Most millisecond pulsars with low-mass companions are in systems with either helium-core white dwarfs or non-degenerate ( “ black widow ” or “ redback ” ) stars .
A candidate counterpart to PSR J1816+4510 was identified by Kaplan et al .
( 16 ) whose properties were suggestive of both types of companions although identical to neither .
We have assembled optical spectroscopy of the candidate companion and confirm that it is part of the binary system with a radial velocity amplitude of 343 \pm 7 { km } { s } ^ { -1 } , implying a high pulsar mass , M _ { psr } \sin ^ { 3 } i = 1.84 \pm 0.11 M _ { \odot } , and a companion mass M _ { c } \sin ^ { 3 } i = 0.193 \pm 0.012 M _ { \odot } , where i is the inclination of the orbit .
The companion appears similar to proto-white dwarfs/sdB stars , with a gravity \log _ { 10 } ( g ) = 4.9 \pm 0.3 , and effective temperature 16 , 000 \pm 500 K. The strongest lines in the spectrum are from hydrogen , but numerous lines from helium , calcium , silicon , and magnesium are present as well , with implied abundances of roughly ten times solar ( relative to hydrogen ) .
As such , while from the spectrum the companion to PSRÂ J1816+4510Â is superficially most similar to a low-mass white dwarf , it has much lower gravity , is substantially larger , and shows substantial metals .
Furthermore , it is able to produce ionized gas eclipses , which had previously been seen only for low-mass , non-degenerate companions in redback or black widow systems .
We discuss the companion in relation to other sources , but find we understand neither its nature nor its origins .
Thus , the system is interesting for understanding unusual stellar products of binary evolution , as well as , independent of its nature , for determining neutron-star masses .