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Alert Notice 718: Monitoring requested for Aquila X-1 (V1333 Aql) outburst

August 26, 2020

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Campaigns and Observation Reports: https://www.aavso.org/aquila-x-1-v1333-aql-2020-august-outburst-01

- Time Sensitive Alerts: https//www.aavso.org/aquila-x-1-v1333-aql-2020-august-outburst

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Dr. Gregory Sivakoff (University of Alberta) and colleagues have requested AAVSO assistance begining immediately in monitoring the x-ray binary Aquila X-1 (V1333 Aql) during its current outburst.

Dr. Sivakoff writes: "Aquila X-1 (V1333 Aql) is the prototypical recurrent neutron star low-mass X-ray binary. It undergoes outbursts about every 9 months to a year. When this occurs, the accretion disk that surrounds the neutron star drains quickly onto the neutron star. Although there is another star 0.5" away and a donor (a K4+/-2) star that feeds the accretion disk, when outbursts occur, the accretion disk (and perhaps to some extent a relativistic jet) gets so bright that it dominates the optical and UV emission. This is what makes this a very variable star.

"[A new outburst is underway] (ATel #13953; ATel #13961). But, it's not too bright. Some light curves of past observations by Las Cumbres Observatory show its behaviour. [As of August 21,] the source is probably V~18 / i'~17 and will likely brighten significantly, maybe by ~3 mag.

"Las Cumbres Observatory observations in V and i' will continue, although I am not sure how often they will occur. We also are getting V, B, U, (and some bluer filters) with Swift Ultraviolet/Optical Telescope no denser than one observation per day. Beware, the A_V extinction is ~ 2, so while bluer filters get closer to the action, our view of them is getting increasingly bloacked.

"So where can you come in? There's been some interesting work connecting the UV/Optical to the X-ray emission, the latter of which traces the inner accretion disk...Fortunately the star peaks currently at an airmass of ~ 1.2 during the night (closer to the night before than the morning...I can see two types of interesting observations, depending on your facility's capacities and your own interests.

   - "Measure the star's photometry in multiple filters, preferably at least once per night, until the source goes back into quiescence or you get several nights of non-detections. This is probably the most useful as it helps us track how the colours of the source change over the outburst. Preferred filter priority is R, B, V, I (BVRI can be replaced with Sloan equivalents). [Observers should] be aware that B might be tough due to reddening...Between V and I, it's really a toss-up of their order.

  - "Measure the star's photometry in one filter multiple times per night (as rapidly as your facility can do so given the star's brightness at the time). This need not be done by lots of people, but having a few of these in different filters allows us to quantify the effect of smaller scaler variability for when we make non-simultaneous colors. Preferred filter priority is V, I, B, R (or Sloan equivalents)...B might be tough due to reddening...Between V and I, it's really a toss-up of their order. It'd be great if we could get both V and I, whether by one person, or split between two people."

Observations are requested beginning now and continuing through three days after V1333 Aql returns to quiescence. While BVRI (or Sloan equivalents) with the priorities given above are preferred, if necessary, unfiltered observations reduced to a Rc zeropoint (CR) or V zeropoint (CV) are acceptable. Typical outbursts of V1333 Aql last from about 30 to 120 days.

Coordinates (J2000): R.A. 19 11 16.05  Dec. +00 35 05.8  (from VSX entry for V1333 Aql)

Charts with comparison stars for V1333 Aql may be created using the AAVSO Variable Star Plotter (VSP). To see the R magnitudes of the comparison stars, be sure to tick the R filter box (at the bottom of the chart request form) when requesting a chart. The R magnitudes will be included in the photometry table that is produced for the chart (they will not be shown on the chart itself).

Please report all observations to the AAVSO International Database using the name V1333 AQL.

This AAVSO Alert Notice was prepared by Elizabeth O. Waagen using material supplied by Gregory Sivakoff.

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