Similar radiation mechanism in gamma-ray bursts and blazars: evidence from two luminosity correlations

# Similar radiation mechanism in gamma-ray bursts and blazars: evidence from two luminosity correlations

## Abstract

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 burst: general - BL Lacertae objects: general - methods: statistical
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## 1 Introduction

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.

## 2 Samples

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

 νLν(5GHz)=4πd2LνfνFbeam(1+z)−α−1, (1)

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 Results

### 3.1 νLν(5GHz)−νpeak 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

 logνLν(5 GHz)=(−0.91±0.03)logνpeak+56.65±0.46. (2)

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 νLν(5GHz)−αRX 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

 αRX=−log(fνR)/log(fνx)log(νR/νx), (3)

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

 αRX=(0.084±0.006)log(νLν(5GHz))+(−2.87±0.24). (4)

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.

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).

We thank the referee for detailed and very constructive suggestions that have allowed us to improve our manuscript. We acknowledge helpful discussions with Y. C. Zou and X. F. Wu. This work is supported by the National Basic Research Program of China (973 Program, grant No. 2014CB845800) and the National Natural Science Foundation of China (grants 11373022, 11103007, and 11033002).

### Footnotes

1. affiliationtext: E-mail: fayinwang@nju.edu.cn (FYW); dzg@nju.edu.cn (ZGD)

### References

1. Amati, L., Frontera, F., Tavani, M, et al. 2002, A&A, 390, 81
2. Band, D., Matteson, J., Ford, L., et al. 1993, ApJ, 413, 281
3. Bannister, K. W., Murphy, T., Gaensler, B. M. & Reynolds, J. E., 2012, ApJ, 757, 38
4. Blandford, R. D., & Levinson, A. 1995, ApJ, 441, 79
5. Chandra, P. & Frail, D. A., 2012, ApJ, 746, 156
6. Dai, Z. G., Liang, E. W., & Xu, D. 2004, ApJ, 612, L101
7. Daigne, F., & Mochkovitch, R. 1998, MNRAS, 296, 275
8. Beloborodov, A. M., 2010, MNRAS, 407, 1033
9. Fardal, M. A., Katz, N., Weinberg, D. H. & Dave, R., 2007, MNRAS, 379, 985
10. Ferguson, H. C., Dickinson, M., & Papovich, C., 2002, ApJ, 569, L65
11. Fossati, G., Maraschi, L., Celotti, A., Comastri, A., & Ghisellini, G. 1998, MNRAS, 299, 433
12. Friedmann, A. S., & Bloom, J. S. 2005, ApJ, 627, 1
13. Gehrels, N., Ramirez-Ruiz, E. & Fox, D. B., 2009, ARA&A, 47, 567
14. Ghirlanda, G., Ghisellini, G. & Lazzati, D., 2004, ApJ, 616, 331
15. Ghirlanda, G., Nava, L. & Ghisellini, G., 2010, A&A, 511, A43
16. Harris, D. E., & Krawczynski, H., 2006, ARA&A, 44, 463
17. Jorstad, S. G., Marscher, A. P., Mattox, J. R., et al. 2001, ApJ, 556, 738
18. Kharb, P., Lister, M. L. & Cooper, N. J., 2010, ApJ, 710, 764
19. Lamb, D. Q., & Reichart, D. E. 2000, ApJ, 536, 1
20. Liang, E. W., Dai, Z. G. & Wu, X. F., 2004, ApJ, 606, L29
21. Lucas Uhm, Z., & Zhang, B., 2013, arXiv: 1303.2704
22. Lyubarsky, Y. 2008, ApJ, 682, 1443
23. Ma, R. Y., Xie, F. G., & Hou, S. J., 2014, ApJL, 780, L14
24. Macquart, J. P., 2007, ApJL, 658, L1
25. Marscher, A. P., Jorstad, S. G., Larionov, V. M., et al. 2010, ApJ, 710, L126
26. Mészáros, P., Rees, M. J., & Papathanassiou, H. 1994, ApJ, 432, 181
27. Nemmen, R. S., Georganopoulos, M., Guiriec, S., et al. 2012, Sci, 338, 1445
28. Panaitescu, A., & Kumar, P., 2001, ApJ, 560, L49
29. Pe’er, A., Mészáros, P., & Rees, M. J., 2006, ApJ, 642, 995
30. Pian, E., Mazzali, P. A., Masetti, N., et al., 2006, Nature, 442, 1011
31. Rees, M. J., & Mészáros, P., 2005, ApJ, 628, 847
32. Sagiv, A., & Waxman, E. 2002, ApJ, 574, 861
33. Sari, R., Piran, T., & Narayan, R. 1998, ApJ, 497, L17
34. Schaefer, B. E., 2007, ApJ, 660, 16
35. Soderberg, A. M., Kulkarni, S. R., Nakar, E., et al., 2006, Nature, 442, 1014
36. Urry, C. M., & Padovani, P. 1995, PASP, 107, 803
37. Usov, V. V., & Katz, J. I. 2000, A&A, 364, 655
38. Wang, F. Y., Bromm, V., Greif. T. H., Stacy, A., Dai, Z. G., Loeb, A. & Cheng, K. S. 2012, ApJ, 760, 27
39. Wang, F. Y., & Dai, Z. G., 2013, Nature Phys., 9, 465
40. Wang, F. Y., Dai, Z. G., & Yi, S. X., 2014, submitted
41. Wang, F. Y., & Dai, Z. G. 2011, A&A, 536, A96
42. Wang, F. Y., Qi, S., & Dai, Z. G., 2011, MNRAS, 415, 3423
43. Wang, J., & Wei, J. Y. 2011, ApJL, 726, L4
44. Wang, J. M., Du, P., Valls-Gabaud, D., et al. 2013, PRL, 110, 081301
45. Woosley, S. E., ApJ, 1993, 405, 273
46. Wu, Q., Zou, Y. C., Cao, X., Wang, D. X., & Chen, L. 2011, ApJL, 740, L21
47. Yonetoku, D., Murakami, T., Nakamura, T. et al., 2004, ApJ, 609, 935
48. Zhang, B. 2011, CRPhy, 12, 206
49. Zhang, J., Liang, E. W., Sun, X. N., et al., 2013, ApJL, 774, L5
50. Zhang, S. N. 2007, HiA, 14, 41
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