AN oil rig worker claimed to spot a top-secret US spy plane which can reportedly hit speeds of 5,000mph.
www.express.co.uk
The thing about the steep triangle is that it is a hypersonic waverider, it would be too inefficient at lower speeds to have a viable operational range. Simple trigonometry of the leading-edge sweep gives you the minimum cruise Mach number (which is also the max for a conventional supersonic wing):
45 deg = tan-1 i.e. Mach 1
63 deg = tan-1 i.e. Mach 2
72 deg = tan-1 i.e. Mach 3
76 deg = tan-1 i.e. Mach 4
79 deg = tan-1 i.e. Mach 5
etc.
How much above that number the cruise speed can go depends on various complications.
We know that the Blackbirds were comfortable until they ran into overheating issues a bit above Mach 3. Even getting there demanded black paint to help radiate the heat. Last I looked, that "bit" was still classified. So a refresh with updated materials and improved knowledge might well extend it significantly.
We also know that the JT-11D (J58) engine intakes were a relatively safe but possibly sub-optimal design to cover such an unprecedented speed range, chosen to minimise technical risk and development time. Add in that time and a more sophisticated two-dimensional intake would be a likely outcome.
Put the two together and you get a Mach 4-ish black triangle at about the right time, still running on the same special fuel - and hence using the same special tanker - as the Blackbirds.
Sweetman, credited in the article, would have known all that perfectly well, hence his willingness to take the sighting seriously.
But was it using a Blackbird-compatible fuelling point? Might it have been not Aurora but an early HAVE BLUE stealth experiment on a special mission? Or even a plywood spoof to discredit real "black triangle" sightings being reported in the States, just as the CIA had promulgated the UFO myth over preceding decades?