Stardev Matched Filter Update

On July 1st, 2021 the two-component SCUBA-2 beam profile parameters were updated in accordance with the results published by Mairs et al 2021. This caused a change in the Matched Filter post-processing.

When using a Stardev installation post-July 1st, 2021, executing the matched-filter method is expected to increase the measured peak fluxes of bright point sources by ~15% relative to older versions of Starlink. A similar factor may apply to faint point sources in otherwise “blank fields” but this requires further testing.

See this software blog post for more details!

 

 

Stardev Matched Filter Change

On July 1st, 2021 the two-component SCUBA-2 beam profile parameters were updated in accordance with the results published by Mairs et al 2021. This caused a change in the Matched Filter post-processing.

When using a Stardev installation post-July 1st, 2021, executing the matched-filter method is expected to increase the measured peak fluxes of bright point sources by ~15% relative to older versions of Starlink. A similar factor may apply to faint point sources in otherwise “blank fields” but this requires further testing.

The plots, below, show comparisons between the 2018A Starlink and most recent Stardev matched filter implementation on Uranus calibrator observations taken between 2016-11-01 and 2021-04-13. Note that the Stardev version consistently returns peak flux values that are ~15-20% higher than before due to updated empirical beam measurements presented in Mairs et al 2021.

The beam patterns for matched filtering purposes are described as a combination of two Gaussian functions:

G_total = α*G_mb + β*G_sec,

where each Gaussian profile, G, is of the form exp[−4*ln(2)*(r/θ)^2], where “r” is the radial distance from the centre and θ is the FWHM of the profile, both measured in units of arcseconds. The first component, G_mb represents the “main beam” response. The second component, G_sec, is an approximation of the “secondary beam” or “error beam”, which describes the flux in the shoulders of the profile. α and β are coefficients describing the relative contribution of each component (the amplitudes). The broad error beam includes factors such as sidelobes due to diffraction, static dish deformations, and dish deformations induced by thermal gradients.

450 microns:

850 microns:

The original SCUBA-2 Beam Parameters were presented by Dempsey et al. 2013:

450 microns:

α = 0.94
β = 0.06
θ_mb = 7.9
θ_sec = 25.0

850 microns:

α = 0.98
β = 0.02
θ_mb = 13.0
θ_sec = 48.0

The updated SCUBA-2 Beam Parameters are presented by Mairs et al 2021:

450 microns:

α = 0.89
β = 0.11
θ_mb = 6.2
θ_sec = 18.8

850 microns:

α = 0.98
β = 0.02
θ_mb = 11.0
θ_sec = 49.1

These values represent the median results after fitting the two-component model to all Uranus calibrator observations since May, 2011 above a significant transmission threshold (see Mairs et al 2021).

The total area of each beam profile, A, can be calculated by:

A = {π/[4*ln(2)]}*[α*(θ_mb)^2+β*(θ_sec)^2].

Comparing the Dempsey et al 2013 beam areas to the Mairs et al 2021 beam areas reveals a ~20% difference at both wavelengths (in practice, there is a spread around this typical 20% value, see plots, above).

Note that previously, it has been common practice in cosmology publications employing “Blank Field” data reduction recipes (e.g. REDUCE_SCAN_FAINT_POINT_SOURCES, REDUCE_SCAN_FAINT_POINT_SOURCES_JACKKNIFE) to apply a correction factor of ~10% in order to compensate for flux lost due to filtering. This 10% factor was derived by inserting a bright Gaussian point source into the raw power versus time stream of individual observations and measuring the response of the model to the filtering during the data reduction process (e.g. Geach et al. 2013, Smail et al. 2014). We recommend repeating this experiment with the new Starlink 2021A matched-filter implementation for your specific data in order to determine whether the correction factor is still necessary.

A Decade of SCUBA-2

We are pleased to announce the publication of a new, comprehensive guideline to calibrating SCUBA2 data obtained from 2011 to the present day:

The Astronomical Journal

arXiv

This work updates the opacity relations used to correct for atmospheric attenuation, summarizes significant changes in flux conversion factor (FCF) values by date and time, details the beam properties at both 450 and 850 microns, presents historical records of standard calibrator fluxes, and includes a case study for Quasar 3C84.

Figure 1: Flux Conversion Factors (FCFs) as a function of time at 450 microns (left) and 850 microns (right). For more information, see Mairs et al. 2021.

Up-to-date SCUBA-2 calibration information can always be found here:

https://www.eaobservatory.org/jcmt/instrumentation/continuum/scuba-2/calibration

A Decade of SCUBA2: A Comprehensive Guide to Calibrating 450 m and 850 m Continuum Data at the JCMT

Abstract

The Submillimetre Common User Bolometer Array 2 (SCUBA2) is the James Clerk Maxwell Telescope’s continuum imager, operating simultaneously at 450 and 850 microns. SCUBA2 was commissioned in 2009-2011, and since that time, regular observations of point-like standard sources have been performed whenever the instrument is in use. Expanding the calibrator observation sample by an order of magnitude compared to previous work, in this paper we derive updated opacity relations at each wavelength for a new atmospheric extinction correction, analyze the Flux Conversion Factors used to convert instrumental units to physical flux units as a function of date and observation time, present information on the beam profiles for each wavelength, and update secondary calibrator source fluxes. Between 07:00 and 17:00 UTC, the portion of the night that is most stable to temperature gradients that cause dish deformation, the total flux uncertainty and the peak flux uncertainty measured at 450 microns are found to be 14% and 17%, respectively. Measured at 850 microns, the total flux and peak flux uncertainties are 6% and 7%, respectively. The analysis presented in this work is applicable to all SCUBA2 projects observed since 2011.

Mahalo,

Steve, on behalf of the JCMT

Nāmakanui Has Arrived in Hilo!

Nāmakanui (pronounced “Naaah-mah-kah-noo-ee”), our newest addition to the JCMT instrumentation suite, arrived in Hilo last week and is now out of the box and being tested in Hilo by staff. The Hawaiian name “Nāmakanui” means “Big-Eyes” and it refers to a type of fish found in and around the islands.

When it is fully commissioned, Nāmakanui will be able to look at the sky using one of three receivers. Each receiver carries the name of a different type of Nāmakanui fish: `Ala`ihi (pronounced “ah-la-ee-hee”; 86 GHz), `U`u (pronounced “oo-oo”; 230 GHz), and `Āweoweo (pronounced “aaah-vay-oh-vay-oh”; 345 GHz). `U`u is the first receiver that will be commissioned.

This instrument will be critical for helping take the next Pōwehi image (the Hawaiian name for the Black Hole image at the centre of M87), hooking into the Event Horizon Telescope network. Additionally, it will be capable of delivering a wide range of fantastic science from studying the earliest stages of star formation and the late stages of stellar mass loss to investigating the gas dynamics of galaxies.  It takes 12 hours to cool down Nāmakanui to its operational temperature (4K) and so far the testing is going very well!

This instrument was built by a team at ASIAA (Taiwan) and is on loan the to the JCMT as a spare for the Greenland Telescope. We are very grateful for the opportunity to collect exciting data with this next-generation instrument!
To learn more about this instrument click here: https://buff.ly/2Zkv1lW

 

JCMT Operations Temporarily Suspended

Dear JCMT Community,

This week, the pending start of the construction of the Thirty Meter Telescope has sparked a protest which has blocked access to Maunakea for all traffic. Yesterday afternoon, the directors of the existing observatories made the joint decision to remove all personnel from their telescope facilities at the summit to guarantee the safety of their staff – the institutions’ top priority. Without guaranteed, reliable access to the telescopes, the Maunakea Observatories have suspended all summit activities (including remote operations) for the time being.

The safety of everyone on the mountain, MKO staff, law enforcement, and protestors is of paramount importance to us. We have voluntarily decided to remove our staff. This is not a decision we came to lightly, but want to emphasize the importance of safety for all staff and facilities.

We are truly grateful to the law enforcement offices who have been working around the clock to ensure the safety of everyone on Maunakea. The safety of our personnel – and of everyone on the mountain – remains our top priority.

We look forward to returning to normal operations as soon as the situation allows.

From the JCMT Team, Aloha and thank you for understanding.

JCMT Transient Survey Team Observes Record-Breaking Flare

On November 26th, 2016, the JCMT Transient Survey Team observed what is estimated to be the most luminous known flare associated with a young stellar object. It is also the first coronal flare discovered at submillimetre wavelengths. The brief flash of light occurred in the direction of a binary system of forming stars known as “JW 566” in the Orion Nebula and it carried ten billion times the amount of energy of the solar flares observed around the Sun.

Left: The Orion Nebula as seen by SCUBA-2 at 850 microns. Right: Two images of
the field surrounded by the green square taken 6 days apart. Small rectangles/triangles show the
positions of known young stars found by other telescopes. On November 20th, 2016, there was
no signal. On November 26th, 2016, the flare was observed while it was already dimming from
its (unseen) maximum brightness.

The flare was discovered by JCMT support astronomer Dr. Steve Mairs using advanced image analysis techniques that had been developed by the Transient Survey team over the past 2 years. The SCUBA-2 observations lasted approximately 30 minutes over which time  the flare faded to half of the brightness measured at the beginning of the scan, indicating the event was short-lived. The flare is thought to be caused by an intense magnetic field re-connection event that energised charged particles to emit gyrosynchrotron/synchrotron radiation.

Press Release: Sky and Telescope, Hawai’i Tribune Herald, Big Island News

Publication: ArXiv, ApJ

The JCMT Transient Survey Team

The JCMT Transient Survey team is an international collaboration of 80 astronomers led by Dr. Gregory Herczeg of Peking (Kavli Institute for Astronomy and Astrophysics) and Dr. Doug Johnstone (National Research Council of Canada). The team has been monitoring 8 star-forming regions in the Milky Way with a monthly cadence since December, 2015. The survey will continue through January, 2020.

The Brightest Quasar in the Early Universe

Observations obtained by the JCMT helped uncover the Brightest Quasar in the Early Universe!

The light from Quasar J043947.08+163415.7 is gravitationally lensed by a dim Galaxy in the foreground, allowing Fan et al. (2019, APJL: 870, 11) to get a good look at this active galactic nucleus at a redshift of z = 6.51 (A distance of ~12.8 Billion Light years!). As the authors note, “This is the first such object detected at the epoch of reionization, and the brightest quasar yet known at z > 5”.

The JCMT is instrumental in observing distant star-forming galaxies. These galaxies have high concentrations of dust that reprocess the starlight such that it is emitted at infrared wavelengths. The light is then redshifted due to the expansion of the universe into the submillimetre regime. The star formation rate is estimated to be 10,000 times higher than that of our own Galaxy, the Milky Way.

Press Release:

Astronomers uncover the brightest quasar in the early universe

Publication:

The Discovery of a Gravitationally Lensed Quasar at z = 6.51


An artist’s rendering of a distance quasar (Credit: ESO/M. Kornmesserhttp://www.eso.org/public/images/eso1122a/)

A bit of history…

The name “Quasar” is a shortened version of the original designation scientists gave to a mysterious signal we didn’t have a scientific interpretation for: a “quasi-stellar radio source” discovered in 1963 by Maarten Schmidt.

Over decades of intensive studies, astronomers have been able to determine that these mysterious signals were coming from intense bursts of light in the hearts of galaxies far, far away.

Most (maybe all!) galaxies contain a supermassive black hole millions to billions times the mass of our Sun. In some of these galaxies, infalling material gets too close to the black hole and it heats up to millions of degrees, exploding outward in a massive release of energy!

We can detect those signals, which we now affectionately call Quasars, at submillimetre wavelengths with the JCMT.

Congratulations to Dr. Xiaohui Fan and his co-authors !

Maunakea Wonders Teachers Workshop

The third Maunakea Wonders Teacher Workshop began on October 17th at the University of Hawai’i’s Department of Education in Hilo. Throughout the second half of October, we have the incredible opportunity to share fun astronomy activities and resources with the future teachers of Hawai’i. Despite the frigid weather, this included on-site tours of the JCMT and UKIRT where we had a blast talking about the different functions of each telescope on the mountain. We are looking forward to more fun and excitement on campus on October 31st – Mahalo nui loa to this vibrant group of educators!

-20181022

Mahalo to Mailani Neal!

The East Asian Observatory wishes to extend a big mahalo to our summer intern Mailani Neal for her excellent work tracking temperature changes across the JCMT’s dish!

We are grateful for all her hard work and we wish her all the best as she enters her final year at Rensselaer Polytechnic Institute in New York!

This gif shows an example of the telescope’s changing temperature over one night in June, 2018:

This analysis is important for understanding calibration observations and the effect of the weather on all the data we collect.

Mahalo, Mailani!

– 20180807