Apep: A Galactic LGRB Progenitor?

Benjamin Pope

NASA Sagan Fellow, NYU

CfA Stars Seminar

I work on very bright stars in Kepler, K2 and TESS, with recent work on K2 transits, the Pleiades, Aldebaran, and red giants in K2 and Kepler.

I am also interested in M dwarfs with planets, and would love to talk about work with Paul Withers and Marissa Vogt (BU) on planetary transits in the radio.

Instead I will be talking about Wolf-Rayet stars today!

Published today in Nature Astronomy:

Anisotropic winds in a Wolf–Rayet binary identify a potential gamma-ray burst progenitor

Collaborators Joe Callingham, Peter Tuthill, Peredur Williams, Paul Crowther.

Slides available at


Wolf-Rayet stars are the last stage in the life cycles of the most massive stars (~>25 Msun)

Thought to be progenitors of Type 1bc supernovae

Rapidly-rotating WRs progenitors of long gamma-ray bursts...

...but at solar metallicity strong line-driven winds cause them to spin down.

WR colliding wind binaries (eg WR + O star) produce dust in a 'pinwheel' spiral, like WR 104. Spiral evolution encodes binary parameters.

Joe Callingham (ASTRON) found a very bright CWB cross-matching Molonglo 843 MHz, 2XMM (X-ray) and WISE (IR) catalogs.


SINFONI J+H band spectra indicated a WR component

IRIS2 AAT spectrum: He 1083 Å line P Cygni profile indicates a ~ 3400 km/s wind speed. Fast!

So we got VLT/NACO (inset, radio contours) and VISIR (main) images...

Whoah! Never seen a pinwheel this complex before. Wanted second epoch to make a video like before.

SED fits and DIB constraints place this at d = 2.4 ± 0.5 kpc. (Bad Gaia parallax.)

At this distance we expect a ~ 10 pix radial expansion.


*Named for Egyptian god of Chaos, dragon enemy of Ra

Difference with second epoch

It expanded by ~ 1.1 pixels - 570 km/s! Far too slow.

Our best model fits a very high opening angle shock, indicating momentum parity between winds. 2x WR?

This is supported by ATNF VLBI radio imaging of the shock - curvature is real

Wind speed discrepancy could be resolved by making it more distant - but it would then be far brighter than η Car!
Alternatively - if there is a fast polar wind and slow equatorial wind...

a signature of rapid rotation known in LBVs but never yet observed in Galactic WRs.

LGRB progenitor?

The Future

This lord of chaos causes problems for our understanding of how WRs form and evolve - why is it spinning so fast?

Binary interaction?

Why is it so bright - what is the nature of the central engine?

We have ongoing radio and IR followup showing more of the same...

we really need ALMA to look for cold dust.

Apep is a new class of colliding-wind binary system with potentially very significant observational value.

This is the beginning of an adventure to understand it!