Pulsar-related Science with theEarly Phase of the LST
Albert Kong
National Tsing Hua University, Taiwan
ICRR, University of Tokyo
University of Oxford
FAN collaboration
K.S. Cheng (HKU), C.Y. Hui (Chungnam), J. Takata (HUST), P.H.T. Tam (SYSU)

Pulsar-related Science in GeV and TeV• Pulsars and their vicinity (including pulsar wind nebula andthe associated supernova remnants) are some of the bestsites to produce GeV and TeV emissions.

• One important implication of GeV and TeV observations ofpulsars is to constrain the emission geometry andmechanisms.

• Fermi, H.E.S.S., and MAGIC have already produced many

important results (e.g., pulsed gamma-rays from pulsars,gamma-rays from globular clusters, long-term gamma-rayvariability of pulsars)• CTA will bridge the gap between Fermi and MAGIC/H.E.S.S.

Early Pulsar Science with LST• Pulsed gamma-rays of pulsars• Can a single LST perform pulsar timing forbright pulsars?

• Spectral shape of bright pulsars• Long-term gamma-ray variability• In this talk, I will focus on gamma-ray emissions

from globular clusters that are primarily from acollection of millisecond pulsars

Stellar Encounters

MSP+WDSupernova

NS

X-ray binary
Accreting MSPWhy Globular Clusters?

Pooley+ 2003
Kong+ 2006

Why Globular Clusters?

Hui+ 2011

Encounter rate

IR photon density

Text

From Scott RansomGamma-ray Emission from GCs• Gamma-ray (>100 MeV) emission is likely to befrom MSPs

• Pulsed curvature radiation arising near the polar
cap and /or in outer magnetospheric gaps (e.g.,
Zhang & Cheng 2003; Harding et al. 2005;Venter& De Jager 2008)• Inverse Compton scattering photons between therelativistic electrons/positrons in the pulsar winds
and the background soft photons (e.g., Bednarek& Sitarek 2007; Cheng et al. 2010)

Why Globular Clusters?

Hui+ 2011

Encounter rate

IR photon density

Gamma-ray Emission from GCs• Most of the gamma-ray MSPs are very nearby (< 1 kpc) => they
are very faint.

• The nearest GCs are several kpc from us. It is almost impossible
to detect individual gamma-ray MSPs in GCs except from apulsation search (e.g., NGC 6624 and M28).

• However, GCs can have tens to hundreds of MSPs because of theenhanced dynamical formation. We can detect the collective
gamma-ray emission with sensitive instruments.

• Terzan 5 and 47 Tuc are the best candidates since they have thelargest number of MSPs in GCs.

• EGRET only obtained upper limits• Fermi/LAT and H.E.S.S./MAGIC can do much better

Theoretical Predictions of MSPs in GCs

Venter et al. 2009

First Gamma-ray detection from a GC (47 Tuc)
Abdo et al. 2009

Text

From Scott Ransom

Fermi 17-month Observations of Terzan 527σ

3.7σ

Kong et al. 2010

Comparison between Terzan 5 and 47 Tuc• If the gamma-rays are from inverse compton scattering (Cheng et al.
2010):• Inverse Compton (IC) scattering between relativistic

electrons/positrons in the pulsar wind of MSPs in the GCs
and background soft photons including cosmic microwave/
relic photons, background star lights in the clusters, thegalactic infrared photons, and the galactic star lights.

• The background soft photon intensity from the Galactic planeat the position of Terzan 5 is roughly 10 times that of 47 Tuc(Strong & Moskalenko 1998)• Terzan 5 should have stronger emission with energies > 10 GeV(Cheng et al. 2010)• Future CTA observations may tell

TeV Observations of GCs• The only confirmed case is Ter 5 detectedby H.E.S.S. (Abramowski+ 2011)

TeV Observations of GCs• Is the TeV emission associated with Ter 5?
TeV centroid is 4 arcmin from the centre of Ter 5, way• Theoutside the half-mass radius• It is marginally within the tidal radius of the GC• The TeV emission is extended

Prospects of CTA• Case study: Ter 5• Based on H.E.S.S. observations: 0.44-24 TeV

photon flux = 1.2x10-12 cm-2s-1 (1.5% of Crab)Ter 5

From Hassan+ 2016, CTA Consortium MeetingWhat about LST only?

Ter 5

two LSTs, Ter 5 may be marginally detected with 50h• Withexposure time
simulations to verify (response files for LST-only• Needobservations are required)• It can be easily detected with three LSTs in 50h
next best target for LST-N is M15 based on H.E.S.S.
• The(Abramowski+ 2013) but an upper limit of 7.2x10 cm s-13-2 -1indicates that a full array is required

NGC6388, M62, and 2MS-GC01 (based on Fermi• NGC6440,
flux from Tam+ 2016) are also worth to try with LST-N but
there are no H.E.S.S. limits.

technique may reveal the pulsations of NGC6624 and• Timing
M28