What is ALeRCE?
ALeRCE is an astronomical alert broker designed to provide a rapid and self-consistent classification of alerts coming from large etendue survey telescopes. It has been ingesting alerts from the Zwicky Transient Facility since 2019 and from the ATLAS telescopes experimentally since 2022. It became one of the seven community brokers for the Vera C. Rubin Observatory in 2021.
ALeRCE uses a pipeline which includes the real-time ingestion, aggregation, cross-matching, machine learning (ML) classification, and visualization of the ZTF alert stream. ALeRCE uses two classifiers: a stamp-based classifier, designed for rapid classification, and a light-curve-based classifier, which uses the multi-band flux evolution to achieve a more refined classification. ALeRCE has three main web interfaces, the ALeRCE explorer, the ALeRCE SN hunter and the ALeRCE watchlist.
The ALeRCE Explorer
The ALeRCE explorer is a web interface made for users to quickly explore the objects contained in the alert stream. You can access it from here: https://alerce.online/.
The ALeRCE explorer is the main tool to explore the astronomical objects recovered from the ZTF alert stream. Its landing page consists of two main sections: the Search and Results sections. The Search section is where users can filter objects by selecting their unique identifier, or by selecting different combinations of classifier, class, class probability, number of detections, and sky coordinates. The Results section is where the results of the filtered objects are shown, sorted by classification probability or other variables. Clicking on an individual object will take the user to the object view page. In the object view page users can see some basic statistics about the object, generate a finding chart, query different catalogs at the position of the object (NED, Simbad, TNS, PanSTARRS, or SDSS), or quickly see basic TNS information about the object. The user can see the object’s light curve, including detections and non–detections, with the capability of plotting the raw difference light curve, a corrected apparent magnitude (which includes the contribution of the reference image), or a folded version of the corrected apparent magnitude using the best-fitting period. You can also superimpose the ZTF data release light curve, derived from the science images, when available. The light curve information can be downloaded as comma separated values (CSV), and every point in the light curve can be hovered over to see more information, or clicked on to show its associated image Stamp. HiPS images and catalogs around the position of the object are shown using Aladin, with superimposed NED and Simbad clickable objects. The science, reference and difference image stamps associated with any point in the light curve can be shown in the Stamps section, where the stamps can be explored by selecting different dates or hovering over them, seen in full screen, or downloaded as fits files. The full Avro packet information can also be explored. The classification probabilities are shown in the Stamp and Light Curve Classifier tabs, where a radar plot is used to show the class probabilities assigned by the light curve or stamp based classifiers, if available. In the Cross Matches section users can see all the cross–matches contained in the catsHTM set of catalogs for a given separation, which can be selected manually with a sliding bar.
The ALeRCE explorer is where most of our web development has been focused, including new tools as requested by our community of users, but also new sources of data which in the future will allow for the multi-stream exploration of astrophysical objects. We are developing a modular data exploration library which will be gradually expanded to include new sources of streaming data.
The ALeRCE SN Hunter
The SN hunter is a tool developed by ALeRCE that helps us to search for SNe in the ZTF public stream. You can access it from here: https://snhunter.alerce.online/. Since it uses the information from the first stamp of a given object, it is specially useful to find SNe in their earliest stages after first light.
The SN Hunter receives all objects classified as SN by the ALeRCE stamp classifier, which means all the objects where the largest probability among the five classes (SN, AGN, VS, asteroid or bogus) is SN. At any given time, you can check this tool and it will display real time information about the objects, shortly after ALeRCE receives their alerts.
You can filter the data in terms of the number of objects shown (from 10 to 1000 [show]), and the time window (from the last 12h to the last 7 days [show]). For a given selection of these filters, you will find a table displaying: the object identifiers, their date of first detection, their probability of being a SN, and the number of detections in the ZTF bands.
A celestial map shows the spatial distribution of all the objects filtered by your selection, where the size of the symbols is proportional to their probability of being a SN, and also the location of the Milky Way and ecliptic are displayed. Note that you can also change the zoom and view of this map [show].
If you click on an object [click], the celestial map centers on it, and additional information is displayed for the object. This includes: the first triplet of discovery stamps (science, reference and the difference between them) and are centered on the detection. There is also an Aladin panel that shows an archival PanSTARRS color image centered on the object coordinates, where you can zoom in or out and also change the image displayed.
More information that you can find here for the discovery includes the object coordinates, magnitude and its band, and date in MJD format. Also you can find information for the closest PanSTARRS object in projection, like its identifier, angular distance to the ZTF object, and a score that indicates if it's a star or a galaxy.
You can also go from this site to the ALeRCE Explorer view for this object, by clicking in the ALeRCE button (or in the object name), as well as to external archives like the NED and SIMBAD databases, and the Transient Name Server (TNS), which is the official IAU mechanism for reporting new astronomical transients, making them available to the worldwide community. And clicking here [click], you can display all the information contained in the ZTF alert packet received by ALeRCE at the time of discovery.
Now, if you log in to the site, for instance with your google account [log in], you can give us feedback about the candidates, in terms of saying to us if you think a given object is possibly a SN or an artifact (or bogus) in the image, using the corresponding buttons here [show buttons]. If you click on one of these buttons [click] a tag will be displayed in the "reported" column. If you change your mind or think you made a mistake you can click the button again [click] to remove your submission.
Some years ago we started using the SN Hunter tool to check, veto and finally report SN candidates to the Transient Name Server, and to date we have sent more than 16k candidates, from which around 2k have been confirmed spectroscopically. This shows how powerful the combination of deep learning and human evaluation can be.
The ALeRCE watchlist
In the ALeRCE watchlist service you can create lists of coordinates, or watchlists, that trigger notifications every time an alert coincides with one of the given positions. A watchlist is useful to crossmatch your favorite targets in real time, getting email notifications when new detections occur. Each watchlist will represent a list of targets or sample, where the position (RA, Dec) of each source in the sky will be determined, along with a radius in arc seconds. The sample corresponds to sources that are interesting in some way and that require a notification system when an alert is generated. Once registered, a user can upload their own watchlists, which can later be modified from the interface. New detections will be notified to your email based on a user-determined frequency.
Use case Jupyter notebooks
We have compiled a list of example jupyter notebook which show how to use our API or directly access our database focused around different science cases. These are available on the ALeRCE Github. In particular, we will use the following repository that you can clone:
https://github.com/alercebroker/usecases/tree/master/notebooks/ADASS_XXXII_Nov2022_tutorial
We will first discuss a notebook showing how to interact with the ALeRCE client, and then three notebooks focused on supernova, variable stars and AGN science, respectively.
ALeRCE client notebook
The ALeRCE client is a library to interact programmatically with the ALeRCE API. It allows you to interact more easily with the API via a Python library. You can query the light curve, probabilities, or features associated with different objects via simple commands in Python.
ALeRCE client starter notebook
ALeRCE SN Starter notebook
This notebook shows how to use the ALeRCE client to query and visualize SN related data. This includes object light curves, features and classification probabilities.
ALeRCE Supernova starter notebook
ALeRCE AGN Starter notebook
The notebook is focused on Active Galactic Nuclei (AGN) mostly using the results from the light curve classifier. After a brief introduction, we will use the ALeRCE client to retrieve the light curve for one particular AGN, and then for a sample of sources. Then, we will learn how to use the ALeRCE database to retrieve large samples of AGN.
The link for this notebook is the following:
ALeRCE ML notebook (bonus)
In this notebook you will explore the data used in the Stamp classifier and an explanation of the model is given. The link to the notebook is the following:
We hope you have enjoyed the tutorial! You can use github issues or email us directly to alercebroker@gmail.com if you have any questions or comments.
Cheers,
The ALeRCE broker team