Constraining the Braking Index and Energy Partition of Magnetar spindown with {\em Swift}/XRT data

Abstract

The long-lasting X-ray plateau emission in long gamma-ray bursts (GRBs) shows observational evidence for ongoing energy injection, which may be from magnetar spindown due to energy released via either magnetic dipole (MD) or gravitational wave (GW) radiation. In this paper, by systematically analyzing the {\em Swift}/XRT light curves detected before 2018 July, we find 45 light curves with a measured redshift that monotonically decay as a smooth broken power law. By assuming that the central engines of these GRBs are newly born magnetars, we measure the braking index $n$ of putative millisecond magnetars, due to MD and GW radiations. The inferred braking indices are not close to 3 or 5, but range between them with a normal distribution ($n_{\mathrm{c}}=4.02\pm 0.11$). We define a dimensionless parameter $\Re$, which is the ratio between the MD and GW components, and find that the energy released via magnetar spindown in most GRBs of our sample is dominated by GW radiation for $P_0=3$ ms and $\epsilon =0.005$ and 0.01. On the other hand, we find that $\Re$ and the braking index $n$ seem to be anticorrelated within a large systematic error at $t=0$, but depend on the values of the parameters $P_0$ and $\epsilon$. These results suggest that the contribution of GW radiation cannot be ignored, and that a larger braking index leads to GWs dominating the energy released during magnetar spindown if indeed magnetars are operating in some long GRBs.

8 pages, 3 figures. ApJ in press, matched the published version

References in corpus (19)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Short Duration Gamma-Ray Bursts with Extended Emission from Proto-Magnetar Spin-Down
• A Comprehensive Analysis of the Swift/XRT Data: II. Diverse Physical Origins of the Shallow Decay Segment
• 45 Years of Rotation of the Crab Pulsar
• The Millisecond Magnetar Central Engine in short GRBs
• The optically unbiased GRB host (TOUGH) survey. I. Survey design and catalogs
• The X-ray afterglow flat segment in short GRB 051221A: Energy injection from a millisecond magnetar?
• A comprehensive analysis of Swift/XRT data: I. Apparent spectral evolution of GRB X-ray tails
• Internal Energy Dissipation of Gamma-Ray Bursts Observed with Swift: Precursors, Prompt Gamma-rays, Extended emission and Late X-ray Flares
• The braking index of millisecond magnetars
• The optically unbiased GRB host (TOUGH) survey. V. VLT/X-shooter emission-line redshifts for Swift GRBs at z~2
• The role of newly born magnetars in gamma-ray burst X-ray afterglow emission: Energy injection and internal emission
• Fallback Accretion onto a Newborn Magnetar: short GRBs with Extended Emission
• Magnetar central engine and possible gravitational wave emission of nearby short GRB 160821B
• The unusual glitch recoveries of the high magnetic field pulsar J1119$-$6127
• Signature of a New-Born Black Hole from the Collapse of a Supra-massive Millisecond Magnetar
• Photosphere emission in the X-Ray Flares of Swift Gamma-Ray Bursts and Implications for the Fireball Properties
• Gravitational wave emission from oscillating millisecond pulsars
• The Braking Index of a Radio-quiet Gamma-ray Pulsar

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• Discovery of Three Candidate Magnetar-powered Fast X-ray Transients from Chandra Archival Data
• Interpreting the X-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars
• On the properties of a newborn magnetar powering the X-ray transient CDF-S XT2
• Comparison of the Characteristics of Magnetars Born in Death of Massive Stars and Merger of Compact Objects With {\em Swift} Gamma-Ray Burst Data
• Constraining the ellipticity of new-born magnetar with the observational data of Long gamma-ray bursts
• GRB 111209A/SN 2011kl: Collapse of a supramassive magnetar with r-mode oscillation and fall-back accretion onto a newborn black hole
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