; and look at the light curve by eye using a graphic tool [137].

There are three scenarios that can happen after the light curve of the object of interest has been plotted;

1. If the light curve shows clear characteristics of periodic stellar variability, the object will be flagged as a variable star;

2. If the light curve presents behaviour characteristic of a microlensing event candidate or other transient event, it will be flagged as interesting. The interesting event requires further analysis by looking at both the unsubtracted and subtracted images, checking for bad pixels, nearby saturated stars, and so forth; or

3. If the light curve can be clearly identified as a microlensing event or other interesting transient event, MOA will use a python program to generate a VOEvent packet that contains all the metadata of this new identified transient event, and then alert this new identified event to the entire microlensing community through email notification and creating a profile for this new identified transient event on their static website.

The pictures shown in Figure 18 are the second and third processes in the observatory during a real observation.

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Figure 18: The left-hand picture is of the raw images captured by the telescope in a single exposure; a total of 10 raw images (the MOA camera is a mosaic of 10 CCD chips) that will be captured after a single exposure, with each raw image being 2K × 4K pixels and corresponding to one CCD chip. The right-hand picture shows the light curve generated from extracting the photometry measurement of an interesting object; the small image on the top right of the light curve is the subtracted image of the interesting object.

In addition, a single transient event may alert more than once, with the second alert also referred to as a “red alert”; a “red alert” only happens when a request of follow up observations of an ongoing transient event is received. In this case, MOA will generate a follow up VOEvent packet, issuing a second notification to the microlensing community worldwide.

In fact, the VOEvent packet is not the only data file that MOA provides to the end users; MOA also provide other data files related to newly detected transient events to end users. These data files include:

Photometry data: This data file works as an external data stream referenced within the VOEvent packet, and includes all the photometry measurements of the transient event. Each individual transient event has its own photometry measurement file. Moreover, this data file will update as more data are obtained by MOA.

Modelling data: The modeling data includes lens parameters of the microlensing event. There are two categories of lens model; the Paczynski model, and the binary model. Both of these models have common parameters, which are —_଴ (impact parameter), WE

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(Einstein timescale, this parameter contains physical properties of the lens) and –_଴ (time of closest approach) [184] [185], but the Paczynski model also includes two more parameters besides these common parameters, which are the baseline flux value and the I magnitude. These parameters result from fitting a model microlensing profile.

On the other hand, the binary model is much more complex than the standard model, therefore, it involves more parameters; these are the values of source star radius, mass ratio, position angle, and separation. However, the binary model is not used in real time notifications, because most of the microlensing events are fitted with single lens and use the standard model. Additionally, detailed modelling of binary events is done offline.

Finder chart: MOA provides the finder chart of a transient event in both GIF (Graphics Interchange Format) and FITS (Flexible Image Transport System) format; the GIF format presents the finder chart of a transient event as a pure image. If the end users want to access the metadata of a transient event from the finder chart, they can open the FITS format using capable graphic tools, such as DS9.

Light curve plot: MOA also provides the light curve plot of transient events to end users; the light curve plot is presented in JPEG format and it might be updated when more photometry measurements are obtained.

These data files described above will not be delivered to end users via the notification system, but are accessible through the corresponding transient event profile Web Page. The MOA data flow system is illustrated in Figure 19.

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Figure 19: The flow chart illustration of the MOA data flow system, this diagram was produced by Edraw Max.