Archive for eLISA

Weekly Update from the Open Journal of Astrophysics – 01/02/2025

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , , , , , , , , , , , , on February 1, 2025 by telescoper

It’s Saturday morning, so once again it’s time for an update of papers published at the Open Journal of Astrophysics. There were no papers to report last week but since the last update we have published four new papers, which brings the number in Volume 8 (2025) up to 11 and the total so far published by OJAp up to 246.

In chronological order of publication, the four papers published this week, with their overlays, are as follows. You can click on the images of the overlays to make them larger should you wish to do so.

First one up is  “A halo model approach for mock catalogs of time-variable strong gravitational lenses” by Katsuya T. Abe & Masamune Oguri (Chiba U, Japan), Simon Birrer & Narayan Khadka (Stony Brook, USA), Philip J. Marshall (Stanford, USA), Cameron Lemon (Stockholm U., Sweden), Anupreeta More (IUCAA, India), and the LSST Dark Energy Science Collaboration. It was published on 27th January 2025 in the folder marked Cosmology and NonGalactic Astrophysics. The paper discusses how to generate mock catalogs of strongly lensed QSOs and Supernovae on galaxy-, group-, and cluster-scales based on a halo model that incorporates dark matter halos, galaxies, and subhalos.

 

You can find the officially accepted version of this paper on arXiv here.

This paper, also published on Monday 27th January 2025, but in the folder Astrophysics of Galaxies, is “The Soltan argument at redshift 6: UV-luminous quasars contribute less than 10% to early black hole mass growth” by Knud Jahnke (MPI Heidelberg, Germany). This paper presents an argument that almost all growth of supermassive black hole mass at z>6 does not take place in UV-luminous quasars.

Here is a screen grab of the overlay, which includes the abstract:You can find the officially accepted version of the paper on the arXiv here.

The third paper to announce, published on 29th January 2025 in the folder Cosmology and NonGalactic Astrophysics, is “A Heavy Seed Black Hole Mass Function at High Redshift – Prospects for LISA” by Joe McCaffrey & John Regan (Maynooth U., Ireland), Britton Smith (Edinburgh U., UK), John Wise (Georgia Institute of Technology, USA), Brian O’Shea (Michigan State U., USA) and Michael Norman (University of California, San Diego). This is a numerical study of the growth rates of massive black holes in the early Universe and implications for their detection via gravitational wave emission.

You can see the overlay here:

 

The accepted version of this paper can be found on the arXiv here.

The last paper of this batch is “Forecasting the Detection of Lyman-alpha Forest Weak Lensing from the Dark Energy Spectroscopic Instrument and Other Future Surveys” by Patrick Shaw & Rupert A. C. Croft (Carnegie Mellon U., USA) and R. Benton Metcalf (U. Bologna, Italy). This paper, published on January 30th 2025, is about extending the applicationof  Lyman-α forest weak gravitational lensing to lower angular source densities than has previously been done, with forecasts for future spectral surveys. It is in the folder marked Cosmology and NonGalactic Astrophysics.

The overlay is here

 

You can find the accepted version on arXiv here.

Incidentally, we currently have 121 papers under review, including 81 under a revise and resubmit request.

That’s all for this week. I’ll do another update next Saturday.

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Lisa Pathfinder – better late than never!

Posted in Science Politics, The Universe and Stuff with tags , , , , , , , on December 3, 2015 by telescoper

Determined to post about something positive after yesterday’s act of collective idiocy by Parliament I find myself given a golden opportunity by today’s successful launch of the Lisa Pathfinder experiment by the European Space Agency.

As space missions go, LISA Pathfinder seems quite a modest one: it is basically a pair of identical 46 mm gold–platinum cubes separated by 38 cm. The idea is to put these test masses in free fall and measure their relative positions as accurately as possible.

After a false start yesterday, LISA Pathfinder was successfully launched in the early hours of this morning and is now en route to the First Lagrangian Point of the Earth-Sun system, about 1.5 million miles from Earth, at the location marked L1 in the diagram:

Lagrange_saddle

The contours show the “effective potential” of the Earth-Sun system, which takes into account the effect of rotation as well as gravity. The five Lagrangian points are the places at which tis effective potential is locally flat, i.e. where its spatial gradient vanishes. Any physics student will know that when the gradient of the potential is zero there is no force on a test particle. What this means is that an object placed exactly at any of the 5 Lagrangian points stays in the same position relative to the Earth and Sun as the system rotates. Put a spacecraft at one of these points, therefore, and it stays put when viewed in a frame rotating around the Sun  at the same speed as the Earth.

It’s not quite as simple as this because, as you will observe the Lagrangian points are not stable: L1, L2 and L3 are saddle-points; a  stable point would be a local minimum. However, around the first three there are stable orbits so in effect a test mass displaced from L1, say, oscillates around it without doing anything too drastic. L4 and L5 can be stable or unstable, in a general system but are stable for the case of the Solar System, hence the tendency of asteroids (the Trojans) to accumulate at these locations.

You may remember that WMAP, Planck and Herschel were all parked in orbits around L2. A spacecraft positioned exactly at L2 is permanently screened from the Sun by the Earth. That might be very useful if you want to do long-wavelength observations that require very cool detectors, but not if you want to use the Sun as a source of power. In any case, as I explained above, spacecraft are not generally located exactly at L2 but in orbit around it. Planck in fact had solar cells on the base of the satellite that provided power but also formed a shield as they always faced the Sun as the satellite rotated and moved in its orbit to map the sky. The choice of L1 for LISA Pathfinder was made on the basis of spacecraft design considerations as it will operate in a very different manner from Planck.

The reason for doing eLISA is to demonstrate the technological feasibility of a much more ambitious planned gravitational wave detector in space originally called LISA, but now called eLISA. The displacement of test masses caused by gravitational waves is tiny so in order for eLisa it is necessary (a) to screen out every effect other than gravity, e.g. electromagnetic interactions due to residual charges, to great precision and (b) to measure relative positions to great accuracy. That’s why it was decided to fly a cheaper technology demonstrator mission, to prove the idea is feasible.

LISA Pathfinder won’t make any science discoveries but hopefully it will pave the way towards eLISA.

It has to be said that LISA Pathfinder has had a fairly troubled history. I just had a quick look at some papers I have dating back to the time when I was Chair of PPARC Astronomy Advisory. Among them I found the categorical statement that

LISA Pathfinder will be launched in 2009.

Hmm. Not quite. It’s obviously running quite a long way behind schedule and no doubt considerably over its initial budget but it’s good to see it under way at last. There will be a lot of sighs of relief that LISA Pathfinder has finally made it into space! Now let’s see if it can do what it is supposed to do!

 

 

 

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