Download Recent Measurements of Millisecond Pulsar Masses

Recent Measurements of Millisecond Pulsar
Masses
Paulo C. C. Freire
Arecibo Observatory / Cornell University
Physics Department / West Virginia University
In this talk...
• Normal Pulsars and Millisecond Pulsars.
• How to Measure Pulsar Masses.
• Why look in globular clusters?
• Mass measurements in globular clusters: The early years.
• Some late surprises!
• Measuring the mass of the PSR J1903+0327 (Parts I & II)
Normal Pulsars and Millisecond Pulsars
• The main difference between normal and
recycled pulsars is not merely the spin period,
which is faster for the second class. Recycled
pulsars also spin down much more slowly.
• Furthermore, many more of them are in binary
systems!
• The fast-spinning objects (P < 8 ms) - the
millisecond pulsars (MSPs) are either isolated
or have light white dwarf companions and
low-eccentricity orbits.
• Some of the slower-spinning recycled pulsars
(8 < P < 110 ms) are either isolated, or they
have a massive white dwarf or a neutron star
companion. The orbits for these systems are
generally eccentric!
Normal Pulsars and Millisecond Pulsars
• The differences among the recycled pulsars
reflect different evolution scenarios!
• If the companion is massive, accretion won’t
last long - mildly recycled pulsar. The massive
companion then goes supernova, producing
a double neutron star system (DNS).
• If companion is not so massive, it will evolve
to a white dwarf. In this case, accretion lasts
much longer! Resulting pulsar should spin
much faster and it should be much more
massive.
From: Duncan Lorimer’s Living Review on Binary and Millisecond Pulsars
Summary
• Pulsars in double neutron star systems are mildly recycled (or not at all for their companions)
• Their orbits are eccentric.
• Millisecond pulsars in binary systems have white dwarf companions.
• Their orbits are circular.
• They could be more massive than members of DNSs.
How to Measure Pulsar Masses
• Five Keplerian parameters can normally be measured from fitting the velocity (or, preferably, the
delay) curves: orbital period (Pb), projected size of the orbit, in light seconds (x), eccentricity (e),
longitude of periastron (ω) and time of passage through periastron (T0). A non-changing
Keplerian orbit is exactly what is predicted by Newtonian gravity.
• Without access to information on transverse velocities, the individual masses of the components
(m1 and m2) and the inclination of the system (i) cannot be measured, but...
• The mass function, a relation between these three quantities, can be measured to excellent
precision, as it depends on two observable parameters:
How to Measure Pulsar Masses
• According to general relativity, these quantities depend on the known Keplerian parameters and
the masses of the two components of the binary.
• We now have three equation for three unknowns! We can determine the masses of the
components and the inclination.
How to Measure Pulsar Masses
• An excellent example is “the” (first)
binary pulsar, PSR B1913+16,
discovered at the Arecibo Observatory
by Russell Hulse and Joe Taylor.
• Mass determination lead to prediction of
the orbital decay due to emission of
gravitational waves: the orbital period
should getting 75.8 microseconds
shorter every year.
From: Weisberg and Taylor, ASP Conf. Series vol. 302, p. 93, 2003
Neutron Star Masses
• Several neutron star masses have been measured precisely. Almost all of these are in DNS
systems, like PSR B1913+16 or PSR J0737-3039. This happens because they have eccentric
orbits, where more relativistic effects can be measured. Their masses range from 1.25 to 1.44
solar masses.
• Because of prolonged accretion, MSPs could be a lot more massive. But are they?
• MSP masses are difficult to determine – in the Galaxy, the orbits are very circular, so normally no
relativistic effects can be used to provide extra equations to solve for m1, m2, i, as for the DNS.
• A precise measurement of the mass of a MSP (PSR J1909–3744, Jacoby et al. 2005) based on
the Shapiro delay yielded a pulsar mass of 1.47 solar masses (Hotan, Bailes and Ord 2006).
• However, there have been claims of more massive MSPs! Unfortunately, no other MSP masses
can be precisely determined for the Galactic systems!
Neutron Star Masses
From: Lattimer and Prakash, 2007, Phys. Rep. 442, 109
• One of the best diagnostics of the EOS for super-dense cold matter is finding the upper limit for
NS masses.
• The existence of massive neutron stars would indicate that matter is highly incompressible.
Why Look for MSPs in Globular Clusters?
• MSPs in globular clusters can be particularly useful to this experiment:
• There are twice as many MSPs known in globular clusters as in the Galactic Disk.
• MSPs in globular clusters can be in eccentric orbits, because of:
• Perturbations from passing stars
• Exchange interactions
• Eccentric orbits allow the measurement of post-Keplerian effects; like the precession of
periastron and (much later) the Einstein delay.
• These, in turn, allow the measurement of MSP masses!
Why Look for MSPs in Globular Clusters?
• So, the question is, How Massive are MSPs in Globular Clusters?
• Let’s first find them, and then time them - and hopefully, in a few cases, find their masses!
Millisecond Pulsars in 47 Tucanae
• We (Paulo Freire, Duncan Lorimer, Michael
Kramer, Fernando Camilo, Andrew Lyne, Richard
Manchester and Nicolo D’Amico) used the
Parkes Multibeam system to search for new
pulsars in 47 Tucanae.
• We found eleven new MSPs! (Camilo et al.
2000, ApJ, 535, 975, Freire et al. 2009, MNRAS,
in preparation)
• We also used these observations to determine
the orbit and timing solution of PSR
J0024-7204H, one of the 11 previously known
pulsars (Manchester et al., 1991, Nature, 352,
219).
ATNF - CSIRO
Millisecond Pulsars in 47 Tucanae
• PSR J0024-7204H is a 3.21-ms
pulsar with an orbital period of
2.350 days and an orbital
eccentricity of 0.0705.
• Assuming a pulsar mass of 1.4
solar masses, the companion has
a minimum possible mass of 0.19
solar masses.
Cover of the ASP proceedings of the “Radio Pulsars” conference (ASP 302),
ed. M .Bailes, D. Nice, D. and S. E. Thorsett. Optical image of 47 Tucanae: AAT
Millisecond Pulsars in 47 Tucanae
• The rate of advance of periastron allows an
estimate of the total system mass for PSR
J0024-7204H: (1.61±0.04) solar masses
(Freire et al. 2003, MNRAS, 340, 1359).
• The pulsar mass can not be larger than 1.52
solar masses, the companion has a mass
larger than 0.16 solar masses.
• This was (as far as I know) the first
measurement shows that the recycling of a
MSP can be accomplished with ~0.1 solar
masses.
• Is this the case for all MSPs?
From: Freire et al., 2003, MNRAS. 340, 1359
A Millisecond Pulsar in NGC 1851
• In 2003, we (Paulo Freire, Yashwant Gupta
and Scott Ransom) used the GMRT to
search for pulsars in 11 GCs.
• One pulsar was found: PSR J0514–4002A,
in NGC 1851. This object has a spin
period of 4.99 ms (Freire et al. 2004, ApJL,
606, 53).
• Previously, nearly all known MSPs had
low-eccentricity orbits. NGC 1851A has an
orbital eccentricity of 0.89, the highest
known at the time!
• This violates the normal rules of stellar
evolution!
From: Freire et al. 2004, ApJ Lett., 606. 53
A Millisecond Pulsar in NGC 1851
• This system was probably formed by an exchange
interaction. These are only likely to happen in environments
with very high stellar densities, like globular clusters.
• The same mechanism could also form a MSP-MSP system
or a MSP-BH system! Such systems would be the ultimate
probes of GR, particularly if they could be timed with the
SKA!
• The high eccentricity allows a measurement of the
periastron advance, indicating a total system mass of
2.453(14) solar masses.
• The pulsar has a maximum mass of 1.5 solar masses
(Freire, Ransom and Gupta, 2007, ApJ, 662, 1177). Again,
not significantly larger than previous neutron star mass
measurements!
From: Freire, Ransom and Gupta, 2007, ApJ 662, 1177
PSR J1911-5958A
• This 3.27-ms pulsar was discovered in 2001, in the Southern globular cluster NGC 6752
(D’Amico et al. 2001, ApJ, 548, 171)
• It has a ~0.2-solar mass companion in a 0.837-day orbit. The eccentricity is < 10–5.
• Bassa et al. (2006, A&A, 456, 295) have estimated the mass of the companion from its observed
optical spectrum and estimated (cluster) distance: 0.175 ± 0.010 solar masses.
• From the orbital variations in the Doppler shift, they could also estimate the mass ratio for this
binary: MPSR/MWD = 7.36 ± 0.25.
• From this, they derive a pulsar mass of 1.34 ± 0.08 solar masses.
• This is consistent with a similar mass determination made by Cocozza et al. (2006, ApJ, 641,
129)
Summary:
• These highly recycled MSPs seem to have masses similar to those of neutron stars in DNS
systems!!!
• Accretion can definitely be achieved with very small amounts of matter, confirming the
result of PSR J1909–3744!
• Is this the case for all MSPs? All we can do is to keep searching for more eccentric binaries,
measure any PK parameters we can, and see what we find.
A Search for Millisecond Pulsars in Terzan 5
• In 2004, we (Scott Ransom, Jason Hessels, Ingrid Stairs, Paulo Freire, Vicky Kaspi, Fernando
Camilo and David Kaplan) used the S-Band receiver of the Green Bank Telescope and the pulsar
spigot to find 30 new MSPs in the globular cluster Terzan 5 (Ransom et al. 2005, Science,
307, 892). All of these have timing solutions now!
NRAO
Picture Above courtesy of Scott M. Ransom
A Search for Millisecond Pulsars in Terzan 5
• Terzan 5 has five of the ten fastest known millisecond pulsars anywhere, including the fastest:
PSR J1748-2446ad (rotating 716 times per second)!
• Six of the new systems in Terzan 5 (I, J, Q, U, X and Z) have orbital eccentricities above 0.3!
• Terzan 5 I is a 9.57-ms pulsar in a 1.33-day orbit with a 0.24-solar mass companion. The orbital
eccentricity of this system is 0.43.
• Terzan 5 J is a 80.3-ms pulsar in a 1.10-day orbit with a 0.39-solar mass companion. The orbital
eccentricity is 0.35.
• The compactness of these orbits allowed the measurement of the rate of advance of periastron
for these two binaries by 2005: they have total masses of 2.17(2) and 2.20(4) solar masses
respectively.
A Search for Millisecond Pulsars in Terzan 5
• If this effect is fully relativistic, two systems (I and J) could host massive pulsars: their median
masses are 1.86 and 1.76 solar masses.
• There is a 95% probability that one of them is more massive than 1.68 solar masses.
• Still, it is possible that these are being seen at low inclination!
From: Ransom et al., 2005, Science, 307, 892
Arecibo Timing of M5B
• Discovered by A. Wolszczan in
1989 using the Arecibo telescope,
PSR B1516+02B was one of the
first MSPs to be found in a globular
cluster (NGC 5904, or M5), for that
reason it is also known as M5B.
• Timing solution found by Anderson
et al. (1997, ApJ, 482, 870). It is a
7.95-ms pulsar in a 6.85-day orbit
with a 0.13-solar mass companion.
The orbital eccentricity is 0.14, the
highest known at the time for a
MSP.
• Precession of periastron was
detected, but no astrophysically
useful mass limits were derived!
NAIC / Arecibo Observatory
Arecibo Timing of M5B
• The Arecibo upgrade, in the late 1990s, has allowed the use of
much higher frequencies and increased bandwidths.
• An Arecibo L-band globular cluster survey started in 2001, this
discovered a total of 13 new MSPs in several globular clusters in
the Arecibo Sky (Hessels et al. 2007, ApJ, 670, 363).
• This found 3 new pulsars in M5. These were timed until the
present (Stairs et al., in preparation)
• Data was also used to time previously known pulsars, PSR
B1516+02A and B at L-band.
• The rate of advance of periastron is now determined with high
significance!
Arecibo Timing of M5B
• The total mass of the system is
remarkably similar to the total masses
of Terzan 5 I and J: 2.29(17) solar
masses.
• Pulsar mass: 2.08+0.18–0.19 solar masses.
There is a 0.77% probability that the
mass of the neutron star is “normal”.
• Pulsar mass is similar to Terzan 5 I and
J. There is a 95% probability that it is
more massive than 1.72 solar masses.
(Freire et al. 2008b, ApJ,679, 1433)
From: Freire et al. 2008b, ApJ, 679, 1433
Arecibo Timing of M5B
• HST archival images of the position of M5B
don’t show any companions down to
magnitude 26.5.
• This implies that, if companion is a MS star,
its mass is below 0.25 solar masses.
• Otherwise, it is a white dwarf.
• In either case, rotational effects are likely to
be negligible! The periastron advance is
therefore very likely to be relativistic.
From: Freire et al. 2008b, ApJ, 679, 1433 (2008)
New Globular Cluster Searches: 35 New MSPs!
• Motivated by the spectacular Terzan 5 results (30 new MSPs to date!), we decided to
use the same observing system to search for pulsars in other globular clusters.
• So far we have found 35 new MSPs in other globular clusters!
• Eleven of these were found in the globular cluster NGC 6626, or M28 (Steve
Bégin, M.Sc. Thesis, Begin et al. 2009, ApJ., in preparation)
• Two of them are in eccentric orbits: PSR J1824–2452C and PSR J1824–2452D.
• PSR J1824–2452C is a 4.159 ms pulsar in a 8.0-day orbit with a 0.25-solar mass
companion. The orbital eccentricity is 0.85! Maximum possible mass: 1.35 solar
masses!
• PSR J1824–2452D is a 79.8-ms pulsar in a 30-day orbit with a 0.37-solar mass
companion. The orbital eccentricity is 0.78!
NGC 6440B
AAT
• The same GBT/S-band/SPIGOT survey has also found 8 new MSPs in NGC 6440 and NGC
6441 (Freire et al. 2008a, ApJ, 675, 670).
NGC 6440B
Image courtesy of Scott M. Ransom
• One of the new discoveries is PSR J1748-2021B, or NGC 6440B. It is a 16.7-ms pulsar in an
eccentric (e = 0.57) 20.5-day orbit with a ~0.1 solar mass companion.
NGC 6440B
• After ~1 year timing this pulsar, we have
obtained a good measurement of the rate of
advance of periastron. If fully relativistic, this
implies a total system mass of ~2.92 ± 0.25
solar masses. It could be a double neutron
star system!
• The companion has only ~0.1 solar masses.
Median of probability for pulsar mass is 2.74
solar masses.
• There is a 99% probability of mass being
larger than 2 solar masses, 0.1% probability
of having a “normal” mass.
• Is this a super-massive neutron star?
From: Freire et al. (2008a), ApJ 675, 670
NGC 6440B
• If it is a DNS binary, there is a 1% chance of finding such a low inclination among the known
DNSs.
• Another possibility is that this is a 2.1-solar mass pulsar like M5B with a massive companion
seen at low inclination! Perhaps a better alternative than new physics!
• We will only know the real masses over the next few years, with increases in the precision of the
advance of periastron, measurements of the relativistic gamma and multi-wavelength studies of
the companion star.
• Hubble Space Telescope time already approved to look for the companion star. If it is a MS star
(a possible origin of a classical contribution), it should be readily detectable in HST images.This
will allow a verification on whether periastron advance is relativistic.
• Ongoing surveys are finding more MSPs, and more objects with eccentric orbits!
• Therefore, next few years are going to be a lot of fun for our group!!!
NGC 6440B
• Confirmation of a very high pulsar mass would have revolutionary implications for the study of
the EOS. It would imply that matter at the center of neutron stars can propagate sound waves at
nearly the speed of light! This would exclude most known models for the behavior of matter at
supra-nuclear densities!
MSP Ensemble
• Strange new trend: more massive
objects have much smaller spin
frequencies!
0
0.5
NGC 6440B
• The likely reason for this is the accretion
of material into the neutron star that was
needed to recycle it into a millisecond
pulsar.
M5B
• This is much wider than observed for the
neutron stars in double neutron star
systems.
Probability density
• If the masses of M5B and NGC 6440B
are confirmed, that will also mean that
millisecond pulsars have a much broader
mass range than previously thought - by
a factor of two!
1
1.5
2
2.5
3
3.5
Pulsar Mass (Solar Masses)
From: Freire et al. 2008b, ApJ, 679, 1433
4
Summary
• Over the last 20 years, many MSPs were found in globular clusters. Their number quadrupled
in the last 10 years, with the discovery of more than 100 new objects.
• An increasing number of MSPs is being found in eccentric binary systems.
• Timing campaigns are now determining post-Keplerian parameters for some of these orbits,
constraining the masses of the components in these binaries.
• Many of these are indistinguishable from the masses of mildly recycled NS, indicating that
accretion does not necessarily add much mass to the NS. However, some MSPs seem to be
significantly more massive than 1.44 solar masses i.e., the range of masses observed for MSPs
appears to be much wider than what is observed for normal NSs.
• Over the next few years, real masses will be measured with much better precision.
• Some of these high masses might pose extreme constraints on the EOS for dense matter.
Measuring the Mass of PSR J1903+0327
Collaborators: Scott Ransom (NRAO), David Champion (ATNF), Patrick Lazarus (McGill),
Fernando Camilo (Columbia), Victoria Kaspi (McGill), David Nice (Bryn Mawr), Ingrid Stairs
(UBC), Ben Stappers (University of Manchester), Julia Deneva (Cornell) and James Cordes
(Cornell)
The PSR J1903+0327 Binary System
With a spin period of 2.15 ms, PSR J1903
+0327 was the first millisecond pulsar
(MSP) discovered in Arecibo’s ALFA pulsar
survey.
It is in a 95-day binary system. Its mc ~ 1.0
Msun companion is by far the most massive
of any MSP with a similar spin period.
It is the first millisecond pulsar in the disk
of the Galaxy to have an eccentric orbit: e
= 0.44. All other MSPs in binary systems
have e < 0.002 (Champion et al., Science,
320, 1309).
It is very difficult to explain the formation of
such a binary system with previous stellar
evolution theory.
From: Champion et al., Science, 320, 1309 (2008)
The PSR J1903+0327 Binary System
The system is even harder to explain if
the companion turns out to be a main
sequence (MS) star as suggested by
the presence of a source with a
coincident position in optical
observations! No previously known
MSP in the Galactic disk has a MS
companion.
Champion et al. (2008) estimate a 2.6%
probability of a chance alignment.
Follow-up at optical wavelengths will
verify this possibility, and possibly allow
an estimate of the mass ratio.
From: Champion et al., Science, 320, 1309 (2008)
From: NRAO / Cornell University Press Release
Measuring the Mass of PSR J1903+0327: Part I
Apart from presenting an evolutionary
puzzle, the unusual characteristics of this
binary system are interesting because
they allow the measurement of postKeplerian orbital parameters that lead to
a precise determination of the masses of
the components of the system.
The eccentricity of this binary system
allowed a measurement of the apsidal
motion even in the early GBT timing.
Assuming GR is correct, this gives us the
total mass of the system.
The large companion mass, coupled with
the good timing precision, allow a
measurement of the Shapiro delay.
This is a truly unusual set of
circumstances for a MSP binary system!
From: Champion et al., Science, 320, 1309 (2008)
Measuring the Mass of PSR J1903+0327: Part I
In Champion et al. (2008), only the
“Shape” of the Shapiro delay (s) was
measured with any degree of
confidence. In most theories of
gravitation, this measures the sine of
the orbital inclination.
Nevertheless, combining the early
apsidal motion and the measured s,
Champion et al. derived a pulsar
mass of 1.74 +/- 0.04 solar masses,
and a companion mass of 1.07 +/0.02 solar masses!
This is potentially interesting for
studies of the EOS for dense matter!
From: Champion et al., Science, 320, 1309 (2008), on-line additional reference material
Measuring the Mass of PSR J1903+0327: Part I
Caveats: If the companion is indeed a
MS star...
... and it is rotating fast, then it could
add a classical term to the observed
apsidal motion (Dicke’s condition). In
that case, we can’t estimate the total
mass of the system from this effect.
... and has a strong wind, there might
be increases in the delay near superior
conjunction due to propagational
effects in the companion’s wind. These
would contribute to the Shapiro delay.
The GBT measurements do not show
this effect within measurement
accuracy.
Image courtesy: Ransom, Camilo (2008)
Measuring the Mass of PSR J1903+0327: Part II
To eliminate (or confirm) these pesky possibilities, and improve the precision on the PK
parameters, we started timing the pulsar regularly at Arecibo at several frequencies:
At 2.0 - 2.4 GHz we can achieve a precision of 0.55 us every 500 seconds for a total
bandwidth of 200 MHz. Pulsar is joining the array being used for detection of gravitational
waves.
This has improved significantly the measurement of the apsidal motion (86.58 +/- 0.25
arcseconds per century), of s (0.977 +/- 0.002, implying an orbital inclination of about 78
degrees) and now allows a measurement of the “range parameter” (r) of the Shapiro delay. In
general relativity, this parameter is proportional to the companion mass. This is very important
to confirm whether (or not!) the apsidal motion is fully relativistic.
Multi-frequency observations (to be obtained around June 2009) at 1.5 and 3.5 GHz will
produce much improved limits on the extra electron column density at superior conjunction.
These measurements can rule out (or not!) any significant contributions to the timing of the
extra gas near superior conjunction.
Results
The improvement in the apsidal motion
and the Shapiro s now leads to a pulsar
mass of 1.67 +/- 0.01 solar masses. This
is 1.75-sigma lower that the
measurement in Champion et al. (2008),
the variation is probably caused by
correlations with other parameters.
The mass measurement has became very
stable after 1 year of S-band Arecibo
data.
These parameters predict a companion
mass of 1.028 +/- 0.004 solar masses.
The companion mass derived from
Shapiro r is 1.00 +/- 0.04 solar masses.
The predicted value is 0.7 sigma above
this - not statistically different.
From: Freire et al., 2009 (in preparation)
Results
The mass predicted from the Shapiro r and s is
1.58 +/- 0.12 solar masses, i.e., we can’t
exclude a “normal” mass of 1.4 solar masses
based on the Shapiro effect alone.
But likewise, we can’t exclude from the
Shapiro effect alone the possibility that the
apsidal motion and s are giving us the real
pulsar mass.
More measurements will improve r and
disentangle it better from astrometric and
other orbital effects. We hope to decide
conclusively for either possibility based on the
Shapiro effect alone.
The proper motion already affects the apsidal
motion at a level similar to its present
measurement uncertainty.
From: Freire et al., 2009 (in preparation)
Summary
Combining the apsidal motion and the Shapiro s, we obtain a pulsar mass of 1.67 +/- 0.01
solar masses.
A precise measurement of the Shapiro r has not contradicted the assertion that the apsidal
motion is relativistic!
This is the most precise mass ever derived for a millisecond pulsar.
This is also the largest pulsar mass that has ever been precisely measured. It is also the only
one that is backed up with extra PK parameters. It shows that during accretion, millisecond
pulsars can become more massive than “normal” pulsars.
More measurements will improve all the PK parameters and show whether the Shapiro delays
are contaminated by dispersive delays or not.
If companion is optically detected, we might measure the mass ratio and test GR precisely in
a system with two components of very different degree of compactness.
Although not spectacular, this is a very solid constraint on the EOS.
Thank you for your time!
• For questions, comments, discussion, contact me at: [email protected], or visit my website at
http://www.naic.edu/~pfreire.
• For an up-to-date list of pulsars in globular clusters, check http://www2.naic.edu/~pfreire/
GCpsr.html
The National Astronomy and Ionosphere Center is operated by Cornell University, under a
cooperative agreement with the National Science Foundation.
The National Radio Astronomy Observatory is a Facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Incorporated.
The Australia Telescope is funded by the Commonwealth of Australia for operation as a
National Facility managed by the CSIRO.
India’s Giant Metrewave Radio Telescope is run by the National Center for Radio
Astrophysics of the Tata Institute of Fundamental Research