AS GAMMA-RAY PULSARS WITH THE MAGIC TELESCOPE

AS GAMMA-RAY PULSARS WITH THE MAGIC TELESCOPE

R. Ordu˜

na and C. Baixeras

Physics Department,Univ. Aut´onoma de Barcelona, 08193 Barcelona, Spain

A. Carrami˜

nana

Instituto Nacional de Astrofisica, Optica y Electr´´ onica, Luis Enrique Erro, 1, Tonantzintla, Puebla, 72840, M´exico

V. Fonseca

Physics Department, Univ. Complutense de Madrid, 28040 Madrid, Spain

for the MAGIC collaboration

Updated collaborator list at

http://hegra1.mppmu.de/MAGICWeb/collaborators.html

june 1, 2004

Abstract. Most of the unidentified gamma ray sources detected near the Galactic plane by EGRET aboard CGRO are expected to be gamma ray pulsars. We present a study about the detectability and identification of some unidentified EGRET sources with the MAGIC telescope. We list some unidentified gamma ray sources from the third EGRET catalogue to be detected with MAGIC taking into account some important conditions such as the variability parameter of the source, spectral index, inclusion in the GeV catalogue (Lamb and Macomb, 1997) and possible associations with known X-ray/radio sources located within the error box of the unidentified gamma ray source. We show the required observation time of these gamma ray pulsar candidates to be detected by MAGIC telescope considering reasonable values of cut- off energy. To be more realistic, we have chosen the zenith angle corresponding to the source culmination in the simulation of the effective area A since the observation time is function of the effective area. In addition to this study, it is very important to consider the extrapolated EGRET flux at MAGIC energies above 30 GeV of the gamma ray pulsar candidates taking the MAGIC sensitivity.

Keywords: Pulsars, Unidentified Gamma-Ray Sources, Cherenkov Telescopes

1. Introduction

The EGRET telescope opened loud pulsars, which are almost certainly

present among the large number of unidentified sources contained in

the 3EG catalog. Other types of gamma-ray sources are certain to be

present in the 3EG catalog, where (Gehrels, 2000) showed evidence of

at least two separate Galactic populations. So, while isolated pulsars

and radio-loud blazars are the dominant types of known gamma-rays

sources, objects like supernova remnants (SNRs), microquasars, X-ray

pulsars and binary pulsars have been proposed as counterparts for

unidentified EGRET sources. The second generation of ground based

γ

-ray telescopes, able to detect gamma-ray photons of energies closer to the EGRET -and GLAST- energy range, should be able to provide fresh information to these questions, like the association between pulsars and unidentified gamma-ray sources.

MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov tele- cope) is a second generation gamma-ray telescope, with its large re- flective area of 234

m²

and high quantum efficiency photomultipliers and it is designed to capture

γ-rays in the ≥

30 GeV domain. In this paper we present the estimated detection time for a selected number of EGRET pulsed gamma-ray sources. To calculate the source required observation time we have used Monte Carlo simulations corresponding to the culmination of each source or we have taken into account the minimum zenith angle of the source during the night. These Monte Carlo simulations are the current estimate of the telescope performance and no use of the gamma/hadron separation methods has been made.

2. The unidentified EGRET gamma-ray sources

Selected list showed that the brighter sources are located at low Galac- tic latitudes, while dimmer sources are concentrated as a large halo around the Galactic center (Gehrels, 2000). Furthermore, (Grenier, 2004) showed the presence of four different populations, taking into account variability and source distribution models. The low-latitude gamma-ray sources are most probably young objects, like gamma-ray pulsars, although studies have also shown formidable evidence in favour of the association between EGRET sources, SNRs and star forming regions. Studies point to a significant number of Population I objects in the parent population of the low-latitude gamma-ray souces, like the correlation analysis with bright giant HII regions performed by (Romero, 2001). The work presented here addresses pulsars hidden among the unidentified gamma-ray sources, which are more likely to be in the group of bright sources, with estimated luminosities around 10

³⁴−

10

³⁶

erg/s, at low latitudes,

|b| <

5 degrees. These unidentified sources have characteristics similar to those of gamma-ray pulsars.

3. Detection of pulsed emission by the MAGIC telescope

The pulsar cutoffs for conservative polar cap models scale as (Nell and

de Jager, 1988)

dNγ

dE

=

K( E

E_n

)

^−Γ

exp (−(

E

E_o

)

^β

) (1)

where

β

is the strength of the cutoff. The most conservative estimate for the detection sensitivity is to take

β >

1, consistent with magnetic pair production above the polar cap, whereas a value of

β <

1 will yield a very optimistic detection rate. The expected rate of triggers of pulsed Cherenkov showers is obtained from the convolution of the collection area

A(E) and the differential spectra given in the eq 2:

Rp

=

Z

A(E)(dNγ

dE

)dE (2)

The expression of the collection area

A(E) has been fitted from our

simulations:

A(E) = aE^b

(e + (

^E_c

)

^d

) (3)

where

a, b, c, d

and

e

are the fit parameters. In order to be conservative in our estimations, we assume a typical polar cap scenario, with

β=2.

According to the EGRET limit, we chose a cutoff energy

E_o

=30 GeV for the selected unidentified gamma-ray sources. We have fixed the normalising energy

En

at 1 GeV so that

K

is the monochromatic flux at 1 GeV, and Γ is the spectral index of the source above 100 MeV given in the Third EGRET Catalogue (Hartman, 1999).

The required observation time is given by

T

=

x²

(R

_p

+

R_b

)/R

²_p

, where

Rp

is the rate of pulsed

γ-ray photons coming from the source

calculated in the eq. 2,

R_b

is the rate of background, including protons, Helium and muons and

x

is the significance of the detection. In our studies, we estimate

R_b

=250 Hz and

x=5σ.

4. The selected unidentified EGRET sources

The chosen unidentified EGRET sources are compatible with the pulsar

hypothesis. All of them should comply with the following criteria: 1) to

be observable from La Palma, with

lat

= 28.8

^o

, 2) to have an spectral

index within the range Γ

≈1.4 - 2.3, typical for pulsar spectra, 3) to

have a variability parameter

δ

compatible with gamma-ray pulsars, we

have used the variability model proposed by (Nolan, 2003) 4) to have

a high value of ˙

E/d²

of the associated radio pulsars, where ˙

E

is the

spin down luminosity and

d

the distance, 5) to have an unidentified

EGRET source associated to a known pulsar, 6) to be listed in the

Lamb and Macomb GeV catalogue, 7) to consider the possibility to

Table I. The selected unidentified EGRET sources and its different parameters

3EG GeV l b F δ Γ Pulsar ass.

J0010+7309 J0008+7304 119.92 10.54 7.87 0.26 1.85 no J1835+5918 J1835+5921 88.74 25.07 45.0 0.15 1.69 no J1837-0606 J1837-0610 25.86 0.40 15.0 0.00 1.82 yes J1856+0114 J1856+0115 34.60 -0.54 12.6 0.71 1.93 yes J2020+4017 J2020+4023 78.05 2.08 12.6 0.06 2.08 no J2021+3716 J2020+3658 75.58 0.33 21.8 0.36 1.86 yes J2227+6122 J2227+6101 106.53 3.18 1.06 0.20 2.24 yes

detect unpulsed high energy emission from the pulsar wind nebulae associated to some unidentified EGRET sources.

As shown in Table I, where

l

is the galactic longitude in degrees,

b

is the galactic latitude in degrees,

F

is the extrapolated flux above 30 GeV (Petry, 2001) given in 10

⁻¹⁰cm⁻²

s

⁻¹

,

δ

is the variability pa- rameter (Nolan, 2003) and Γ is the spectral index above 100 MeV.

All the sources selected are low-latitude ones, except the mid-latitud sources 3EG J0010+7309 and 3EG J1835+5918, and four out of the six gamma-ray bright EGRET pulsars have wind nebula, further sup- portive evidence exists for most of the objects selected here.

Table I contains the four EGRET sources selected which are po-

sitionally coincident with some radio pulsars, 3EG J1837-0606, 3EG

J1856+0114, 3EG J2021+3716 and 3EG J2227+6122 have pulsar as-

sociations within their EGRET error boxes, the associated pulsars are

shown in the table II, but with pulsations not been found in the gamma-

ray data. 3EG J1856+0114, 3EG J2021+3716 and 3EG J2227+6122

have pulsar wind nebulae (PWN’s), which might be significant gamma-

ray sources by themselves, while 3EG J1837-0606 does not have a

nebula associated.

γ-radiation above 200 GeV could be detected from

the pulsar wind nebulae, although the unpulsed component of the pul-

sar itself cannot be excluded considering that the

γ-ray nebula is the

result of inverse Compton scattering of the electrons injected by the

pulsar into the surrounding interstellar medium (ISM) of low magnetic

field (Aharonian, 1997). The TeV fluxes can be estimated assuming

that the

γ

-rays are produced in the X-ray nebula. Taking into account

that the energy fluxes of many X-ray nebulae are between 10

⁻¹²

and

10

⁻¹¹

erg cm

⁻²

s

⁻¹

while the flux sensitivity of the MAGIC telescope

is 10

⁻¹⁰

cm

⁻²

s

⁻¹

at 30 GeV, that opens an interesting line in the

discovery of the X-ray nebulae around many radio pulsars since if the

origin of these X-ray nebulae is via synchrotron emission, it would imply

Table II. Pulsar associations and the gamma-ray efficiency

3EG PSR Lγ/E(≥˙ 100M eV) logE/D˙ ²(ergs⁻¹kpc⁻²)

J1837-0606 J1837-0604 0.08 0.88

J1856+0114 J1856+0113 0.08 3.7

J2021+3716 J2021+3651 0.18 0.24

J2227+6122 J2229+6114 0.04 2.37

the existence of the accompanying

γ-ray nebulae due to IC scattering

of the relativistic electrons.

The three EGRET sources selected with no coincident known ra- dio pulsar, 3EG J0010+7309, 3EG J2020+4017 and 3EG J1835+5918, might be related to radio-quiet pulsars. (Brazier, 1998) and (Slane, 2004) have studied the X-ray remnant CTA-1, coincident with 3EG J0010+7309, a nebula with a pulsar-like point source. 3EG J2020+4017, the brightest of the non variable unidentified EGRET sources, also has a X-ray nebula, the

γ-Cygni supernova remnant.

3EG J0010+7309 and 3EG J2020+4017 show a hard Geminga-like spectra (Mirabal and Halpern, 2001) and (Halpern, 2002), large gamma to X-ray flux ratios and no pulsed emission is known up to now. One principal hypothesis about these two sources is that they might be radio-quite Geminga class objects. Their variability is quite consistent with the identified pulsars. On the one hand, the low expected cut off energy, around 3GeV - 5GeV, is the only objection to be detected by MAGIC telescope, on the other hand, the position of 3EG J2020+4017 is coincident with the

γ−Cyg

SNR, so that, it is a good gamma-ray pulsar candidate.

3EG J1835+5918 is an intriguing

γ-ray pulsar candidate. The γ-

rays properties of this source are very similar to those of Geminga and other EGRET pulsars, its soft X-ray flux is at least 50 times fainter that of Geminga and similar EGRET pulsars (Mirabal, 2000). Some gamma-ray studies show that if 3EG J1835+5918 is a pulsar, it must be either older or more distant than Geminga and may be a highly beamed gamma-ray transmitter.

However, it is important to know that four of the EGRET pul-

sars are known to have wind nebulae. We cannot forget that all these

unidentified EGRET sources have been detected at GeV range (Lamb

and Macomb, 1997), this fact is quite important to be considered in

the detection capability of the telescope.

Table III. Observation times for 10 GeV and 30 GeV.

3EG K θmin(^o) T(hours) T(hours)

(10⁻⁸cm⁻²s⁻¹GeV⁻¹) Ethr= 10GeV Ethr= 30GeV

J0010+7309 5.2 44.3 33 135

J1835+5918 9 30.5 0.6 7

J1837-0606 5.5 35 3.1 50

J1856+0114 7.4 27.5 4.1 58

J2020+4017 11 11.5 3.7 90

J2021+3716 11.5 8.5 0.9 17.5

J2227+6122 4.8 32.5 65 1300

5. The required Observation time

We have calculated the estimated minimum required observation time from the effective area simulating gamma-ray showers at the culmina- tion zenith angles for each unidentified source. The culmination angle is calculated from the following equation:

θmin

=

|dec−lat|

(4)

where

dec

is the declination of the source and

lat

is the geographic latitude of the telescope (Lat = 28.8

^o

). In table III we show the used values to calculate the observation time for two threshold energy values (10 GeV and 30 GeV).

In the table III we have calculated the required observation time taking into account a threshold energy of about 30 GeV and 10 GeV.

During the first operation stage of the telescope, the expected threshold

energy is estimated to be about of 30 GeV, in a second operation stage

of the telescope, thanks to the new high quantum efficiency PMT’s,

the new threshold energy is expected to be not lower than 10 GeV. On

the one hand, 3EG J1835+5918 and 3EG J2021+3716 are the sources

that require the minimum observation time of all the listed sources,

on the other hand, 3EG J0010+7309 and 3EG J2227+6122 require a

large observation time to be detected, however, the pulsed emission of

these sources could be detected during some nights considering a low

energy threshold of the telescope. 3EG J1837-0606, 3EG J1856+0114

and 3EG J2020+4017 could be detected with an observation time of

about 50 h, 58 h and 90 h respectively at 30 GeV. These observation

times can be much lower taking into account 10 GeV threshold energy.

6. Conclusions

3EG J1835+5918 seems to be the most promising candidate out of the seven unidentified sources since we will need only 30 minutes or 7 hours to detect it considering 10 GeV and 30 GeV threshold energy respec- tively. The following better candidate for the MAGIC response is 3EG J2021+3716, the 10th bright source in the GeV domain with pulsar wind nebulae, with a detection time of 1 hour or 17.5 hours for 10 GeV or 30 GeV respectively. This source has a low zenith angle, therefore, it could be easily detected during one night, while 3EG J1835+5918 has the culmination angle at 30.5 degrees what increases the required observation time for a detection (Fonseca, 2003).

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