Similar radiation mechanism in gamma-ray bursts and blazars: evidence from two luminosity correlations
Active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) are powerful astrophysical events with relativistic jets. In this Letter the broadband spectral properties are compared between GRBs and the well-observed blazars. The distribution of GRBs are consistent with the well-known blazar sequence including the and correlations, where is defined as the broadband spectral slope in radio-to-X-ray bands, and is defined as the spectral peak frequency. Moreover, GRBs occupy the low radio luminosity end of these sequences. These two correlations suggest that GRBs could have a similar radiation process with blazars both in the prompt emission and afterglow phases, i.e., synchrotron radiation.
Gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) are both powered by relativistic jets from accreting black holes (Gehrels et al. 2009; Urry & Padovani 1995). The central engines of GRBs are argued to be stellar-mass black holes (Woosley 1993) and for AGNs the central engines are supermassive black holes. GRBs are most powerful explosions with isotropic-equivalent energy erg in the universe (Zhang 2011), and can be detected out to very high-redshift universe (Lamb & Reichart 2000; Wang et al. 2012). So GRBs can probe high-redshift universe, including dark energy (Dai, Liang & Xu 2004; Schaefer 2007; Wang & Dai 2011). Blazars include two subtype of AGNs, i.e., flat-spectrum radio quasars (FSRQs) and BL Lac objects (BL Lacs). A subclass of AGNs, e.g., super-Eddington accreting supermassive black holes, are also proposed to be a standard candle (Wang et al. 2013).
The radiation mechanism of balzars is well constrained. The spectral energy distribution (SED) of blazars is well understood, including the low-energy (infrared-soft X-ray) bump and the high-energy (MeV-GeV) bump. The synchrotron radiation can account for the low-energy peak, while the MeV-GeV peak is produced by inverse Compton radiation. But for GRBs, the radiation mechanism for the prompt emission is still highly debated. The spectrum of prompt emission can be modeled by the “Band function” (Band et al. 1993), whose origin is still unknown (but see Lucas Uhm & Zhang 2013). Some studies (Mészáros et al. 1994; Daigne & Mochkovitch 1998) proposed that synchrotron radiation is the leading mechanism. Other mechanisms are also proposed (Pe’er et al. 2006; Rees & Mészáros 2005; Beloborodov 2010). The radiation mechanism of afterglows is well understood (Sari et al. 1998). The observed afterglow radiation is well explained by synchrotron radiation (Sari et al. 1998; Panaitescu & Kumar 2001).
Some studies (Zhang 2007; Wang & Dai 2013; Wang et al. 2014) have proposed that the mechanisms in different scale outflow or jet systems may be the same. Some works have been done on comparison between GRBs and AGNs. Wang & Wei (2011) compared the spectral properties of blazars and optically bright GRB afterglows, and found that GRB afterglows have the same radiation mechanism as BL Lac objects. A similar correlation of the synchrotron luminosity and Doppler factor between GRBs and AGNs has been found (Wu et al. 2011). Nemmen et al. (2012) suggested that the relativistic jets in AGNs and GRBs have a similar energy dissipation efficiency. Ma et al. (2014) extended the analysis of Nemmen et al. (2012) by adding X-ray binaries and low-luminosity AGNS. Wang & Dai (2013) found that the GRB X-ray flares and solar X-ray flares have similar distributions, which indicate that the X-ray flares of GRBs are due to a magnetic reconnection process. These similar distributions also exist in X-ray flares from black hole systems with (Wang et al. 2014). Zhang et al. (2013) found that the prompt emission of GRBs may be produced by magnetic dominated jets.
In this Letter we compare the broadband spectral properties of GRBs and blazars, including the and correlations, where is the radio-to-X-ray spectral slope. For a GRB, is the peak frequency of spectrum of prompt emission, while is the low peak of spectrum for a blazar. The aim of this Letter is to explore a possible similarity in radiation mechanism between GRBs and blazars. this Letter is organized as follows. In section 2, we present the sample of blazars and GRBs. The fitting results are given in section 3. Section 4 gives conclusions and discussions.
Chandra & Frail (2012) compiled a sample of GRB radio afterglow observations from 1997 to 2011. This catalog consists of 304 GRBs with radio observations. We select 43 GRBs with redshift measurements. The sample is listed in Table 1. For each GRB, the name of GRB and redshift are presented in Columns 1 and 2, respectively. X-ray flux measured in 0.3-10 keV energy band is given in Column 3 observed by Swift at the time of Column 6. For bursts observed by BASTE, the energy range is between 2 keV and 10 keV. We use the typical GRB spectrum to convert the flux to 1 keV. The observed radio flux (Column 4) and frequency (Column 5) at the time of Column 6 are also provided. Column 7 is the jet opening angle. For GRBs without opening angle determination, we assume a typical value 5 degree. The derived collimation-corrected radio luminosity at 5 GHz is given in Column 8. Column 9 gives , which is the peak energy of the prompt spectrum. The parameters from Column 2 to Column 7 are taken from Chandra & Frail (2012). We use the value of from Wang, Qi & Dai (2011). In the calculation, we use the peak frequency in the GRB rest frame. The collimation-corrected radio luminosity is calculated as
where is the luminosity distance, is radio flux at 5 GHz, is the beaming factor and is the spectral slope (Sari et al. 1998). We adopt in the slow cooling case. The observed radio flux is converted to flux at 5 GHz using spectral slope . In this work, we assume the cosmological parameters: km s Mpc, , and .
We use the spectral properties of balzars from Fossati et al. (1998). This sample consists of all the parameters that we require, including redshift, X-ray flux at 1 keV, radio flux at 5 GHz, and synchrotron peak frequency. Fossati et al. (1998) found a power spectral sequence for the blazars despite the difference in the continuum shapes among different sub-classes of blazars. This sequence indicates that the radio luminosity is anti-correlated with the synchrotron peak. A plausible interpretation is that relativistic jets radiate via synchrotron and inverse Compton processes if the physical parameters (i.e. magnetic field) vary with luminosity.
3.1 correlation in blazars and GRBs
The radio luminosities at 5 GHz are anti-correlated with the synchrotron peaks of blazars, as found by Fossati et al. (1998). This correlation has not been studied in GRBs so far. We investigate this correlation in GRBs for the first time. Figure 1 shows the correlation of blazars and GRBs. The black and open dots represent blazars and GRBs, respectively. There is a tight correlation between and as expected from the blazar sequence (Fossati et al. 1998). The correlation coefficient is at a significance level from Spearman rank-order statistical test. From this figure, we can see that the GRBs occupy the low-luminosity region of this correlation. For both blazars and GRBs, the best fitting result is
The correlation coefficient is improved to with probability . The correlation coefficient has an obvious enhancement after adding the GRB sample. GRB 060218 may deviate from this correlation. The possible reason is that this GRB usually called X-ray flash has a low peak energy (Soderberg et al. 2006; Pian et al. 2006).
3.2 correlation in blazars and GRBs
The broad-band spectral slope is also correlated with luminosity at 5 GHz in blazars (Fossati et al. 1998). We also investigate this correlation in GRBs. The broad-band spectral slope is defined as
where GHz, and keV. The X-ray flux at keV can be obtained as follows. From Column 3 of Table 1, X-ray flux measured in 0.3-10 keV energy range can be obtained. The X-ray flux usually evolves as (Sari et al. 1998), with is the power-law index of accelerated electrons distribution. The X-ray flux in 0.3-10 keV is . After obtaining the value of , the flux at keV can be derived.
Figure 2 shows a correlation between the spectral slope and radio luminosity at 5 GHz for blazars (black dots) and GRBs (open dots). The correlation coefficient is with probability using Spearman rank-order statistical test for blazars. After combining GRBs and blazars, the fitting result is
The Spearman’s rank correlation coefficient is with probability . GRBs also occupy the the low-luminosity end of this correlation.
4 Conclusions and Discussions
The physics of GRBs are poorly understood, i.e., the radiation mechanism of prompt emission, the value of Lorentz factor, jet composition, and central engine (Zhang 2011). Wang & Dai (2013) found similar frequency distributions between X-ray flares of GRBs and solar X-ray flares, which may indicate the magnetically dominated jets in GRBs. In this Letter we compile 43 GRBs with well X-ray and radio observations. Two new correlations between GRBs and blazars may provide a new clue as to the radiation mechanism of GRB prompt emission and afterglows. For example, our clear and correlations suggest that the radiation mechanism of GRBs in prompt and afterglow phases and blazars is similar, namely, synchrotron radiation. Moreover, GRBs are occupy the low-luminosity region of these correlations.
In this Letter, we use the radio luminosities during the GRB afterglow phase. Although some models predict that bright radio emission may be generated within about 10 s of the initial explosion of a GRB (Usov & Katz 2000; Sagiv & Waxman 2002; Shibata et al. 2011), but a detection of prompt radio emission has some impediments, such as scattering (Macquart 2007; Lyubarsky 2008). Bannister et al. (2012) have searched for prompt radio emission from nine GRBs at 1.4 GHz, and found single dispersed radio pulses with significance in a few minutes following two GRBs. Unfortunately, the probability of GRB origin is only 2%. There has been no confirmed evidence for detection of GRB prompt radio emission up to now. So we use the radio emission of an afterglow in this Letter. Theoretically, internal shocks produce the GRB prompt emission, and external shocks produce the afterglow emission. The radiation mechanism of an afterglow is well constrained, i.e., synchrotron emission. But the prompt emission related to is not well understood. If some similar correlations between and exist in GRBs and blazars, the radiation mechanism of prompt emission is argued as synchrotron emission. Until now, the location of blazar gamma-ray emission regions are still uncertain (Marscher et al. 2010), since some theories locate blazar gamma-ray emission regions close to the black hole/accretion disk (Blandford & Levinson 1995) while the others place them at parsec scales in the radio jet (Jorstad et al. 2001). The X-ray emission from kiloparsec-scale blazar jets has been observed (Harris & Krawczynski 2006). Meanwhile, the radio emission region of blazars spans kiloparsec scale. Kharb et al. (2010) found that a few blazars exhibit only radio core emission. But the X-ray emission region and radio emission region do not fully overlap. So the high-energy and radio emissions of blazars also originate from different regions. So a comparison of the correlation between GRBs and the blazar sequence is reasonable. Liang et al. (2004) found that the peak energy evolves with isotropic-equivalent luminosity from the time-resolved spectra. They also found that the correlation also holds for time-resolved spectra and time-integrated spectra. Ghirlanda et al. (2010) studied the time-resolved spectra of Fermi GRBs. The peak energy correlates with the luminosity within individual bursts (Ghirlanda et al. 2010). Moreover, the time-resolved correlation is very similar for all the bursts and has a slope similar to the correlation defined by the time-integrated spectra of different bursts detected by several different satellites. The time-integrated value of is widely used in luminosity correlations of GRBs, for example (Amati et al. 2002), (Yonetoku et al. 2004) and (Ghirlanda et al. 2004) correlations. So if some correlation is due to a similar physical mechanism, this correlation holds no matter whether the time-resolved or time-integrated values are used.
Fossati et al. (1998) found that is anti-correlated with the synchrotron peak luminosity for blazars. For GRBs, Liang et al. (2004) found that is positively correlated with the isotropic-equivalent luminosity (about total luminosity). But the luminosity in the correlation for blazars is the synchrotron peak luminosity, not the total luminosity. Because there are two peaks in a blazar spectral energy distribution and there is no correlation between and in GRBs (Chandra & Frail 2012), from our simple analysis above we cannot conclude that GRBs have a different correlation compared with that for blazars. In this paper, we find that GRBs occupy the low radio luminosity end of the blazar sequence, which is similar to that of Wang & Wei (2011).
|erg cm s||JY||GHz||days||erg s||keV|
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