The Swift Gamma-Ray Burst Explorer Paolo D’Avanzo INAF-Osservatorio Astronomico Di Brera
INAF, Osservatorio Astronomico di Roma
-
Upload
cain-keith -
Category
Documents
-
view
87 -
download
1
description
Transcript of INAF, Osservatorio Astronomico di Roma
INAF, Osservatorio Astronomico di Roma
XI Advanced School of Astrophysics, Brazil, 1-6 September 2002
The first MSP in an interacting binary: J1740-
5340
and in a globular cluster!
is observed during the radio-ejection phase?(Burderi D’Antona &
Burgay 2002)
The lack of sub-ms pulsars
As we have seen, mass transfer in the progenitors of binary MSP should be able to spin up pulsars to periods well below the minimum observed,
1.56ms
This may mean that sub-ms pulsars are hidden in short Porb systems, or that many NS have collapsed to BH, or, most probably, that not enough matter is accreted. But we must find a mechanism more efficient than the “propeller” to get rid of matter in these low B systems, in which Rm~RNS
The radio-ejection When the radiation
pressure of the rotating magnetic dipole becomes large enough, it prevents accretion directly at the inner Lagrangian point! (Ruderman et al. 1989,
Burderi et al 2001)
Requirement:1) Mass transfer must stop (or be very much
reduced) to allow the radio pulsar switch on2) Ps short enough that Ppsr > Pmatter
Pulsar pressure:P psr ~Ps
-4r-2
Disc pressure:Pdisc ~ Lacc
(17/20)r-(21/8)
The equilibrium in the disk
Either the magnetic pressure acts, or the radio pulsar pressure, if the disk terminates outside the light cilinder
(dyn/cm2 ) valid for a Shakura-Sunayev accretion disk=viscosity n~1 f~1
radio-ejection Fluctuation of mass transfer: disc pressuregoes down, radio pulsar switches on
Accretion resumes. If matter enters at this point, P_disk>P_psrand accretion goes on
If the Roche lobe is out, P_disk<P_psrand the pulsar prevents accretion
Burderi et al. 2001,ApJL 560, L71
The critical Porb depends on a very high power of the spin
The radio-ejection is much more efficient than the “evaporation” proposed to destroy the binarycompanions: in fact, mass loss is driven by theAML losses and/or nuclear evolution and not by the pulsar energy
Binaries
in NGC 6397
Taylor et al. revealed in the PC field of the HST data possible binary objects, BY Dra and candidate He-WDs. The red dot is the companion of the MSP J1740-5340
The MSP J1740-5340 in NGC6397
Orbital period 32.5 hr Mass function 0.0026442
Spin period 3.65 ms M companion > 0.19 Mo
Period deriv. 16.87e-20 Pulsar power 1.4x1035 erg/s
Radio eclipse T=0.4 Porb R eclipse= a x sin(0.4 )
~4.4 1011cm
Radio freq. 1326 Mhz1454 Mhz
R2(Roche) 1.3 1011cm
Delays of pulses:
up to 0.8ms t_delay~-2 (free free abs.)
Ne~6.4e18cm-3 Mdot~10-10
D’Amico et al. 2001, ApJL 561, L89
The search for the optical
counterpart
Ferraro et al. 2001: search for variability at the MSP orbital period in the HST arrchive data, at the radio location
PSR J1740-5340: the optical component
The radio eclipses last for ~40% of the orbit: matter is flowing around the system, being present at a radius
larger than the secondary’s Roche lobe. The optical light modulation indicates a non spherical companion
1) Intrinsic wind from a MS star? Not expected (Lithium problem)
2) Bloated low mass companion, pulsar evaporated: energy requirements much too large
Burderi, D’Antona and Burgay 2002 ApJ 574, 325
Evolution of the system
The optical counterpartof the MSP J1740-5340 is NOT a He-WD as in most MSP, but it is a quasi-MS mass losing star! How is it possible that we SEE the radio MSP, even if only for 60% of the orbit?
Roche lobe overflow, inhibited by radio-ejection seems to remain the only possible model.
J1740-5340 is in a radio-ejection phase
If we take the system parameters, and we put them in this analytic expression, we find Pcrit=39hr,
While the system period is 32.5hr: in view of the steep dependence of Pcrit, especially on Ps, this coincidence seems compelling. And, it may be that J1740-5340 can be pushed again into accretion
A necessary ingredient: intermittent mass transfer
If the mass transfer is stationary, the radio pulsar can not become active, and the system can not enter in the radio ejection phase:
This is where we need the irradiation mostly
Artist view of
the system
PSR J1740-5340:an interactingcompanion to a MSP
Code ATON 1.1 by Mazzitelli 1989, D’Antona et al. 1989
Metallicity Helium content 0.0002 Y=0.23 Masses (Mo): 1.4 + 0.85 (close to end of core H
burning) Initial orbital period: 14.27 hr t~10GyrColor transformations: from Castelli et al. 19971) Conservative mass transfer: 2) Conservative until P=25hr, then mass loss from
system3) Systemic AML (Magnetic braking –Verbunt &
Zwaan with f_vz=1, 2, 0.3) and Gravitational radiation
4) Role of specific angular momentum losses5) Irradiation (following Tout et al, 1989;
D’Antona and Ergma 1993)6) We followed the NS spin evolution due to the
mass transfer and checked that at P=3.5ms we are in the condition of radio-ejection
Reproducing the optical
companion of PSR J1740-
5340
Open square: 22.5hr
Open triangle: 32.5hr
Blue: f_vz=1Green: f_vz=0.3
The initial orbital period is close to Pbif
We are accumulating evidence that in GCs many systems start their evolution close to thebifurcation period:
1) It is necessary for J1740-5340, to be able to reproduce its HR diagram location at the correct period of 32.5hr
2) It is convenient to explain the short orbital period systems, especially X1820-303 in NGC 6624 which has the shortest period of 11m
3) But this may be common also outside GCs: the two MSP in XTE J1751-306 and XTE J0929-314 have Porb~40m, compatible only with partially H-rich degenerate companions
Self-consiste
nt evolution
to He-WD
Red: f_vz=1Blue: f_vz=0.3Magenta:f_vz=2Black: irradiated
The internal helium profile
From the start ofmass transfer to the WD formationin the conservative case
Mass transfer is faster in the
irradiated case
This frozens the chemical evolution, as we see from thehydrogen profile
The irradiated case
Comparison between
the H- profiles
Consequences:•Much longer evolutionary times for the remnant of irradiated evolution
•In any case, very long H-burning times
AML during the radio-ejection phase
Until the systems transfer mass, it is appropriate to consider a conservative or quasi-conservative evolution. But as soon a s the radio-ejection begins, there is loss of AML associated to the loss of mass. How large is the specific AML? Numerical simulations in this case would help.
Remnant helium white dwarf mass
The final mass depends slightly on the mass loss rate (and then on the specific aml). It is very difficult that we may obtain a mass smaller than 0.2Mo.
This is also true if we consider irradiation.
Are the Taylor et
al. objects
He-WDs?The three most luminous stars: may be
The other objects have much larger radii than the minimum mass possible WD remnant of binary evolution
What is this
sequence of objects
at constant R~0.04 Rsun?
Are the He-wd
candidatequiescent CVs?
Townsley and Bildsten 2002 suggest that we are in the presence of accreting WDs in which Tc(core) is determined by compressional heating due to accretion
Mass transfer and AML
Conservativeevolution can bea good assumption only until the mass is accreted. In theradio-ejection phase the mass loss rate depends on the loss of angular momentum
Evolution of
orbital period
While conservative evolution or evolution with small specific aml leads to long orbital periods, during the radio-ejection phasethe period may increase only slightly, or even decreaseOrbital period (hr)
Log
Mdo
t