Likely Detection of γ-Ray Pulsations of PSR J1717+4308A in NGC 6341 and Implication of the γ-Ray Millisecond Pulsars in Globular Clusters

Abstract

We report our analysis results for the globular cluster (GC) NGC 6341 (M92), as a millisecond pulsar (MSP) J1717+4308A has recently been reported found in this GC. The data used are from the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi). We detect γ-ray pulsations of the MSP at a 4.4σ confidence level (the corresponding weighted H-test value is ∼28.4). This MSP, the fourth γ-ray pulsar found in a GC, does not have significant off-pulse emission and has γ-ray luminosity and efficiency 1.3 × 1034 erg s−1 and 1.7%, respectively. In order to have a clear view on the properties of the known GC γ-ray MSPs, we reanalyze the Fermi-LAT data for the other three ones. These four MSPs share the properties of either having high E ̇ (∼1036 erg s−1) or being in the GCs that contain only limited numbers of known MSPs. In addition, we find that PSRs J1823−3021A and B1821−24, in NGC 6624 and NGC 6626, respectively, have detectable off-pulse γ-ray emission and PSR J1835−3259B in NGC 6652 does not. Using the obtained off-pulse spectra or spectral upper limits, we constrain the numbers of other MSPs in the four GCs. The results are consistent with the numbers of the radio pulsars reported in them. While at least in NGC 6624 and NGC 6626, the contribution of other MSPs to their observed γ-ray emission can not be ignored, our study indicates that the presence of a bright MSP could be the dominant factor for whether a GC is detectable at γ-rays or not.

A Possible γ-Ray Pulsation from PSR J1740−5340B in the Globular Cluster NGC 6397

“…This is also consistent with the off-pulse TS map in the phase-resolved analysis (shown in the right panel of Figure 4). Similar results have been reported for PSR J1835 −3259B in NGC 6652 and PSR J1717+4308A in NGC 6341 by Zhang et al (2022bZhang et al ( , 2023, in which the pulsed γ-ray component also contributes the most of total observed γ-rays from the GCs.…”

“…No significant difference was found between this γ-ray PSR J1740−5340B and the previous ones detected in GCs reported in Freire et al (2011), Johnson et al (2013, Wu et al (2013), Zhang et al (2022bZhang et al ( , 2023 in terms of the γ-ray SEDs and the numbers of known MSPs, in summary those four MSPs are younger MSPs. Three of them have higher spin-down luminosities of (0.4 − 2.2) × 10 36 erg s −1 (Freire et al 2011;Johnson et al 2013;Wu et al 2013;Zhang et al 2022b), they are among the more brighter MSPs known than the typical ones.…”

“…For this MSP, its spin period and the spin-down rate are P = 1.83 ms and P 1.61 10 18 = ´- s s −1 , respectively, reported in Gautam et al (2022), its E  is ∼4.3 × 10 35 erg s −1 . Recently, Zhang et al (2023) reported the fourth γ-ray pulsation of PSR J1717+4308A in NGC 6341 with the radio rotational ephemeris reported in Pan et al (2020). Above mentioned reports seem to indicate that γ-ray emissions from GCs primarily arise from an individual MSPs hosted within them.…”

Characterizing the Gamma-Ray Emission Properties of the Globular Cluster M5 with the Fermi-LAT

“…They are all energetic pulsars with relatively high spin-down power and are bright in gamma rays, indicating that they probably dominate the gamma-ray emission from the host GCs. We have been unable to confirm the gamma-ray pulsations from PSR J1717+4308A in NGC 6341 (M92) reported by Zhang et al (2023b).…”

Detection of Increasing γ-Ray Emissions from 4FGL J1718.5+4237 with Fermi-LAT

“…Here we present the γ-ray emissions of 4FGL J1718.5+4237 from 4FGL-DR4 (Ballet et al 2023), which may be associated with a blazar, NVSS J171822 +423948, with a redshift of ∼2.7. Zhang et al (2023) carried out an analysis for γ-rays from the globular cluster NGC 6341 with Fermi-LAT, and found a significant γ-ray excess presented with a maximum likelihood test statistic (TS) of ∼40 (see their paper for more details). Motivated by their findings, we conducted a detailed data analysis for ∼15.25 yr Fermi-LAT observations on the γ-ray excess and discovered a high-energy γ-ray emission counterpart of a blazar, NVSS J171822+423948, exhibiting increasingly bright features in GeV γ-rays.…”