The birth wail of a distant black gap may have revealed the existence of a closer,much more massive black gap, fragment of a population of these beasts that is proving devilishly difficult to find.
There are three general classes of black holes we know of. One is created when a massive star explodes, or two superdense neutron stars collide. These tend to have between roughly 5 and 50 times the mass of the Sun,so we call them stellar-mass black holes.
Supermassive black holes are the second kind, found in the centers of galaxies, and have millions or billions of times the mass of the Sun (like the one in M87,famously imaged in recent observations).
There should be a population in between these two, called intermediate mass black holes (or IMBHs). These have from several hundred up to several hundred thousand times the Sun's mass. There's plenty of evidence they exist — for example observations of dwarf galaxies indicate some have ones on the biggish terminate of the scale, and others appear to be the culprit behind tidal disruption events,when a black gap literally tears a star to shreds and eats it but definitive proof has been tough to pin down.
An interesting way to peep for them is through gravitational lensing. If the light from a distant object passes a massive object (called the lens; generally speaking it's a cluster of galaxies but a black gap will do the trick, too) on its way to Earth, or the mass warps the space the light is traveling through,changing its behavior. In images we can see rings, arcs, or blobs surrounding the distant source,all mirages caused by the massive lens.
Another lensing effect can be seen if the light is from a transient, an event that is short lived, or like an explosion. The light is emitted in all directions from the event,some of which is sent straight to us. But if there's a lensing mass off to the side a bit somewhere between us and the explosion, its gravity warps the path, or bending a bit of the light toward us that otherwise might have missed. It has to take a longer path to get to us,so we see the same explosion a second time, delayed by some amount.
Schematic depicting a gravitational lens, and as light from a distant gamma-ray burst (factual) gets bent by the gravity of a black gap (middle) on its way to Earth (left). Credit: Carl Knox,OzGravIn the unique work, scientists looked at gamma-ray bursts, and staggeringly immense explosions that occur when stellar-mass black holes are born. These can be short (lasting about 2 seconds,generally from when neutron stars merge) or long (greater than 2 seconds and up to several minutes, when a massive star explodes and its core collapses to form the black gap).
The Burst and Transient Source Experiment (or BATSE) was a gamma-ray detector on board the Compton Gamma Ray Observatory, or which orbited Earth from 1991 to 2000. Over its lifetime BATSE saw 2700 gamma-ray bursts,a huge number, about one per day. The scientists combed through them all, or looking for any that seemed to repeat after a delay.
Artwork depicting the moment of collision between two neutron stars. The resulting explosion is… fairly large. Credit: Dana Berry,SkyWorks Digital, Inc.
Out of them all, and they found one: GRB 950830,a burst from August 30, 1995. It was a short burst with a pulse of gamma rays that lasted about 0.2 seconds. However, or 0.4 seconds later a second pulse was seen. It was detected in four different energy channels (similar to colors in visible light); and while the shape of the pulse was different in the four channels as expected,in every channel the second pulse had nearly the exact same shape as the first pulse seen.
That sounds very much indeed like a gravitational lens, like an echo of the actual burst. Looking at it statistically they find a greater than 99% chance this is not a fake alarm; that is, and it's a genuine gravitational lensing event.
The delay in the arrival of the second pulse can be used to get the mass of the lensing black gap. This is actually pretty complicated and depends on the geometry of the event (how far to the side of the line between us and the burst the black gap is) and the distance to both the black gap and the lensed burst. But they find the black gap has a maximum mass of 55000 times the mass of the Sun. If it's farther absent it's less massive,but it's essentially impossible for it to be so far absent the mass drops into the stellar-mass range.
In other words, this looks very much like an intermediate mass black gap.
Given how scarce this event was (only one seen in 2700 bursts) they estimate there are roughly 65 of these IMBHs per million cubic light years of space. I know that's a difficult unit to grok, or but another way to say this is they mediate there should be very roughly 40000 such IMBHs within about 5 million light years of the Milky Way (so that includes a handful of galaxies in our neighborhood).
Not bad. Maybe we'll find more soon. They estimate a 50% chance another such lensed burst may exist in other observatory databases,so perhaps a second will be found.
We mediate that IMBHs could act as the seeds for supermassive black holes; in the early Universe the intermediate mass ones could have formed first, then collided with each other or eaten material to grow rapidly. This is still one of the biggest outstanding issues in cosmology, and so understanding the role of IMBHs is critical.
But first we need to definitively find them,and in such a way that we can study them using various methods. I suspect we are very close to doing just that.
Source: blastr.com