Discovery of eleven millisecond pulsars using jerk searches on unassociated Fermi-LAT sources

Motivated by the successful targeted searches of unassociated Fermi-LAT $\gamma$-ray sources in finding new millisecond pulsars (MSPs), we observed 72 such sources using the Green Bank Telescope (GBT) at 820MHz (Ransom et al. 2011; Hessels et al. 2011; Cromartie et al. 2016). Our main aim was to discover more tight binary systems (P$ _{B}$ $\le$ 1 day) colloquially known as "spiders" since they may hold the key to unlocking mysteries of pulsar evolution (Roberts 2011, 2013). Short orbital periods of spiders induce a significant apparent period derivative through the Doppler shift even if the observation lasts for only a few percent of the orbital period. If the observation lasts for more than $\sim$5% of the orbital period, then the second time derivative, or "jerk" becomes important. To mitigate these effects, we analyzed all the observations using a novel jerk search algorithm (Andersen \& Ransom 2018). These computationally expensive searches, along with 30 min of integration time for most of our sources, assured high sensitivity to tight orbit binary systems. The searches were done on a 12,000 core high performance cluster at the author's institution, using a search pipeline based on pulsar search software PRESTO (Ransom 2011). We present the discovery of eleven new MSPs as a result of these searches. Eight of the new MSPs were found through jerk searches while three were bright enough to be seen in preliminary acceleration searches. Among these is PSR J0418+66 (P = 2.91 ms) which was also independently discovered as a $\gamma$-ray pulsar and thus has a 11-year timing solution which shows that it is an isolated pulsar. Timing solutions remain to be obtained for the rest of the ten MSPs. However, for PSR J0312-09 (P = 3.704 ms) and PSR J0843+67 (P = 2.846 ms), we have orbital solutions through preliminary timing analysis. PSR J0843+67 is in a 7.35 day orbit with a companion mass M$ _{c}$ $\ge$ 0.27 M$ _{\odot}$. PSR J0312-09 is in a 2.34 hr orbit with companion mass M$ _{c}$ $\ge$ 0.009 M$ _{\odot}$, which makes it a black widow. It was discovered as a highly accelerated system with a significant jerk component. Folding the discovery observation without specifying the jerk component resulted in a smeared fold with a significance below the detection threshold in our pipeline. Hence, the cost of a jerk search was justified in the discovery of this unique binary system. PSR J1102+02 and PSR J1904-11 show exceptionally narrow pulses, indicating suitability for pulsar timing arrays. PSR J1624-39 has an inter-pulse at a phase offset of 0.5 from the main pulse, suggesting a likelihood for it to be an orthogonal rotator. This would make it an especially good candidate for pulsar evolution studies (Novoselov et al. 2020). Accounting for re-detection of already known pulsars among the observed sources, our survey had a detection fraction of 20.8%, comparable to that of other surveys of unassociated Fermi-LAT $\gamma$-ray sources (Cromartie et al. 2016). The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and DOE (United States), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, and JAXA (Japan), and the K.A.~Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also gratefully acknowledged. This work performed in part under DOE Contract DE-AC02-76SF00515. Fermi LAT research at the U.S. Naval Research Laboratory is supported by NASA.