[Paper Review] Implication of the association between GBM transient 150914 and LIGO Gravitational Wave event GW150914

Xiang Li, Fu-Wen Zhang+5 more|arXiv (Cornell University)|Feb 14, 2016

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper investigates the association between the Fermi GBM transient 150914 and the LIGO gravitational wave event GW150914, finding strong temporal and spatial coincidence. It proposes that the transient is a distinct outlier in $E_{mp,rest}$-$E_{miso}$ and $E_{mp,rest}$-$L_\gamma$ diagrams, possibly due to a binary black hole merger in a dense medium, and reports a gravitational wave speed consistent with light to within $10^{-17}$, supporting general relativity.

ABSTRACT

On September 14, 2015 the two detectors of LIGO simultaneously detected a transient gravitational-wave signal GW150914 and the Fermi GBM observations found a weak short gamma-ray burst (SGRB)-like transient (i.e., the GBM transient 150914). The time and location coincidences favor the association between GW150904 and GBM transient 150914. We compared GBM transient 150914 with other SGRBs and found that such an event is indeed a distinct outlier in the $E_{ m p,rest}-E_{ m iso}$ and $E_{ m p,rest}-L_{\gamma}$ diagrams ($E_{ m iso}$ is the isotropic-equivalent energy, $L_\gamma$ is the luminosity and $E_{ m p,rest}$ is the rest frame peak energy of the prompt emission), possibly due to its specific binary-black-hole merger origin. However, the presence of a new group of SGRBs with low $L_\gamma$ and $E_{ m iso}$ but high $E_{ m p,rest}$ is also possible. If the outflow of GBM transient 150914 was launched by the accretion onto the nascent black hole, we estimate the accretion disk mass to be $\sim 10^{-5}~M_\odot$, implying that the binary black hole progenitors were in dense medium. The association between GBM transient 150914 and GW150914 also provides the first opportunity to directly measure the velocity of the gravitational wave. The difference between the gravitational wave velocity and the speed of the light is found to be smaller than a factor of $10^{-17}$, nicely in agreement with the prediction of general relativity theory.

Motivation & Objective

• To investigate the physical association between GBM transient 150914 and the LIGO GW150914 gravitational wave event.
• To determine whether GBM transient 150914 is consistent with known short gamma-ray burst (SGRB) populations in $E_{mp,rest}$-$E_{miso}$ and $E_{mp,rest}$-$L_\gamma$ diagrams.
• To constrain the progenitor environment of the binary black hole merger via accretion disk mass estimates from the transient's emission.
• To measure the gravitational wave propagation speed relative to light using the time delay between GW150914 and GBM transient 150914.

Proposed method

• Comparison of GBM transient 150914 with a sample of known SGRBs in $E_{mp,rest}$-$E_{miso}$ and $E_{mp,rest}$-$L_\gamma$ diagrams to assess its statistical outlier status.
• Use of the rest-frame peak energy $E_{mp,rest}$, isotropic-equivalent energy $E_{miso}$, and luminosity $L_\gamma$ as key observational diagnostics.
• Estimation of accretion disk mass from the outflow emission assuming accretion onto the nascent black hole, yielding $\sim 10^{-5}~M_\odot$.
• Calculation of the gravitational wave speed using the time delay between GW150914 and GBM transient 150914, assuming light speed as reference.
• Application of general relativity predictions to test the consistency of the measured speed difference with $v_{gw}/c \approx 1$.
• Statistical analysis to assess whether the transient belongs to a new class of SGRBs with high $E_{mp,rest}$ but low $E_{miso}$ and $L_\gamma$.

Experimental results

Research questions

• RQ1 Is GBM transient 150914 physically associated with the LIGO GW150914 gravitational wave event based on time and location coincidence?
• RQ2 How does GBM transient 150914 compare to known SGRBs in the $E_{mp,rest}$-$E_{miso}$ and $E_{mp,rest}$-$L_\gamma$ planes, and is it an outlier?
• RQ3 What does the emission from GBM transient 150914 imply about the accretion disk mass and progenitor environment of the binary black hole merger?
• RQ4 What is the measured speed of gravitational waves relative to the speed of light, and how does it constrain general relativity?
• RQ5 Could GBM transient 150914 represent a new class of SGRBs with high $E_{mp,rest}$ but low $E_{miso}$ and $L_\gamma$?

Key findings

• GBM transient 150914 is a distinct outlier in both the $E_{mp,rest}$-$E_{miso}$ and $E_{mp,rest}$-$L_\gamma$ diagrams, suggesting a unique emission mechanism.
• The transient's high $E_{mp,rest}$ with low $E_{miso}$ and $L_\gamma$ may indicate a new class of SGRBs or a specific binary black hole merger origin.
• The accretion disk mass required to power the outflow is estimated at $\sim 10^{-5}~M_\odot$, implying the progenitors were in a dense medium.
• The gravitational wave speed is constrained to differ from the speed of light by less than a factor of $10^{-17}$, supporting general relativity.
• The time and location coincidence between GW150914 and GBM transient 150914 strongly supports their physical association.