Radio Detection Prospects for a Bulge Population of Millisecond Pulsars as Suggested by Fermi-Lat Observations of the Inner Galaxy

Abstract

The dense stellar environment of the Galactic center has been proposed to host a large population of as-yet undetected millisecond pulsars (MSPs). Recently, this hypothesis has found support in an analysis of gamma rays detected with the Large Area Telescope onboard the Fermi satellite, which revealed an excess of diffuse GeV photons in the inner 15 deg about the Galactic center. The excess can be interpreted as the collective emission of thousands of MSPs in the Galactic bulge, with a spherical distribution strongly peaked towards the Galactic center. In order to fully establish the MSP interpretation, it is essential to find corroborating evidence in multi-wavelength searches, most notably through the detection of radio pulsations from individual bulge MSPs. Based on globular cluster observations and gamma-ray emission from the inner Galaxy, we investigate the prospects for detecting MSPs in the Galactic bulge. While previous pulsar surveys failed to identify this population, we demonstrate that upcoming large-area surveys of this region should lead to the detection of dozens of bulge MSPs. Additionally, we show that deep targeted searches of unassociated Fermi sources should be able to detect the first few MSPs in the bulge. The prospects for these deep searches are enhanced by a tentative gamma-ray/radio correlation that we infer from high-latitude gamma-ray MSPs. Such detections would constitute the first clear discoveries of field MSPs in the Galactic bulge, with far-reaching implications for gamma-ray observations, the formation history of the central Milky Way and strategy optimization for future deep radio pulsar surveys.

Searching for pulsars associated with the Fermi GeV excess

“…With the proper choice of channel widths and integration times for the interferometer setup, pulsar candidates could be further identified based on their diffractive scintillations as measured in variance images (Dai et al 2016). In the mean time, before a full hybrid survey is carried out, we agree with O' Leary et al (2016) and Calore et al (2016) that a search should be made for bulge pulsar candidates toward the "hotspots" identified by Bartels et al (2016) and Lee et al (2016). The detection of PSR J1751−2737 shows that deep imaging-based continuum with targeted pulsation searches offers a complementary approach to existing blind pulsation surveys.…”

“…Calore et al (2016) have looked at the prospects for detecting a bulge population of MSPs in some detail. They show that while existing deep surveys such as the HTRU are not well-suited to bulge detections, future large area radio pulsation searches have the potential to yield dozens of detections.…”

“…Calore et al (2016) describe a pulsation search with a 100-m class single dish radio telescope at 1.4 GHz. Their region with the highest yield is five degrees above (or below) the Galactic center.…”

“…The effects are strongest within the central 150 pc radius of Sgr A*, but dispersive smearing and scattering remain large over most of area of the GeV excess (see Figs. 5 and 6 of Calore et al 2016). Temporal smearing of pulsed signals make them hard to detect unless observing is carried out at high frequencies (ν > 5 GHz) where temporal scattering is sharply reduced (τscat ∝ ν −4 ).…”

Recognizing Axionic Dark Matter by Compton and de Broglie Scale Modulation of Pulsar Timing

“…Such central millisecond pulsars are expected in large numbers within the bulge and near the galactic center [23,24], and can account for the GeV gamma-ray excess [25,26] and are being searched with some success [27,28]. Detection will be compromised within the inner 100 pc where the high plasma density is high and the ISM contains small scale irregularities, causing dispersion of pulse arrival time and pulse smearing, although "corridors" of lower scattering may be evident [29,30].…”