2XMMp and Galactic neutron stars Natalie Webb Stéphanie Dupuy
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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
Results
346 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!
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