472 research outputs found
New mass limit for white dwarfs: super-Chandrasekhar type Ia supernova as a new standard candle
Type Ia supernovae, sparked off by exploding white dwarfs of mass close to
Chandrasekhar limit, play the key role to understand the expansion rate of
universe. However, recent observations of several peculiar type Ia supernovae
argue for its progenitor mass to be significantly super-Chandrasekhar. We show
that strongly magnetized white dwarfs not only can violate the Chandrasekhar
mass limit significantly, but exhibit a different mass limit. We establish from
foundational level that the generic mass limit of white dwarfs is 2.58 solar
mass. This explains the origin of over-luminous peculiar type Ia supernovae.
Our finding further argues for a possible second standard candle, which has
many far reaching implications, including a possible reconsideration of the
expansion history of the universe.Comment: 7 pages including 2 figures and supplementary information; accepted
for publication in Physical Review Letter
Maximum mass of stable magnetized highly super-Chandrasekhar white dwarfs: stable solutions with varying magnetic fields
We address the issue of stability of recently proposed significantly
super-Chandrasekhar white dwarfs. We present stable solutions of magnetostatic
equilibrium models for super-Chandrasekhar white dwarfs pertaining to various
magnetic field profiles. This has been obtained by self-consistently including
the effects of the magnetic pressure gradient and total magnetic density in a
general relativistic framework. We estimate that the maximum stable mass of
magnetized white dwarfs could be more than 3 solar mass. This is very useful to
explain peculiar, overluminous type Ia supernovae which do not conform to the
traditional Chandrasekhar mass-limit.Comment: 10+1 pages including 4 figures and 1 table; version accepted for
publication in JCA
Strongly magnetized cold electron degenerate gas: Mass-radius relation of the magnetized white dwarf
We consider a relativistic, degenerate electron gas at zero-temperature under
the influence of a strong, uniform, static magnetic field, neglecting any form
of interactions. Since the density of states for the electrons changes due to
the presence of the magnetic field (which gives rise to Landau quantization),
the corresponding equation of state also gets modified. In order to investigate
the effect of very strong magnetic field, we focus only on systems in which a
maximum of either one, two or three Landau level(s) is/are occupied. This is
important since, if a very large number of Landau levels are filled, it implies
a very low magnetic field strength which yields back Chandrasekhar's celebrated
non-magnetic results. The maximum number of occupied Landau levels is fixed by
the correct choice of two parameters, namely the magnetic field strength and
the maximum Fermi energy of the system. We study the equations of state of
these one-level, two-level and three-level systems and compare them by taking
three different maximum Fermi energies. We also find the effect of the strong
magnetic field on the mass-radius relation of the underlying star composed of
the gas stated above. We obtain an exciting result that, it is possible to have
an electron degenerate static star, namely magnetized white dwarfs, with a mass
significantly greater than the Chandrasekhar limit in the range 2.3-2.6M_Sun,
provided it has an appropriate magnetic field strength and central density. In
fact, recent observations of peculiar Type Ia supernovae - SN 2006gz, SN
2007if, SN 2009dc, SN 2003fg - seem to suggest super-Chandrasekhar-mass white
dwarfs with masses up to 2.4-2.8M_Sun, as their most likely progenitors.
Interestingly our results seem to lie within the observational limits.Comment: 28 pages including 7 figures; section 4.1 significantly modified,
section 4.7 and Appendix including Figure 7 added; version appear to Physical
Review
Modified Einstein's gravity as a possible missing link between sub- and super-Chandrasekhar type Ia supernovae
We explore the effect of modification to Einstein's gravity in white dwarfs
for the first time in the literature, to the best of our knowledge. This leads
to significantly sub- and super-Chandrasekhar limiting masses of white dwarfs,
determined by a single model parameter. On the other hand, type Ia supernovae
(SNeIa), a key to unravel the evolutionary history of the universe, are
believed to be triggered in white dwarfs having mass close to the Chandrasekhar
limit. However, observations of several peculiar, under- and over-luminous
SNeIa argue for exploding masses widely different from this limit. We argue
that explosions of the modified gravity induced sub- and super-Chandrasekhar
limiting mass white dwarfs result in under- and over-luminous SNeIa
respectively, thus unifying these two apparently disjoint sub-classes and,
hence, serving as a missing link. Our discovery raises two fundamental
questions. Is the Chandrasekhar limit unique? Is Einstein's gravity the
ultimate theory for understanding astronomical phenomena? Both the answers
appear to be no.Comment: 14+1 pages including 2 figures and 1 table; version published in JCA
New mass limit of white dwarfs
Is the Chandrasekhar mass limit for white dwarfs (WDs) set in stone? Not
anymore -- recent observations of over-luminous, peculiar type Ia supernovae
can be explained if significantly super-Chandrasekhar WDs exist as their
progenitors, thus barring them to be used as cosmic distance indicators.
However, there is no estimate of a mass limit for these super-Chandrasekhar WD
candidates yet. Can they be arbitrarily large? In fact, the answer is no! We
arrive at this revelation by exploiting the flux freezing theorem in observed,
accreting, magnetized WDs, which brings in Landau quantization of the
underlying electron degenerate gas. This essay presents the calculations which
pave the way for the ultimate (significantly super-Chandrasekhar) mass limit of
WDs, heralding a paradigm shift 80 years after Chandrasekhar's discovery.Comment: 6 pages; received Honorable Mention in the Gravity Research
Foundation 2013 Awards for Essays on Gravitation; version accepted for
publication in IJMP
Radiatively Inefficient Accretion Flow Simulations with Cooling: Implications for Black Hole Transients
We study the effects of optically thin radiative cooling on the structure of
radiatively inefficient accretion flows (RIAFs). The flow structure is
geometrically thick, and independent of the gas density and cooling, if the
cooling time is longer than the viscous timescale (i.e., ). For higher densities, the gas can cool before it can accrete
and forms the standard geometrically thin, optically thick Shakura-Sunyaev
disk. For usual cooling processes (such as bremsstrahlung), we expect an inner
hot flow and an outer thin disk. For a short cooling time the accretion flow
separates into two phases: a radiatively inefficient hot coronal phase and a
cold thin disk. We argue that there is an upper limit on the density of the hot
corona corresponding to a critical value of , the ratio of the cooling time and the free-fall time. Based on our
simulations, we have developed a model for transients observed in black hole
X-ray binaries (XRBs). An XRB in a quiescent hot RIAF state can transition to a
cold black-body dominated state because of an increase in the mass accretion
rate. The transition from a thin disk to a RIAF happens because of mass
exhaustion due to accretion; the transition happens when the cooling time
becomes longer than the viscous time at inner radii. Since the viscous
timescale for a geometrically thin disk is quite long, the high-soft state is
expected to be long-lived. The different timescales in black hole transients
correspond to different physical processes such as viscous evolution, cooling,
and free-fall. Our model captures the overall features of observed state
transitions in XRBs.Comment: 17 pages, 10 figures; version accepted to MNRAS; runs with thermal
conduction included in this revisio
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