2XMMp and Galactic neutron stars

Natalie Webb Stéphanie Dupuy

Why search for X-ray emitting neutron stars?• Almost 2000 known neutron stars (NS) (1627 pulsars, ~70 NS X-ray binaries, the magnificent seven …)• Expect a NS to be born every 30-100 yrs in Milky Way • => 108-109 NS in our Galaxy (Neuhäuser & Trümper 1999) – depends on star formation• Only youngest detected as radio pulsars• Expect ~1000 radio quiet NSs for every pulsar (Kulkarni & van Kerkwijk 1998)• 7 X-ray emitting radio quiet NSs found with Rosat

NS detection important for: star formation rate physics of dense matter stellar core collapse

From Rosat to XMM-Newton

XMM-Newton has >10x collecting area to detect 100 cts from a NS, Lx = 1 x 1031 erg s-1, 1.9 kpc Rosat ~ 30 ks XMM-Newton ~ 3 ks

Dramatic increase of observable population!

Median flux of 2XMMp sources is ~10x fainter than the Rosat All Sky Survey limit=> Huge resource with which to detect NSs

X-ray emitting, dim, isolated neutron stars (XDINs)

(Zane et al. 2005)Radio quietX-ray to optical flux ratio (104) Low hydrogen column densities (nH

~1020 cm-2)

Quiescent neutron star low mass X-ray binaries (qNSLMXBs)

Without assuming mass MNS = 1.190.21

0.69 Msolar

T = 1.130.470.25 x106K

R = 7.55.10.60 km

2 = 1.12, 45 d.o.f.

Adapted from Gendre, Barret & Webb (2003a)

2XMMp (and all preceding versions) Used temperatures and nH of XDINs and qNSLMXBsDetermined ranges of fluxes in 2XMMp bands using:• xspec blackbody model • neutron star atmosphere model (Zavlin et al. 1996) colour diagrams ratios fluxes

screening important

Check: Found all observed XDINs/qNSLMXBs observed in 2XMMp

Results346 candidate NSs based on X-ray properties98 candidates without optical counterparts 55% false sources => 44 NS candidatesProposed 3 candidates to be observed in AO6/VLT

A strange object …

0.2-0.5 keV: 1.730.87 x 10-16 erg cm-2 s-1

0.5-1.0 keV : 1.490.02 x 10-13 erg cm-2 s-1

1.0-2.0 keV : 5.190.16 x 10-14 erg cm-2 s-1

2.0-4.5 keV : 3.370.26 x 10-16 erg cm-2 s-1

4.5-12.0 keV : 0.421.13 x 10-16 erg cm-2 s-1

No optical counterpart

kT1=0.0390.002 keVkT2=0.1680.002 keV

2=2.17 (111 dof)

All looking good ?

Flux ~ 7x10-13 erg cm-2 s-1

kT1=0.0390.003 keVkT2=0.1950.050 keV

2=7.8 (103 dof)

… but

Flux ~ 5x10-12 erg cm-2 s-1

kT1=0.080.05 keVkT2=0.440.02 keVkT3=1.310.03 keV

2=4.54 (215 dof)

… and

Rosat (early ’90s)

Flux ~ 1.5x10-12

erg cm-2 s-1

… and in fact …

The bright star is the optical counterpart (M2 V)B=9, V=7.5, proper motion=4.8’’/yr, d=2.5 pc!!!

Although,

The temperatures seem to be low for a flare star

But no significant radial velocity (N’dever et al 2002)

Summary

43 candidate neutron starsHoping for three to be followed up with XMM/VLTOne very strange object!